Eclipse Documentation

Eclipse

Simple Eclipse Predictor

August 27, 1999

Introduction

This small DOS program displays the principal characteristics of solar and lunar eclipses for a given year or range of years. These characteristics include the type of the eclipse, degree of totality, and the time and location on earth of the midpoint of the eclipse.

This is not a mapping program, and thus does not show the path of an eclipse, the beginning and ending times, etc. It serves as a quick reminder of when eclipses will be, or have been. For full details, there are many good reference books in print, as well as very comprehensive guides published by NASA on upcoming eclipses, including up-to-date weather predictions.

How To Run

Eclipse is run as a DOS program from the command line, and has two optional parameters and two optional switches, for example:

    eclipse 1999 2001 /s /m

This will show all solar and lunar eclipses from 1999 to 2001. The two years are optional, and if both are omitted, information on just the current year is shown.

The two switches (/s and /m) turn on the display of eclipses of the sun and of the moon, respectively. If both are omitted, eclipses of both are shown. (Hint: this means, to see just lunar eclipes, use just the "/m" switch.)

So, to see all eclipses for this year, simply type:

    eclipse

Interpreting the Output

Here is a sample run for the year 2000, and comments on the output.

The date and time of the midpoint of the eclipse is shown both as a conventional date and time, and as the equivalent Julian Day. (The Julian Day is commonly used in astronomical calculation as a single number that conveniently expresses an exact moment in time. Julian Days are easier to subtract, etc., than conventional dates and times.)

The date and time is shown based in Universal Time, or Greenwich Mean Time. To convert to local time, add or subtract the appropriate number of hours for your time zone.

The midpoint of the eclipse is specified simply as latitude and longitude. Use any globe or map to interpret this. If you know the values for your own home, a little mental arithmetic will tell you approximately how far away this is (and I've found that it's often quite far indeed!).

C:\eclipse 2000


Lunar eclipse, maximum at 2451564.699129  2000 1 21   4:47
Type         Total  Magnitude    1.328118
Longitude W  71.686394
Partial duration 101.013309, total duration 38.386062

The first eclipse of the year is a total or umbral lunar eclipse, at 4:47 AM in Greenwich, England, or 12:47 AM on the east coast of the United States. At mid-eclipse, the moon is "over" (highest in the sky at) longitude 71.686, which is fairly close to Boston, Massachusetts.

The earth's shadow on the moon has a central, dark portion called the umbra, and a lighter outer ring called the penumbra. An umbral eclipse is one where the moon spends some of its time entirely within the umbra. In this eclipse, the duration of the umbral phase is a little over 38 minutes, so totality begins about 19 minutes before 4:47 and ends 38 minutes later, from 4:28 to 5:06.


Solar eclipse, maximum at 2451580.034990  2000 2 5  12:51
Type         Partial  Magnitude    0.575314
Latitude     -90.000000  Longitude W  12.596322

Fifteen days later, the year's second eclipse is a partial solar eclipse. (Eclipses of the sun and moon occur together, about 15 days apart.) This one is visible at the south pole (latitude -90), and is a partial eclipse because the umbra of the moon's shadow misses the earth, "south" of the south pole.


Solar eclipse, maximum at 2451727.317104  2000 7 1  19:37
Type         Partial  Magnitude    0.481009
Latitude     -90.000000  Longitude W  114.157279

Groups of eclipses happen twice a year, and this year the first eclipse of the second group is very similar to the previous one, a partial eclipse at the south pole.


Lunar eclipse, maximum at 2451742.081103  2000 7 16  13:57
Type         Total  Magnitude    1.773148
Longitude W  -150.803080
Partial duration 117.700185, total duration 53.064378

About two weeks later, the corresponding lunar eclipse is another total eclipse, quite a bit longer than the one in January, with its midpoint at about the longitude of Sydney, Australia.


Solar eclipse, maximum at 2451756.592597  2000 7 31   2:14
Type         Partial  Magnitude    0.598132
Latitude     90.000000  Longitude W  -146.665193

Finally, another partial eclipse of the sun, this one "north" of the north pole.

So, the year 2000 has no total solar eclipses. There were some spectacular ones in the 1990s, and the next one in the continental United States is in the year 2017. Finding this one is, as they say, left as an exercise for the reader!

The Author

I wrote Eclipse to answer occasional questions such as "when is the next one?" or "do you remember a lunar eclipse a couple of years ago?" I hope you enjoy the program, simple though it is.

Please send any comments, complaints, or bug reports to me: Stephen Bryant.

Thank you.