PC Planetarium
Version 2.36

Copyright (C) 1988-1998, Stephen C. Bryant
All rights reserved

October 15, 1998



SKYMAP displays the sky as it would appear at a given place and time. The display has much in common with the monthly star maps published in various astronomy magazines, but at your choice of time and place. You can use it to check on current celestial events, to preview the sky as it will appear somewhere or sometime else, or to view the heavens in ways that are impossible in the real world.

Our goal is to show how the sky will look to you, and how it changes with time. We are not so concerned with the exact coordinates needed to point a telescope at a particular star or planet (although this is provided by SKYMAP), instead, we want to see the sky as a whole, quickly and easily, and to watch it as time passes.

SKYMAP includes built-in information on the stars down to magnitude 5.5, the sun, the moon, and the planets.

It is possible to select only the brightest stars, and to turn individual planets on and off.

In addition, many new objects may be added to the display by storing their coordinates in separate data files. Some samples are included with SKYMAP.

Given standard orbital elements, such as those published in Sky & Telescope, SKYMAP can show the positions, orbits and magnitudes of comets and asteroids.

Any date and time may be selected, and successive displays may be generated at regular intervals.

To help visualize why members of the solar system appear where they do, a separate display maps the solar system as it would look from far away. This display can be rotated and viewed in 3-D mode.

One strength of the program is its speed. To see how the sky changes over time, the maps of the sky are recalculated and redisplayed continually: much effort has gone into minimizing the recalculation time.

We assume that you are familiar with the basic concepts and terminology of astronomy. If not, check in any guide to the stars for help, although you should be able to use SKYMAP anyway.

Hardware and software requirements
This program runs on an IBM-compatible PC with at least EGA video capabilities. For memory, 512K is required. No math coprocessor is necessary, but the program will recognize and use one if present. (In fact, SKYMAP was originally developed without a coprocessor, and runs quite well without one.) The operating system is MS-DOS, version 3.1 or higher.
SKYMAP is distributed on one diskette. On the diskette is a program called INSTALL.EXE which will move SKYMAP to your hard disk. To use, place the diskette in floppy drive A: or B:, and type either A:INSTALL or B:INSTALL, as appropriate. INSTALL will ask where you want the software installed, creating a new directory if necessary. It will then copy all of the SKYMAP files to that directory.

Using SkyMap

To run, simply type SKYMAP and press ENTER. The program will display a map of the sky as it appears right now together with some descriptive information. Initially, the display shows the stars down to magnitude 4.0, the five visible planets, the sun and the moon.

Some things, such as location, time zone, and colors, can be set in a permanent startup file. If you have not done any setup work, SKYMAP will show the sky as it appears from latitude 40 degrees North, longitude 75 degrees West (which is in New Jersey) at the current date and time (from your computer's clock) in Eastern Standard Time.

These values can all be changed at will, and can be saved in a settings file for regular use.

SKYMAP shows the sun and moon as well as the eight other planets of the solar system. Controls which apply to the planets also apply to the sun and the moon. For simplicity, therefore, we will use the term "planets" to refer to the primary members of the solar system, including the sun and moon.

The SKYMAP display
SKYMAP displays the entire sky as it would appear at a certain place and time. It uses what is called "stereographic projection" to place the celestial objects on the screen. Many printed star maps use the same projection method, and for good reason: it looks "right" over the whole map without obvious distortion.

Some early astronomers thought of the heavens as a series of concentric spheres revolving around the earth. The outermost one held the fixed stars and the inner ones held the planets (five of them), the sun, and the moon. For our purposes, we will simplify even this model, and imagine that all of these heavenly bodies lie on a single sphere, with us, the observers, at the center.

Our sky map is a projection of that sphere, or at least the portion we can see, onto a flat surface: the computer screen. Most people know that to make a map of the earth on a flat piece of paper involves some distortion, perhaps a considerable amount. The same is true of a map of the sky: to make it fit we have no choice but to stretch portions of the map. The trick is to do this as unobtrusively as possible.

Stereographic projection tends to preserve shapes over moderate areas - thus constellations look familiar anywhere on the map. Some distortion is unavoidable. This is evident when comparing two different sections of the map: stars look farther apart at the edges (near the horizon) that they do at the center (near the zenith).

As an demonstration, try following the Big Dipper once around the sky. It will appear larger at the horizon than it does overhead, but it looks like the Big Dipper all the way around. (To do this, start SKYMAP and press the right arrow key one or more times: this will make the display advance through time.)

On the SKYMAP display, the horizon is shown as a blue circle, stars as white spots of different sizes (depending on magnitude) and planets as larger spots, each in a different color.

Some information is displayed on the right hand side of the screen. These items will be described in more detail when we cover the control keys, but for now:

Date and time Shows the current date and time given by the computer's internal clock.
Latitude That of the observer.
Longitude That of the observer.
Magnification Relative enlargement of the display.
Elevation How far above the horizon the display begins.
Facing The direction that the observer seems to be facing (imagining the lower part of the screen to be on the horizon in front of the observer).
Stars Brightness cutoff: only stars whose magnitude is at least this value are displayed.
Planets Shows which planets (including the sun and the moon) are being displayed at any time. The display color of each planet is shown here as well.
Asteroids/comets Shows whether comets and asteroids are being displayed, and in what color.
Objects Shows object categories and colors.
Sun elev The distance of the sun above or below the horizon (its elevation).
Date, Time The date and time of the map being displayed.
UT Universal Time of the display, or Greenwich Mean Time.
LST Local Sidereal Time of the display.
Rate The displayed map is recalculated continually. The date and time of each successive display advances by this rate (which may be negative).
SidR The rate in sidereal time. Sidereal seconds are slightly shorter than ordinary seconds.
To use the SKYMAP menu bar, press and release the ALT key and then use the arrow keys to move through the menu items. Press ENTER or the underlined letter to select an item. All menu items have a hot key which may be used to activate the menu item when the menu is not displayed. The menu bar may be hidden or redisplayed by pressing Alt-B.

