Version 2.0c
solunar is a command-line utility for Linux and similar platforms,
that displays sunrise and sunset times and related data at a specified
location on a specified day. You can specify the location as a city or as
geographical coordinates. The timezone can also be specified in terms
of a city, or as an offset from UTC. By default, solunar uses the local
time of the specified city. solunar can display a summary of
solar and lunar events for a selected year.
Internally, version 2.0 is completely different from earlier releases --
it is a complete reimplementation. In this version I have separated
the astronomical computations completely from the input and output,
into an independent library.
Not only should this make it possible to use the astronomical library
in other programs, solunar should be easier to maintain. The
main problem with the previous version of solunar was that it had
grown up incrementally over a period of twenty years, and had become
impossible to modify without breaking existing functionality.
Although this new version does not yet have all the same functionality
of the old one, it should be much easier to extend.
Despite the internal reorganization, there should be relatively few differences from a user perspective.
The display timezone can be set explicitly using the --tz option.
You can set it to a city or to a UTC offset (see notes below on how
the offset is interpreted).
--tz must now be specified explicitly
when the location is given in
lat/long coordinates, rather than a city.
The program no longer defaults to using the host
system's timezone. The special timezone "sys" can be used to indicate
the host system timezone, for compatibility with previous versions. However,
the program will display a warning when this is done.
Default values can now be set in a configuration file, either at the user or the system level. This is particularly useful for setting a home city.
The display format has changed. I'm hoping that it is both clearer to read, and also easier for other programs to parse.
The program displays the dates of changes to daylight savings time, to the extent that the host system's timezone database has the correct information.
There is a JSON output mode, for use in scripts. JSON is easy to parse in most scripting languages. I have not settled on the exact format of the JSON output yet, and I'm open to suggestions.
All internal calculation now uses Unix time_t objects, rather than the mixture of Unix times, julian days, and custom date representations the old version had. This isn't a change that users will notice, but I hope it will make timezone handling more consistent.
Although it gives me no pleasure to say this, I should point out that, as a completely new implementation, I will probably have to fix many of the bugs again, that I had to fix in the previous version. Most of these bugs concerned timezone handling, particular in the southern hemisphere.
solunarsolunar (version 1.0) is already in the repositories of several Linux
distributions, but this new version will probably not be, so you'll have
to build it from source. This should be trivially easy on Linux, so
long as you have gcc and make available:
$ make
$ sudo make installsolunar will build on Linux, and is reported to build on OS/X. It
will build for Android (for use in a terminal) with the Google
native development tools. For Android you'll need to add position-independent
code flags to both the build and link:
$ EXTRA_LDFLAGS=-pie EXTRA_CFLAGS=-fPIC -fPIE make It builds on Windows under Cygwin, and I would expect it to build with the
Windows Linux subsystem (WSL) but I haven't tried it. It won't run on Windows
under MinGW, even if it builds, because there is no timezone database. Problems
may also be encountered on very minimal Linux systems like Alpine, for the same
reason. However, you can install the timezone database on Alpine using apk add tzdata.
solunar stores timezone information for cities in libsolunar/src/cityinfo.h.
Timezone information changes occasionally. If you're building on a system with
an up-to-date /usr/share/zoneinfo/zone.tab you can optionally run
parse_zoneinfo.pl to generate a new cityinfo.h. All the recent changes of
which I'm aware have been in naming -- Kyev to Kiev, etc.
-a,--ampm
Show 12-hour AM/PM times rather than 24-hour clock times.
-c,--city={name}
Specify a full or partial city name. Full names are of the form
"Europe/London", but it is usually easier to give a partial name:
--city=paris. Matching is case-insensitive, and the program will warn if
there is more than one match. Use --city-list to get a (long) list of known
cities.
Specifying a city sets the geographical location (latitude and longitude) and also the timezone for local time display. The latitude and longitude can be overridden whilst still keeping the city's name and timezone. Alternative, the timezone can be overridden, while keeping the same location.
-d,--date={date}
The date can be specified in any of the following formats:
Note that solunar will not accept dates in formats where the separator is a
forward-slash character, because there is too much opportunity for confusion
between the US and European varieties.
If no year is specified in the date, the current year is assumed. If no date is specified at all, the results will be for the current day (relative to the specified timezone).
-f,--full
Display full, rather than summary, results. Not all functions display more data in 'full' mode.
-j,--json
Outputs all data in JSON format, for parsing by other programs.
--list-cities
Print to standard out the full list of cities.
--log-level={0..4}
For debugging purposes, set the logging level. The default level is
-l,--latitude={-90..90}
Specifies the latitude in degrees. Positive latitudes are north of the equator, negative south. The latitude can be a decimal number.
