Solar position & radiation calculations.

This gem provides functions to compute solar position, rise & set times for a given position & time, as weel as solar radiation and radiation on a sloped surface.


Add this line to your application's Gemfile:

gem 'solar'

And then execute:

$ bundle

Or install it yourself as:

$ gem install solar


Compute solar passages at a date and Earth's position.

The position is specified as longitude and latitude in degrees:

longitude =  1.5 # 1 degree 30 minutes East
latitude  = 42.0 # 42 degrees North

The date as a Date object: ruby date =, 3, 22)

The +passages+ method returns the time of sun rise, transit and set. Transit refers to the moment the sun crosses the local meridian, i.e. to local solar noon.

rise, transit, set = Solar.passages(date, longitude, latitude)
puts rise    # => Sat, 22 Mar 2014 05:54:29 +0000
puts transit # => Sat, 22 Mar 2014 12:00:54 +0000  
puts set     # => Sat, 22 Mar 2014 18:07:51 +0000

Solar position in horizontal coordinates

We can also compute the local relative position of the sun for a given instant and place.

Now, instead of a +Date+ we need to specify a +Time+; e.g. we can specify the given date at 1 pm like so:

time = date.to_time + 13*3600.0

The +position+ method returns the sun's elevation and azimuth in degrees at the given time and place:

elevation, azimuth = Solar.position(time, longitude, latitude)
puts elevation # => 48.710153789164785
puts azimuth   # => 179.6564173373824

Azimuth is measured on the horizontal plane clockwise from North. Elevation is the measured from the horizontal plane to the Sun.

It's complementary angle is the solar zenith, the angular distance from the local zenith to the sun:

zenith = 90.0 - elevation
puts zenith # => 41.289846210835215

Day and Night

We can query for the day/night situation for a given time and place:

situation = Solar.day_or_night(time, longitude, latitude)
puts situation.inspect # => :day

This method returns +:day+, +:night+ or +:twilight+. Twilight refers to the period when the sun has set (appears under the hozizon) but the sky is not completely dark.

situation = Solar.day_or_night(Time.utc(2014,3,22,18,10), longitude, latitude)
puts situation.inspect # => :twilight

Actually there are different definitions of twilight and you can differentiate between them with the +:detaild+ option:

situation = Solar.day_or_night(
  Time.utc(2014,3,22,18,10), longitude, latitude, detaild: true
puts situation.inspect # => :civil_twilight
situation = Solar.day_or_night(
  Time.utc(2014,3,22,18,40), longitude, latitude, detailed: true
puts situation.inspect # => :nautical_twilight
situation = Solar.day_or_night(
  Time.utc(2014,3,22,19,30), longitude, latitude, detailed: true
puts situation.inspect # => :astronomical_twilight
situation = Solar.day_or_night(
  Time.utc(2014,3,22,20), longitude, latitude, detailed: true
puts situation.inspect # => :night

Solar Radiation

The +radiation+ method can compute the radiation (W per square meter) on at a given time and location on a horizonta plane:

global_radiation_h = Solar.radiation(time, longitude, latitude)
puts global_radiation_h # => 1021.7400285752376

In this case whe're assuming a clearness index of 1.0, i.e. clear skies.

It can also compute the radiation on an inclined surface, defined by its slope (angle in degrees from 0--horizontal to 90--vertical) and aspect (horizontal clockwise angle from North):

r = Solar.radiation(time, longitude, latitude, slope: 10, aspect: 0)
puts r # 852.5943696877531

But this method was not created to give estimates of the radiation, but to adjust measures of the global radiation on the horizontal to what a sloping surface would get. For this, we need to provide the measured radiation on a horizontal plane as +:global_radiation+:

r = Solar.radiation(
  time, longitude, latitude,
  slope: 10, aspect: 0,
  global_radiation: 432
puts r # => 410.89951605660417


Please refer to the code documentation for more information.

  • TODO: explain how to use ActiveSupport date & time methods, time zones, etc.
  • TODO: explain the use of zenith/elevations to work with civil/nautical/astronomcial etc.
  • TODO: more information about the uses of +radiation+


After checking out the repo, run bin/setup to install dependencies. Then, run rake test to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment. Run bundle exec solar to use the gem in this directory, ignoring other installed copies of this gem.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and tags, and push the .gem file to


Bug reports and pull requests are welcome on GitHub at[USERNAME]/solar. This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the Contributor Covenant code of conduct.


Copyright (c) 2012-2015 Javier Goizueta. See LICENSE.txt for further details.