Here is a listing of the menu items. Each of these will be described in the Control Keys section of this manual.


        Find Object                     Alt+L
        Turn Cursor On                  Alt+C
	Change Settings                 Alt+S
        Write Coordinates               Alt+W
        Run Dos Commands                Alt+D
        Set Menu Bar Off [or On]        Alt+B
        Set Text Display Off [or On]    Alt+T
        Exit SKYMAP


        Show Solar System               F2
        Jupiter's Moons                 J
        Night Vision                    Alt+N
        Set 3-D Mode On [or Off]        Alt+3
        Set Viewpoint Far [or Near]     Alt+V


        Set Latitude                    F3
        Set Longitude                   F4
        Set Date                        F5
        Set Time                        F7
        Set Time to Now                 Home
        Sidereal Time                   F8
        Universal Time                  F6


        Set Update Rate                 F9
        Set Sidereal Rate               F10
        Real-time Rate                  PgDn
        Set Rate to Zero                End

        Magnitude Limit                 M
        Select Planets                  Alt+P
        Asteroids                       A
        Comets                          C
        Deep Sky Select                 Alt+O
        Magnification                   Alt+M
        Elevation                       Alt+E
        Facing Direction                Alt+F


        Hotkey Summary                  F1
        About SKYMAP                    Alt+A

Control Keys

This section describes the keys used to control SKYMAP. There are quite a few of them: try those that sound interesting and see what happens. The HELP key is F1, and shows a summary of all the control keys.

As soon as it finishes displaying one map, SKYMAP starts calculating the next one. When you use one of the control keys to change the display, SKYMAP usually restarts its calculations.

To change some settings, you enter a new value over the old one. While doing so, calculations are temporarily suspended. To make a change, select the field, use the arrow keys to move the cursor within it and type over the current information. To accept the changes, press ENTER. To quit and revert to the original value, press ESC.

An asterisk (*) is placed next to settings that have been changed. When the display is next updated, the asterisk is removed.

List of Control Keys
For reference, here is a list of the various control keys:
    F1		Help / exit help
    F2		Sky map / solar system map
    F3		Set latitude
    F4		Set longitude
    F5		Set date
    F6		Set Universal Time
    F7		Set time
    F8		Set local sidereal time
    F9		Set rate
    F10		Set rate in sidereal time

    Right	Increase rate
    Left	Decrease rate
    Up		Increase magnitude limit
    Down	Decrease magnitude limit
    Home	Set time to now
    End		Set rate to zero
    PgDn	Real time toggle

    0 - 9	Select/deselect planets

    A		Asteroid display toggle
    C		Comet display toggle
    E		Earth anchor (solar system display)
    H		Hold display/resume
    J		Jupiter's moons
    M		Set magnitude limit
    P		Planet trails on/off
    R		Refresh display
    S		Star trails on/off

    Alt-B	Turn menu bar on or off
    Alt-C	Write coordinate file (positions of planets)
    Alt-D	DOS commands
    Alt-E	Set elevation above horizon
    Alt-F	Set "facing direction"
    Alt-L	Locate an object by name
    Alt-M	Set magnification of display
    Alt-N	Night Vision mode
    Alt-O	Turn object classes on or off
    Alt-P	Turn on the "Pointer" or cursor
    Alt-S	Change settings
    Alt-T	Turn text display on or off
    Alt-V	Solar system viewing distance toggle
    Alt-X	Exit

    Alt-Left	Rotate display 15 degrees clockwise
    Alt-Right	Rotate display 15 degrees counter-clockwise
    Alt-Up	Elevation - Raise the display horizon by 6 degrees
    Alt-Down	Elevation - Lower the display horizon by 6 degrees

    Alt-3	3-D Mode toggle
Of this rather large number of control keys, you will find that in practice you use a relatively small and obvious group of them.
Observer's location

    F3		Set latitude (degrees and minutes)
    F4		Set longitude (degrees and minutes)
Press F3 to change the observer's latitude on earth, and F4 to change his longitude. Both values are entered in degrees and minutes, although you should not worry about getting them exactly right - the nearest degree will work well.

Latitudes are entered in the range 90 (North Pole) to -90 (South Pole). The range of longitudes is 180 (west of Greenwich) to -180 (east of Greenwich).

The initial latitude and longitude can also be set on the Settings Screen (see below).

These keys are also used to set the orientation of the solar system map (see below).

Observer's view of the sky
SKYMAP shows the whole sky within a horizon circle on the screen, with the southern horizon at the bottom. To change the orientation and scope of the display, use the control keys described here. With them, you can choose a direction to be facing, an elevation above the horizon, and a magnification factor.

    Alt-F	Set Facing direction (degrees)
    Alt-Left	Rotate display 15 degrees clockwise
    Alt-Right	Rotate display 15 degrees counter-clockwise
The standard orientation of SKYMAP is to show the southern horizon at the bottom of the screen. The direction that the observer is facing is 180 degrees (North = 0, East = 90, Southeast = 135, etc.) You can change this by pressing "Alt-F" and entering a new direction. To look east, for example, change "Facing" to 90 degrees.

The permissible values are 359 to -359 degrees.

The two keys Alt-Left and Alt-Right increment and decrement, respectively, the facing direction by 15 degrees. This rotates the display clockwise or counter-clockwise. (On some computers, the Alt-Left and Alt-Right keys do not function.)

