A simple bash script to display the moon phase, today or on some specific date.
moonphase.sh ["date string"]
Don't forget the quotation marks if the date string contains spaces.
The output shows the moon phases as a percentage and name. For example:
$ moonphase.sh "jan 28 2023"
Jan 28 2023: 19%, first quarter
The date string is anything that can be parsed by the date utility,
which is locale-specific. If no date is given, the utility assumes
today.
To a good approximation, there is a fixed, known time between corresponding moon phases. The 'synodic month' is the time in Earth days between corresponding phases. It's about 29.5 days. The value used in the calculation is 2551443 seconds.
The utility knows the exact moon phase at some reference time. It doesn't matter what that reference time is, but it shouldn't be too far in the future or the past. The default is Jan 15, 2023, when the moon was at 0.75, third quarter. It doesn't matter what moon phase is used, so long as it's known exactly. The accuracy of the final result depends critically on the accuracy of this reference value.
We use the GNU date utility to convert the selected date and the
reference date to Unix epoch times. These are times in seconds after the
beginning of 1970, but the details are unimportant -- we just subtract
the to epoch time to get the time in seconds between the selected time
and the reference time.
Then we just divide this time difference by the synodic month in seconds, to get the number of moon phases in the elapsed time. We then multiply this figure by 100, and add it to the reference phase, which is a percentage. This result might be greater than 100%, because the elapsed time might be many moons. However, moon phases are cyclic, so we just reduce the result modulo 100%, to get the final result as a percentage.
We then display the percentage along with a string representation of the moon phase. The program recognizes eight phases: new, waxing crescent, etc. So we divide the 100% range of the moon cycle into eight blocks of 100/8%=12.5%, centered on the full moon at 50%
The calculation is not perfect, by any means. I've compared it with the results at timeanddate.com, and the results match pretty well in the time range 2023-2026. The main source of error are probably
- the use of integer arithmetic throughout. I've used the standard
exprutility to do the math, which only works with integers, as does the bash shell itself; - the duration of the synodic month itself is only an average, as the moon's orbit of the Earth is subject to small gravitational fluctuation;
- the reference time is only given as a date, but moon phases occur at specific points in time. There's little point striving for better accuracy in this area, because published moon tables do not give sufficient detail.
moonphase.sh is Copyright Kevin Boone (c)2024, released under the terms
of the GNU Public Licence, version 3. It's just an educational example
that demonstrates specific aspects of bash shell programming -- please don't
use it for anything critical.