diff --git a/docs/building/sources/constants.rst b/docs/building/sources/constants.rst
index c62d638b..b98b9324 100644
--- a/docs/building/sources/constants.rst
+++ b/docs/building/sources/constants.rst
@@ -3,3 +3,132 @@
 ###########
  constants
 ###########
+
+.. warning::
+
+   This source may be renamed ``forcings``.
+
+Although this source is called `constants`, its purpose is to provide
+fields with values that only depend on the grid cell and/or the time.
+
+The is a mean to provide the model with with space and time information
+during training and inference.
+
+latitude
+   Each grid point has the value of its latitude in degrees.
+
+   Range of values: :math:`[-90, 90]`
+
+cos_latitude
+   Each grid point has the value of :math:`cos(latitude/180*\pi)`.
+   Unlike the **latitude** field, this field is periodic in space.
+
+   Range of values: :math:`[-1, 1]`
+
+sin_latitude
+   Each grid point has the value of :math:`sin(latitude/180*\pi)`.
+   Unlike the **latitude** field, this field is periodic in space.
+
+   Range of values: :math:`[-1, 1]`
+
+longitude
+   Each grid point has the value of its longitude in degrees. Currently,
+   the longitude are not normalised and the values are the ones provided
+   by the source used as template.
+
+   Range of values: :math:`[-180, 360)`
+
+cos_longitude
+   Each grid point has the value of :math:`cos(longitude/180*\pi)`.
+   Unlike the **longitude** field, this field is periodic in space.
+
+   Range of values: :math:`[-1, 1]`
+
+sin_longitude
+   Each grid point has the value of :math:`sin(longitude/180*\pi)`.
+   Unlike the **longitude** field, this field is periodic in space.
+
+   Range of values: :math:`[-1, 1]`
+
+ecef_x
+   Each grid point has the value of the **x** coordinate of the
+   `Earth-Centered, Earth-Fixed (ECEF) coordinate system <ECEF>`_. The
+   Earth is assumed to be a perfect sphere with a radius of 1.
+
+   Range of values: :math:`[-1, 1]`
+
+ecef_y
+   Each grid point has the value of the **y** coordinate of the
+   `Earth-Centered, Earth-Fixed (ECEF) coordinate system <ECEF>`_. The
+   Earth is assumed to be a perfect sphere with a radius of 1.
+
+   Range of values: :math:`[-1, 1]`
+
+ecef_z
+   Each grid point has the value of the **z** coordinate of the
+   `Earth-Centered, Earth-Fixed (ECEF) coordinate system <ECEF>`_. The
+   Earth is assumed to be a perfect sphere with a radius of 1.
+
+   Range of values: :math:`[-1, 1]`
+
+julian_day
+   The Julian day is the fractional number of days since the 1st or
+   January at 00:00 of the current year. For example, the Julian day of
+   1st or January at 12:00 is 0.5. Every grid point has the same value
+   of the Julian day at the given date.
+
+   Range of values: :math:`[0, 365)` on a non-leap year and :math:`[0,
+   366)` on a leap year
+
+cos_julian_day
+   Each grid point has the value of
+   :math:`cos(julian_day/365.25*2*\pi)`. Unlike the **julian_day**
+   field, this field is periodic in time.
+
+   Range of values: :math:`[-1, 1]`
+
+sin_julian_day
+   Each grid point has the value of
+   :math:`sin(julian_day/365.25*2*\pi)`. Unlike the **julian_day**
+   field, this field is periodic in time.
+
+   Range of values: :math:`[-1, 1]`
+
+local_time
+   Each grid point has the value of the local time in hours. The
+   computation of the local time is solely based on the longitude of the
+   grid point (i.e. no time zone information is used). The local time is
+   computed as the fractional part of the longitude in hours, starting
+   from 0 at the Greenwich meridian. So, for example, a grid point with
+   a longitude of 0 will have a local time of 12.5 at 12:30 UTC.
+
+   Range of values: :math:`[0, 24)`
+
+cos_local_time
+   Each grid point has the value of :math:`cos(local_time/24*2*\pi)`.
+   Unlike the **julian_day** field, this field is periodic in time.
+
+   Range of values: :math:`[-1, 1]`
+
+sin_local_time
+   Each grid point has the value of :math:`sin(local_time/24*2*\pi)`.
+   Unlike the **julian_day** field, this field is periodic in time.
+
+   Range of values: :math:`[-1, 1]`
+
+insolation
+   This is an alias for the **cos_solar_zenith_angle** field.
+
+   Range of values: :math:`[?, ?]`
+
+cos_solar_zenith_angle
+   This is an alias for the **insolation** field.
+
+   Range of values: :math:`[?, ?]`
+
+toa_incident_solar_radiation
+   Top of atmosphere incident solar radiation in W/m^2.
+
+   Range of values: :math:`[?, ?]`
+
+.. _ecef: https://en.wikipedia.org/wiki/Earth-centered,_Earth-fixed_coordinate_system