The facing direction can also be set with the /f command line switch.

    Alt-E	Set elevation above horizon
With its elevation set to 0 degrees, SKYMAP show the horizon in the direction you are facing at the bottom of the screen. To look higher up in the sky, select a new elevation. This is a number of degrees above the horizon, and specifies where the bottom of the screen is relative to the horizon. To see the sky starting at 15 degrees above the horizon, for example, set the elevation to 15.

The elevation can be set from -45 to 90 degrees.

    Alt-Up	Raise the display horizon by 6 degrees
    Alt-Down	Lower the display horizon by 6 degrees
SKYMAP initially places the horizon at the bottom of the screen. These two control keys can be used to raise or lower the observer's viewing angle, by moving the horizon line down or up. (On some computers, these two keys do not function.)

When SKYMAP first starts, objects are displayed if they are above an imaginary line 18 degrees below the horizon. (The sun falls to this level at the end of astronomical twilight.) To raise or lower this level, press Alt-Up or Alt-Down. (On some computers, these two keys do not function.)

    Alt-M	Set magnification of display
When showing the whole sky, the magnification of the display is 1.00. To see an enlarged portion of the sky, set the magnification to a larger value, up to a maximum of 10. (The minimum value is 0.5, a compressed view of the sky, which can be used to see objects below the horizon.)

The point at the center of the bottom of the display area remains fixed when you change the magnification: it is determined by the facing direction and the elevation. It is often easiest to set this point at low magnification and then change to a higher magnification.

When you first change the magnification, the new positions of stars and planets are estimated and displayed immediately. Once they have been recalculated accurately, you may see them shift slightly on the screen.

Date and time
In addition to his location on earth, the observer looks at the sky on a particular date and time. Date and time are shown in several different formats; change whichever field is most convenient, and related fields will be updated as well.

    F5		Set date (day month year)
Enter the date with a three letter month. For example: 22 JUL 1981

    F7		Set time (hours:minutes:seconds)
This is the actual local time, and uses a 24 hour clock.

    F6		Set Universal Time
Universal Time is the astronomical equivalent of Greenwich Mean Time, and is often used because it is independent of the observer's location. Local time is equal to Universal Time minus your time zone.

The time zone is the number of hours West (positive) or East (negative) of Greenwich Mean Time. In the United States, for example, this value is 5 hours for Eastern Standard Time and 4 hours for Eastern Daylight Time. This value is can be changed in the Settings Screen (see below).

    F8		Set local sidereal time (hours:minutes:seconds)
The earth rotates on its axis in one sidereal day. Every 24 sidereal hours, the same stars would be overhead, no matter what time of year.

Because the earth is also moving around the sun, what we think of as a day is actually about 4 minutes longer than a sidereal day - it takes that long for the sun to return to the same position in the sky.

To set Local Sidereal Time, press F8 and enter it using a 24 hour clock.

    Home	Set time to now
Resets the display's date and time to the time given by the computer's internal clock.
Update rate
SKYMAP refreshes the display as often as possible. The date and time of each successive display changes by an amount called the "rate." The rate can either be a fixed amount (like 30 minutes) or can be tied to the actual clock on the computer.

    F9		Set rate (+/- days hours:minutes:seconds)
    F10		Set rate in sidereal time (+/- days hh:mm:ss)
These set the rate of change between successive displays to a fixed amount of either ordinary clock time or sidereal time. The rate can be positive or negative (to go back in time) and can be any reasonable number of days, hours, minutes and seconds.

It is interesting to set the sidereal rate to several days, and watch the planets move against the fixed background of stars.

Note also that 366 sidereal days is quite close to a year.

    Right  	Increase rate
    Left   	Decrease rate
Pressing the Right and Left arrow keys will increase or decrease the rate by 15 ordinary minutes (not sidereal minutes).

When viewing the solar system map, these keys change the rate by one day instead of 15 minutes.

    End		Set rate to zero
Press the End key to reset the rate to zero.

End will also turn "real time" mode off.

    PgDn	Real time toggle
Press PgDn to enter "real time" mode, and again to leave it.

In "real time" mode, the rate at which the display advances tied to the computer's clock. If it takes about 0.7 seconds to recalculate the next display, the "rate" will be effectively 0.7 seconds.

When you enter real time mode, the difference between the display time and the actual current time is noted. For successive displays, SKYMAP maintains the same difference as closely as possible, thus there is no drift due to changing conditions (such as increasing the number of stars being displayed).

Once in real time mode, any time you change the display's date or time, SKYMAP will advance in real time from the new setting. Thus, if you press PgDn to enter real time mode and then Home to set the time to "now," you can watch the sky as it actually changes through time (it's slow, but you CAN see stars and planets shift from one position to the next).

Selecting objects to display

    0 - 9	Select/deselect planets
    A		Select/deselect asteroids
    C		Select/deselect comets
The Sun, Moon, and nine planets are turned on and off with the number keys. The planets are numbered from 1 to 9 (Mercury to Pluto), with the exception of the Earth, which of course is the third planet. The number 3 is used for the moon, and 0 for the sun. Comets and Asteroids are selected with the letters "C" and "A."

    0		Sun
    1		Mercury
    2		Venus
    3		Moon
    4		Mars
    5		Jupiter
    6		Saturn
    7	 	Uranus
    8		Neptune
    9		Pluto

    A		Asteroids
    C		Comets
When starting up, SKYMAP shows the visible bodies, from the Sun to Saturn. Press the appropriate number to turn an individual body on or off. If present, Comets and Asteroids are turned on initially.