-o,--longitude={-180..180}
Specifies the longitude in degrees. Positive longitudes are east of of the meridian, negative south. The longitude can be a decimal number.
Note that latitude and longitude can be specified along with a city
name, to use the city's timezone information. Without a city name,
the latitude and longitude must be used with the --tz option
to set a timezone.
-t,--tz={timezone}
Sets the timezone in which results will be displayed. This option also
affects the interpretation of input dates, in a rather subtle way.
In general solunar displays information for a 24-hour period
beginning and ending at midnight. When a date is entered on the
command-line, the 24-hour period is assumed to be on that date
in the specified timezone. Otherwise, the 24-hour period is
relative to the timezone of the specified city.
In general, any name in the city list can also be used as a timezone,
except that the --tz switch will not accept abbreviations.
On most Linux systems, the timezone can also be one of the conventional
strings: GMT, UTC, EST, MST, WET, etc. These can be used with
offsets, of which the most useful are probably UTC+N and UTC-N.
N is a number of hours offset from UTC (GMT), which can be fractional.
Be aware that the UTC offset is for the selected location, not for the user's system -- the meanings of the + and - signs might not be intuitive, and should be tested.
A special form of the tz option is the string sys. This overrides
the timezone in a selected city with the system's timezone. That is,
all times are displayed correct for the user's system, regardless of
the selected location. In some circumstances, the program will
display a warning when this option is used, as it is likely to
be inappropriate.
Note that solunar does not, indeed can not, warn about an
incorrect timezone name -- the C function used to manipulate the
timezone does not report any errors.
-y,--year={year}
Print a year summary of events with astronomical significance, such as equinoxes, and festival days derived from a lunar calendar, such as Easter.
This option can optionally be used with a timezone, city, latitude, and longitude, in any combination. The effects are subtle and, if nothing else is specified, the location is London, with the system timezone, for the current year.
The location and timezone affect the display of events that have
notional exact times, rather than just particular days, like the solstices.
The latitude, however specified, affects the ordering of the solstices
-- the winter solstice is in June in the southern hemisphere.
There is no general agreement about whether the term "vernal"
(spring) equinox should be used in the southern hemisphere, where
it usually occurs in autumn. solunar assumes the vernal equinox is
in March, whatever the location.
solunar attempts to work out the start and end of daylight
savings time -- not from astronomical calculations (because there
aren't any) but from the system's timezone database. These calculations
are not particularly exact, and are subject to the vagaries of
politics.
solunar reads the configuration files /etc/solunar.rc and
$HOME/.solunar.rc. Any of the long-form command line options
can be specified, in the form option=value. The command-line
arguments override these settings. So, for example, to set the
default city to London, add the line city=London to the RC file.
The calculations are all based on published algorithms that are in the public domain, 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 solunar can
do if the platform data is incorrect. I've recently discovered that some
Linux distributions require timezone data to be installed separately.
Issues of interpretation include uncertainty about exactly
what positions of the sun in the sky constitute sunset and
sunrise (and similar
considerations for the moon.)
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 take the Zenith angle as 90 degrees and 50 minutes, so
solunar does the same. However, the --full switch
will display sunset according to other popular zeniths.
In particular, civil twilight usually has
a zenith 6 degrees below conventional
sunset, and denotes the time during which outdoor activities are reasonably
practicable. Nautical and astronomical twilight have zeniths 12 and 18 degrees
below conventional sunset respectively. 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.
Although there will always be at most one sunset on a given day, and one sunrise, there can be zero, one, or two moonrises and sets. So to capture all these events we have to consider the position of the moon at a series of time intervals, and then determine the horizon-crossing points, interpolating if necessary (at least, I have not been able to find a better way to do this). This means that there is even more scope for disagreement in lunar event times than solar events. Published sources seem to vary by +/- ten minutes or so.
For the purposes of display and computation, the moon's perigee is taken
to be a distance less than 370000 km from the earth. By that definition,
there are 5-6 perigee days in a lunar month. When the moon is full at perigee, this
configuration is commonly known as a "supermoon". solunar displays
the dates of supermoons, but be aware that there is no clear definition of
what 'full' amounts to in this context.
Version 0.1.0, May 2012
Version 0.1.1, June 2013
Version 0.1.2, June 2014
Version 0.1.2a, March 2015
Version 0.1.3, January 2016
--syslocal switch; various bug fixes.Version 0.1.3a, January 2016
Version 0.1.3b, May 2017
Version 0.1.3c, December 2018
Version 2.0a, July 2020
Version 2.0b, November 2022
Version 2.0c, June 2024
Please report bugs through GitHub. Please provide enough information for me to reproduce the problem including, in particular, your home timezone.
solunar is maintained by Kevin Boone, and released for general use under
the terms of the GNU Public Licence, v3.0.