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Solunar - a simple command-line utility for calculating Sun and Moon
rise and set, and related times
solunar is a simple, command-line utility for rapidly displaying
sun- and moon-related data, such as sunset times and equinoxes. It builds under Cygwin on Windows, and on most Linux platforms.
solunar produces output that can be readily parsed by scripts,
although it is reasonably human-readable. The utility
contains a database of city locations, so locations can be specified
by name, rather than latitude and longitude, although specific latitude
and longitude can also be specified if required.
Solunar can also calculate the dates of lunar-related
events such as Easter.
Get sun and moon rise and set times, etc., in London, on the current day:
$ solunar -c london
Selected city Europe/London
Using location 51:30N,000:07W
Date/time Mon May 21 13:52:52 2012 local
Date: Monday 21 May 2012
Moon phase: 0.02 new
Partial city names can be given, so long as the name is unambiguous. To see
a full list of city names, run
Get sun and moon rise and set times, etc., in Paris, on January 1st, this year:
solunar -c paris -d "jan 1"
Get detailed information for location
solunar -c paris -d "jan 1"
Get brief information for location 51.0 degrees north, 1.5 degrees west:
solunar -l 51.0,-1.5 -d "jan 1"
Note that southerly latitudes are negative, as are westerly longitudes.
For more information on the accepted lat/long formats,
solunar -l help.
Note also that when latitude and longitude are given, the times displayed
are in the default timezone, whether or not the specified point is in
that timezone. If it isn't, you probably need to use the
switch to get the times in UTC, then convert appropriately. In practice,
the use of
-c to set a city is more convenient, unless there
really is no city within a hundred miles or so of the location.
To get a list of equinoxes, etc., for the current year:
This also displays the dates of festivals like Easter which are derived from
the lunar calendar. However, this dating of Easter is the conventional
Western one, and not all locations observe the same date, even when they
For a full list of command-line switches:
# make install
solunar uses GNU-cc specific methods of handling
time and date, and requires a Posix timezone database. Consequently it
won't build under MinGW (and won't work even if it can be made to build).
Interestingly it will build for Android using the Native Development
Kit, and does seem to work correctly. Of course, that's only useful if you
routinely use the command prompt on Android.
solunar may build
on other Posix-like plaforms, perhaps with changes to the Makefile.
Notes on the calculations
The calculations are all based on published algorithms, and I have checked
them against reliable data sources so far as possible. There are
potential sources of error that the program can't control, and some
issues of interpretation.
The most troublesome potential source of error is the system's timezone
database. By default, results are worked out in UTC and then converted to
the local time of the specified city. This relies on the system's own
time and date being correct, and having a proper understanding of daylight
savings changes. In general, these issues seems to be handled reasonably well
in modern Linux distributions, but there's not much
do if the platform data is incorrect.
Issues of interpretation include those relating to uncertainty about exactly
what position of the sun in the sky constitudes sunset.
The sun is not a uniform disc, so there has to be a convention for the
angle of zenith that we take as sunset. Most publications that give sunset
times seem to take the Zenith angle as 90 degrees and 50 minutes, so
solunar does the same. However, the
will make it display sunset according to other popular zeniths.
In particular, civil twilight has a zenith 6 degrees below conventional
sunset, and denotes the time before which outdoor activities are reasonably
practicable. Nautical and astonomical twilight have zeniths 12 and 18 degrees
below conventional sunset. In practice, many parts of the world will experience
no astronomical sunset for at least part of the year. Some, of course,
experience no sunset at all for part of the year.
- Events like equinoxes are displayed to the nearest minute but, in fact,
it's hard to get an accuracy of better than 30 minutes or so. These are
annual events, after all.
- The --days switch produces a list for the current year. To get a list for a
different year, you will need to specify a full date in that year, even though
only the year part will be used.
- Times are displayed either as UTC, or local to the specified city. There is
no way to get the times of events in one location relative to a different
solunar is not intended
to be a general timezone converter.
- If a date is specified but no time, then the time is taken to be 2AM, local
time. So, for example, the distance from the earth to the moon will be given at
2AM on the specificed day. In practice, this seems highly unlikely to be
significant. The use of 2AM, rather than midnight, is so the user is less
likely to be surprised when selecting a day at the boundary between daylight
savings changes. Midnight local time might end up being in an unexpected
solunar is copyright (c)2005-2012 Kevin Boone, and is released
under the GNU Public Licence, version 2.0. That means, essentially, that
you are welcome to use this program however you see fit, at your own risk, so
long as you do not redistribute it without acknowledging the original
The algorithms used in the program are published in various textbooks and,
so far as I know, there are no particular legal restrictions on their
use and distribution.