Note that the /p command line switch can be used to set the list of planets to be displayed when starting up (see below).

    Alt-O	Select objects by category
The display of different categories of fixed objects can be turned on and off using Alt-O. This places the cursor on the "Objects" display line. Pressing the following keys will turn the display of the corresponding category on or off:

    G		Galaxies
    L		Globular clusters
    O		Open clusters
    N		Nebulae
    M		Messier objects
    X		Other fixed objects
    S		Stars
Each category has a separate color, as shown on the "Objects" display line.

When finished selecting object categories, press ENTER.

    Alt-L	 Locate objects
    Alt-C	 Display cursor (pointer)

    Cntl-Left    Move cursor East
    Cntl-Right   Move cursor West
    Cntl-Up      Move cursor North
    Cntl-Down    Move cursor South
Alt-P displays a cursor (or pointer) at the center of the display. It is shaped like a plus sign with a gap in the middle. The four "control cursor" keys move the pointer. Note that the pointer moves along lines of right ascension (left/right) and declination (up/down), which may take a moment to get used to. When the pointer is centered on an object, the name of the object will be displayed, if available, and underneath the name, it's magnitude. Object names are read from the data files, so these files must be present for this feature to work.

To locate an object, press Alt-L. You will be prompted for the name of the object. Type the name and press Enter. The data files will be scanned for an object with this name (ignoring spaces and capitalization). If found, the pointer will be moved to the object and its name and magnitude displayed.

To return to normal operation, press ESC.

    M		Set magnitude limit
    Up		Increase magnitude limit
    Down	Decrease magnitude limit
SKYMAP shows all stars that are at least as bright as a given magnitude. The level of this brightness cutoff is initially 4.0 (meaning that stars fainter than 4.0 will not be displayed) and can be modified with these three keys.

Press "M" to set the magnitude limit to a specific value: you will be able to enter a positive or negative value with two digits after the decimal point.

In addition, the Up and Down arrow keys increase and decrease the magnitude limit by 0.1.

The /m command line switch can be used to set the magnitude limit to a different value when starting SKYMAP

Special commands

    P		Planet trails on/off
    S		Star trails on/off
Usually, each time a star or planet is displayed at a new location, it is erased from its old location. By pressing "P" or "S", however, this erasure can be suppressed, giving the effect of a trail left by each planet (sun, moon, comet, asteroid) or star.

When these options are active, the phrases "P trail" or "S trail" are shown on the screen. These keys are toggles, so that if "planet trails" are on, pressing "P" will turn them off again.

    H		Hold display/resume
To freeze the display, press "H". Calculations will continue in the background. Press "H" again to resume and display the next screen.

    R		Refresh display
To clear the screen and re-display the map, press "R".

Note that refreshing the display will not refresh any planet or star trails, only the current positions.

    Alt-N	Night Vision toggle
Press Alt-N to switch to Night Vision mode, where the screen is displayed entirely in red. Press Alt-N again to return to the normal set of colors.
Jupiter's moons

    J		Jupiter's moons toggle
Press "J" to display Jupiter and the four Galilean moons as they appear from Earth. Jupiter will be shown as a small circle at the top of the screen, and the moons as colored spots. As time passes the moons appear to move horizontally, with some slight vertical movement, since their orbital planes are not parallel to that of the Earth. For simplicity, the four moons are shown with the colors of the first four members of the solar system (Mercury to Mars).

To turn off the Jupiter display, press "J" again.

Writing coordinates file

    Alt-C	Write coordinates file
Press Alt-C to write the coordinates of the currently selected planets, comets and asteroids to a file. These values are generally accurate to within a few of minutes of arc. The name of the file is that given with the /out switch, or if that is omitted, SKYMAP.OUT.

The coordinates written for each active body are:

    Right Ascension (hours and minutes)
    Declination	(degrees and minutes)
    Altitude (degrees and minutes, when visible)
    Azimuth (degrees and minutes, when visible)

    F1		Help / exit help
Press F1 to see a summary of the control keys. The help screen hides the sky map, but calculations proceed in the background and you can use any of the other control keys to change settings.

Once in HELP, press F1 again to exit.

DOS commands

    Alt-D	DOS commands
Press Alt-D to enter the DOS command interpreter temporarily. Since SKYMAP continues to use a good deal of the computer's memory, not all programs or DOS commands will run in this context, although many useful ones will have no trouble.

To return to SKYMAP, type "exit" at the DOS prompt.


    Alt-X	Exit
Press Alt-X to leave SKYMAP.

Solar System Map

SKYMAP can also display the solar system as it might appear from a distance. The orientation of the solar system can be rotated in three dimensions, so that the orbits of the planets can be viewed from any angle. The orbits of Mercury, Mars, and Pluto are especially interesting.

In addition to the keys described here, most of the regular control keys function on this display as they do on the main sky map.

    F2		View solar system / sky map
Press F2 to toggle between the two displays. Each time you enter the solar system display it is scaled so that the orbit of the outer-most of the currently selected planets will fit on the screen.

When the Moon is selected on the sky map display, the position of the Earth is shown on the solar system display.

Solar system control keys

    F3		Set latitude (degrees and minutes)
    F4		Set longitude (degrees and minutes)
F3 is used to set what might be thought of as the "latitude" of an observer viewing the solar system from far away. Initially, the solar system is presented as if seen from the "north pole" of the solar system, 90 degrees above the ecliptic. Enter 0 degrees to view the solar system from the plane of the ecliptic (the Earth's orbit) or a number between 90 and -90 to set the angle to some intermediate value: 10 degrees is a good number to try.

