Lunar Co-longitude program

Lunar Co-longitude

Colong.exe is a native Windows application which displays the lunar colongitude (position of the morning terminator) for dates between 1989 and 2050. The program uses the Date and Time Zone settings in Windows to compute and display local time and UTC, automatically compensating for Daylight Savings Time.

The program is free, but the author reserves all rights. To minimize download time, Colong.exe (and its companion help file) is stored as


The algorithm used to calculate the co-longitude is from a book of astronomical algorithms by Belgian mathematician Jean Meeus, and was valid for dates from 1989 through 2020.

The algorithm was reproduced in Antonin Rukl's excellent "Atlas of the Moon", which is currently out of print (truly unfortunate, since this was a remarkable reference).

In February of 2020 the author of Colong.exe modified the program to extend support through 2050.

application main form

Colong.exe was named the winner of the 1998 Computing Challenge Award of the Computing Section of the Association of Lunar and Planetary Observers:

Computing Challenge Award

The Firefighter Calendar is now maintained at

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