F4 is used to set the hypothetical observer's "longitude." With the initial value of 0 degrees, the Earth's orbit reaches the highest point on the screen at the Vernal Equinox, on about March 21. To rotate the display around the Earth's orbit, set the longitude to another value.

    Right	Increase rate
    Left	Decrease rate
These keys change the rate by 24 hours when viewing the solar system map. You may also set the rate directly with the F9 and F10 keys.
Earth anchor mode

    E		Earth anchor toggle
On the usual solar system map, the earth revolves around along with the other planets. Press "E" to anchor the earth to a fixed area of the screen: below the sun. This may allow you to see more clearly how the planets appear to move within the solar system as viewed from Earth.

In this mode, the hypothetical observer's "longitude" is updated to reflect the Earth's motion around the sun.

When this mode is active, the phrase "E anchor" appears on the screen. Press "E" again to return to the normal, unanchored mode.

Near/far viewpoint

    Alt-V	Near/Far Viewpoint toggle
The solar system map can be viewed from two locations in space:

    Near - from an apparent distance of about twice the width 
           of the display

    Far  - from an apparent distance that is very far away
The difference is that from the near viewpoint, there is a small effect of perspective. From the far viewpoint, there is no such effect.

The calculations for the far viewpoint are somewhat simpler, so this mode is slightly more rapid.

SKYMAP starts up in Near mode; press Alt-V to toggle between the two modes.

3-D solar system

    Alt-3	3-D mode toggle                   
You can use common red and blue glasses (or other bi-colored glasses) with SKYMAP to see the solar system in three dimensions.

When 3-D mode is enabled, bodies are displayed on the solar system map as pairs of red and blue spots. This can be surprisingly effective, especially when viewing comets moving relative to the planets. You may find that the 3-D effect is more obvious if you dim the lights.

Press Alt-3 to toggle between the 3-D and ordinary solar system displays.

Since the 3-D effect depends also upon our sense of perspective, it only works with the Near viewpoint set (see Alt-V).

Command Line

    skymap F1 F2 ... Fn /set /out /f /m /p /vmode /clear
Several options can be specified to start up SKYMAP in special ways. These parameters and switches can be entered in any order with any reasonable spacing. If you use the same switch more than once, the last instance will be used and earlier ones ignored.

Note that in the examples below, we use the "=" character to give values to certain switches. The ":" character may be used as well, and in fact is necessary if you call SKYMAP with nested batch files (one batch file calling another which then runs SKYMAP).

Switches may begin with either the "/" or the "-" character.

    /set=file	settings file
To start SKYMAP with special setting for latitude, longitude, etc., you may store the parameters in a file, and then use the /set switch to read that file. For example, suppose you have a settings file called HAWAII.SET that contains the latitude, longitude, and time zone for an observer in Hawaii. You would start up SKYMAP by typing

    skymap /set=hawaii.set
and skymap would start up showing the sky as seen from Hawaii. The default extension for the settings file is ".SET" - which may be omitted.

If this switch is not used, SKYMAP uses the file SKYMAP.SET in the current directory, if present.

The settings file is described in detail below.

    /out=file	output file
When Alt-C is pressed, the current coordinates of the planets are written to the output file. The name of this file may be given with this switch. For example, if you type

    skymap /out=planets.loc
SKYMAP will open the file PLANETS.LOC (creating it if it does not already exist) and will write new coordinates to the end of the file whenever you press Alt-C.

If this switch is not used, SKYMAP uses the file SKYMAP.OUT in the current directory.

    /f=90	observer's direction
The ordinary display shows South at the bottom of the screen. To show another direction there, use the /f switch, giving the direction as the number of degrees starting from North = 0.

For example, to place East at the bottom of the screen, type

    skymap /f=90
The default value is 180 (South).

    /m=4.1	magnitude limit for stars
Used to set the initial magnitude limit. If you use the /m switch without any value, the limiting magnitude will be set to that of the dimmest star.

The default value is 4.0.

    /p=45	select planets
Used to select solar system objects to be displayed. See the list above under the heading "0 - 9 Select/deselect planets".

The default value is 0123456 (sun, moon, and the visible planets).

To select no planets at all, use /p=x, and for all, use /p without any value at all.

    /vmode=m	set video mode
SKYMAP tries to determine what sort of video display your computer has, and adjusts the display appropriately. If it chooses the wrong mode, or if you want to try other video modes, use the /vmode switch to force SKYMAP to use another mode (SKYMAP will ignore any problems it encounters if you force it with this switch - the display may never appear).

For example, typing

    skymap /vmode=16
on a machine with a VGA monitor will force SKYMAP to use video mode 16 (the high resolution 16-color EGA mode) instead of the default VGA mode.

    /clear	no built-in stars
SKYMAP has a built-in list of over 1600 stars. To display no stars at all, or to use another list of your own, you can erase the built-in list by giving the /clear switch on the skymap command.

    F1 F2 ... Fn	files with additional stars, etc.
You can add to the standard list of stars by giving SKYMAP the names of one or more coordinate files. These files can include other stars or objects of other types, as described below.

You may list them separately on the command line, or use the usual DOS wildcards to specify groups of files. For example,

    skymap n.dat o.dat l.dat
would add the contents of three files (N.DAT, O.DAT, and L.DAT) to the list of objects to be displayed, while

    skymap *.dat
would do the same with all files with the .DAT extension. The extension ".DAT" is the default, so the first example could be run with the command:

    skymap n o l

Data Files

As described above, you can supplement the built-in list of stars with lists of stars or other objects such as galaxies, nebulas, etc. For each "fixed" object (stars, galaxies, etc.), four pieces of information are needed: type, right ascension, declination, and magnitude. For each object in orbit around the sun (comets and asteroids) six to eight pieces of information are needed to define the orbit. Spacing between fields is not important, but the number of fields is important: don't skip any.

In addition to the formats described below, you can include "comments" in a data file. Any line that begins with a "!" sign is ignored and can be used to record notes about the file as a whole or entries within it.

Stars and Other Fixed Objects
As an example, consider the file BD.DAT that contains the stars of the Big Dipper:

    Type     Ascension   Declination   Magnitude  Identification

    S	    11  1 48	    56 23 0	     2.4	 BET-UMA
    S	    11  3 42	    61 45 0	     1.8	 ALP-UMA
    S	    11 53 48	    53 42 0	     2.4	 GAM-UMA
    S	    12 15 24	    57  2 0	     3.3	 DEL-UMA
    S	    12 54  0	    55 58 0	     1.8	 EPS-UMA
    S	    13 23 54	    54 56 0	     2.1	 ZET-UMA
    S	    13 47 30	    49 19 0	     1.9	 ETA-UMA
There are five fields, two of which consist of three numbers each:

One of the following; objects of different types can be displayed in different colors:

	S - Star
	N - Nebula
	L - Globular cluster
	O - Open cluster
	G - Galaxy
	M - Messier object
	X - Other

	C - Comet
	A - Asteroid
For comets and asteroids, there is a special format described below.

    Right ascension - three numbers:  hours, minutes, and seconds.
Any of these three may be a decimal number, but all three fields must be present. Thus, the following two entries would be equivalent, because 30 minutes 36 seconds is equal to 0.51 hours:

	20    30  36
	20.51  0   0

    Declination - three numbers:  degrees, minutes, and seconds.
Decimal numbers may be used as they are for right ascension. Also, since declinations can be negative, a minus sign "-" is used on the degrees field for objects south of the equator (even when the number of degrees is zero).

    Magnitude - a decimal number.
Magnitudes are currently used only for stars, but SKYMAP requires this field for all objects. For those other than stars, you may as well enter zero (the value doesn't matter).

In addition, you can put the name of the object or a description at the end of the line, after the magnitude. This is not required by SKYMAP, but will allow you to locate the object by name, and may also be helpful in maintaining a data file.
Comets and Asteroids
Because they are in orbit around the sun, comets and asteroids are described by entirely different data elements. As an illustration, the following values specify the orbit of Comet Austin (as of Feb. 27, 1990).

!                                         Longitude
! Time of       Perih.   Eccen-  Arg. of  Ascend.  Incli-  
! Perihelion    Distance tricity Perih.   Node     nation  Ident.
! ------------- -------- ------- -------- -------  ------- ------
C 1990 4 9.9761 0.349957 1.00038 61.5546  75.2132  58.9596 Austin
For known comets and asteroids, these values are available in many reference books. Current information on recently discovered or recently recovered bodies is often given in Sky & Telescope magazine, just to name one source.

Some references give other orbital elements than these: most of the conversions to the elements used by SKYMAP are easily calculated. The ones used here are appropriate for both closed and open orbits.

Orbital elements are sometimes given for standard equinox 1950.0, but SKYMAP uses 2000.0 as its standard equinox. See below for a discussion of equinoxes and precession. For now, you should know SKYMAP assumes that orbital elements are for equinox 2000.0.

An entry for a comet or asteroid consists of seven or more fields. You do not have to have a precise understanding the definitions of these fields to use SKYMAP. However, several of the books given in the bibliography will explain them if you are curious.

SKYMAP will handle 256 comets and asteroids at once.

As mentioned above, one of the following:

	C - Comet
	A - Asteroid
The following six values describe the simple orbit of any body relative to the Sun:

    T - time of perihelion
Four numbers, in the format "YYYY-MM-DD-HH.HHHH", giving the year, month, day and hour that the object passes through perihelion. This value is given in Ephemeris Time, which for our purposes, is the same as Universal Time or Greenwich Mean Time. If desired, a decimal number of days may be entered instead: "YYYY-MMM-DD.DDDD".

    q - perihelion distance
The distance, in astronomical units (AU), of the object from the sun at perihelion.

    e - eccentricity
A parameter that describes the shape of the orbit:

	0 < e < 1		ellipse
	e = 1		parabola
	e > 1		hyperbola

    ap - argument of perihelion
The angle between the ascending node and perihelion, in degrees.

    lan - longitude of the ascending node
The ecliptical longitude, in degrees, of the point at which the object passes through the plane of the ecliptic in a northerly direction.

    i - inclination
The inclination of the orbit, in degrees. When i > 90, the orbit is said to be retrograde.

A name or description, for documentation purposes. This name can be used to find the object using the Alt-L "Locate" key.
Other Orbital Elements
The orbital elements described above were chosen because they apply to solar orbits of any shape and also because they are commonly used published orbital elements. Other orbital elements are sometimes given, and SKYMAP provides for some of these. This section describes these extensions to the the standard set of orbital elements.

To specify elements other than the standard ones, a "field definition" line is given in the data file before the first non-standard comet or asteroid data line. This line begins with the letter "F", and then names the fields that will be given in subsequent data lines. These field names must be one of the following:

    T		Time of perihelion
    q		Perihelion distance, A.U.
    e	 	Eccentricity
    ap		Argument of Perihelion, degrees
    lan		Longitude of the Ascending Node, degrees
    i	 	Inclination, degrees

    a		Semi-major axis, A.U.
    M		Mean Anomaly, degrees
    n	  	Mean motion, degrees per day
    epoch	Epoch of the orbital elements

    H		Absolute magnitude parameter
    G		Second magnitude parameter
Of these, the first six are the standard orbital elements. The "F" line for an orbit expressed with these elements would look like this (using Comet Hale-Bopp as an example):

F T		     q	   e	      ap	     lan      i
C 1997-4-1-9.4	.916702 .996348 130.4405 282.4733 88.87970 Hale-Bopp
Each field name has been placed above the corresponding data for the sake of clarity, although this is not required. Note also that the name of the object has no field name: anything after the last defined field is considered part of the identification.

SKYMAP supports several alternative orbital elements, each replacing one or more of the standard ones.

If instead of "q" (the perihelion distance), the orbital elements include a value for "a" (the semi-major axis), the value of "q" can be calculated as:

    q = a * (1 - e)
The elements for Hale-Bopp, using "a" instead of "q", are as follows:

F T		     a	   e	      ap	     lan      i
C 1997-4-1-9.4	251.014 .996348 130.4405 282.4733 88.8797 Hale-Bopp
From this information, SKYMAP can determine the standard set of orbital elements.

If instead of the "T" (the time of perihelion), the orbital elements give values for the "epoch" (a given instant of time) and "M" (the Mean Anomaly at "epoch"), the time of perihelion can be calculated using the formula:

    T = epoch - M / n
where "n" (the mean motion per day) is calculated from "a" as

    n = 0.9856076686 / (a * sqrt(a))
Here are sample values for Hale-Bopp, using these elements:

F epoch      M       a       e       ap       lan      i
C 1995-10-10 359.866 251.014 .996348 130.4405 282.4733 88.8797
Again, with this information, SKYMAP can determine the standard elements, and thus the orbit of the object.

SKYMAP can also estimate the visual magnitude of a comet or asteroid given two magnitude parameters, which are called "H" and "G". These two parameters may be included in the "F" line and data lines along with the other orbital elements.

    H - the absolute visual magnitude
    G - a constant relating magnitude to distance from earth
For comets, the first parameter may be called "g" and the second one "kappa" in some references. Note that this "g" is equivalent to SKYMAP's "H", and "kappa" is the same as "G" in SKYMAP.

In older references to asteroid magnitudes, parameters "g" and "kappa" may also be given, but in this case, "kappa" is not truly equivalent to SKYMAP's "G" parameter. (The switch to the new convention for asteroids was made in 1985, and the older convention is not supported in SKYMAP.)

Standard Data Files
SKYMAP is distributed with several data files:

    A.DAT		Asteroids
    C.DAT		Comets
    G.DAT		Galaxies
    N.DAT		Nebulas
    O.DAT		Open clusters
    L.DAT		Globular clusters
    M.DAT		Messier objects

    STARS50.DAT		Stars down to magnitude 5.0
    STARS55.DAT		Stars from magnitude 5.01 to 5.50

    BD.DAT		Big Dipper

    EQUATOR.DAT		Celestial equator
    ECLIPTIC.DAT	The ecliptic
These are all text files. There is one file for each of the supported object types. STARS50.DAT and STARS55.DAT contain the same information as the built-in list of stars. You should not modify these two files.

To see the sky with the Messier objects, enter the command

    skymap m.dat
To clear the built-in list of stars and read the data files for non-stellar objects, enter the command

    skymap /clear ?.dat
Precession and equinoxes
Due to the influence of the Moon and the Sun, the Earth's axis precesses slowly, so that the celestial "North Pole" moves in a circle once every 26,000 years. This might seem so slow that we could neglect it, but a moment's thought shows that this comes to about one degree during a human lifetime, and therefore cannot be dismissed so quickly.

To be precise, celestial coordinates must always be given with reference to a stated "equinox," or coordinate system. Those that are used most often today are 1950.0, 2000.0, and in some cases, the "equinox of date." 1950.0 and 2000.0 refer to standard equinoxes which are used in nearly all atlases.

SKYMAP uses equinox 2000.0 for the coordinates of the fixed objects, as do the newer atlases. However, comets are usually described in 2000.0 coordinates, and SKYMAP makes this assumption.

Older references use older equinoxes, such as 1950.0 or earlier. To override the default value of 2000.0, you can change the input equinox by adding a line to a data file such as

    E 1950.0
SKYMAP will then interpret the orbits of any following comets or asteroids as being stated in equinox 1950.0. Note, however, that at the beginning of each data file, the comet/asteroid equinox is reset to 2000.0, so you must include "equinox lines" in each data file that has orbits stated in any other equinox.

The correction from 1950.0 to 2000.0 is a small one, but is noticeable, especially when viewing a magnified display. It would be more significant if you were to try entering a comet whose orbit had been described, say, in the 19th century.

Settings File

Certain parameters are stored permanently in a settings file. The default name for this file is SKYMAP.SET, but other files can be specified with the /set switch as described above. This section describes the contents of SKYMAP.SET or other settings files, and how to use the settings screen.
Changing settings
Press Alt-S to bring up the settings screen. On the computer, this screen will show the actual colors of each item:

		 SKYMAP Settings - Press F1 for help

	   Latitude:	 40 00       Horizon limit:   -18
	   Longitude:	 75 00	     Initial mag:    4.00
	   Timezone:	  4:00	   
	   Aspect:	1.0000	   
	»  0  Background (0)	   8  Text
	   4  Horizon		   8  Help text

				  12  Messier objects
	  15  Stars		   1  Galaxies
	  			   4  Nebulae
				   5  Globular clusters
	  14  Sun		   6  Open clusters
	   5  Mercury
	  13  Venus		  10  Comets
	  11  Moon & Earth	   9  Asteroids
	  12  Mars
	  10  Jupiter		   1  Other
	   2  Saturn
	   3  Uranus		   4  3-D left eye
	   9  Neptune		   1  3-D right eye
	   6  Pluto		   5  3-D both eyes
Here you can change four numeric parameters used when SKYMAP starts running, as well as the colors for text, lines, and objects displayed (these items are described in the next section). Press F1 while in the settings screen to see a list of the settings screen control keys.

There are two kinds of fields on the settings screen: numbers and colors. Use the Up and Down arrows to move between fields.

Change colors using the Left and Right arrows. You will see both the color itself and the corresponding color number. (The background color, which would be invisible, is indicated by "(0)" next to the item name.)

Change numbers by moving within them using the Left and Right arrows, and entering new digits over the old ones. If you change your mind and want to bring back the original value of a number, press ESC while still in the number field. Press Enter to actually change the number.

To restore all of the settings to the values they had when you entered the settings screen, press the letter "R" (for restore).

There are two ways to leave the settings screen. Press Enter to write the changed settings to the current settings file (see the /set switch above). To see how the new settings will look without making a permanent change in the settings file, press ESC instead.

Details of the settings file
A settings file contains a list of numbers. Many are used to control colors. The standard file SKYMAP.SET is included here for reference. The layout is free-form; all that matters is the sequence of numbers (line breaks may be inserted anywhere). To make your own settings file, you should copy SKYMAP.SET and run SKYMAP with the new copy.


	40.0  75.0  5.0  1.0
	 0  0
	 0  1  7  7
	15  0  0
	14  5 13 11 12 10  2  3  9  6
	 2  1  4  5  6 10  9
	 4  1  5
       -18  4.0  16

	latitude  longitude  timezone  aspect
	[two obsolete values]
	graphics and text colors
	star color and two more obsolete values
	planet colors
	nebula colors, etc.
	other color
	3-D mode colors
        display horizon, initial magnitude, video mode
The first line of SKYMAP.SET contains the initial settings for:

   latitude	40.0
   longitude	75.0
   timezone	 5.0
   aspect	 1.0
Although SKYMAP shows latitude, longitude, and timezone in conventional units (degrees, hours, and minutes), they are given as decimal numbers in the settings file. This is for simplicity's sake in the program, and is not much of a hardship for the user. For example, in Labrador, where standard time is 3 hours and 30 minutes west of Greenwich Mean Time, you would enter the timezone as 3.5 rather than 3:30.

The fourth parameter, aspect, controls the shape of the horizon. With the default setting of 1.0, SKYMAP draws an ellipse that fills the left hand portion of the screen (the number of vertical and horizontal pixels depends on the video mode). This should look like a circle, but if it doesn't, you can make the ellipse narrower by changing the aspect to a number less than 1.0 or wider (actually, shorter) with a number greater than 1.0.

The two numbers on the second line are no longer used, but are kept in the settings file for compatibility with older versions of SKYMAP.

The next six lines of SKYMAP.SET contains color settings. For reference, here are the numbers of the sixteen standard EGA colors:

     0	  black
     1	  blue
     2	  green
     3	  cyan
     4	  red
     5	  magenta
     6	  brown
     7	  white
     8	  gray
     9	  light blue
    10	  light green
    11	  light cyan
    12	  light red
    13	  light magenta
    14	  yellow
    15	  bright white
The third line sets various graphics and text colors:

    background	 0  (black)
    lines	 1  (blue)
    text	 7  (white)
    help text	 7  (white)
The fourth line sets the color of stars, and has two more values that are no longer used by SKYMAP, again kept for compatibility.:

    stars	15  (bright white)
The fifth line sets planet colors:

    sun 	14  (yellow)
    mercury	 5  (magenta)
    venus	13  (light magenta)
    moon	11  (light cyan)
    mars	12  (red)
    jupiter	10  (light green)
    saturn	 2  (green)
    uranus	 3  (cyan)
    neptune	 9  (light blue)
    pluto	 6  (brown)
The sixth line sets colors for other celestial objects which may be supplied via auxiliary data files:

    Messier objects	 2  (green)
    Galaxies		 1  (blue)
    Nebulas		 4  (red)
    Globular clusters	 5  (magenta)
    Open clusters	 6  (brown)
    Comets		10  (light green) 
    Asteroids		 9  (light blue)
The format of data files is described in an earlier section.

The seventh line gives a color to use for anything else. The file equator.dat is an example: it uses the usual data file conventions to draw a dotted line around the celestial equator.

    Other		 1  (blue)
The eighth line specifies the colors to use in 3-D mode. The 3-D effect is created by passing a specially colored two dimensional image through two colored filters, which the brain then interprets as a three dimensional image. The usual color scheme is red and blue, but red and green glasses may also be found. (Note that polarized glasses will not work with SKYMAP.)

    Left eye		 4  (red)
    Right eye		 1  (blue)
    Both eyes		 5  (magenta)
The "left eye" color should be visible through the left lens, but not the right, and vice versa for the "right eye" color. The "both eyes" color should be visible through both lenses. If you are not using standard red and blue glasses, try experimenting with these settings.

    Display Horizon     -18
    Initial Magnitude   4.00
    Video Mode          16
The last line specifies three other values. The Display Horizon and Initial Magnitude values are specified on the settings screen. The value for Video Mode is a record of the video mode used to start SKYMAP.

A Final Note

I sincerely hope that you enjoy using SkyMap. I originally wrote it for my own use, but others have found it a useful tool, both for a quick preview of tonight's sky, or as an observational planning tool.

Many enhancements have been made in response to your feedback. Please send any comments, complaints, or bug reports to me: Stephen Bryant.

Thank you.

Copyright © 1998, Stephen Bryant. All Rights Reserved.