analemma

analemma, a Fortran90 code which evaluates the equation of time, based on a C program by Brian Tung.

The program can compute and plot the equation of time, the declination, and an analemma curve for various orbital parameters. The analemma is the curve traced by the position of the sun, measured at clock noon, over a year.

The program creates data and command files which must be processed by gnuplot():

        gnuplot < analemma_commands.txt
      

Usage:

analemma ecc lon obliq

• ecc, the eccentricity;
• lon, the perihelion longitude in degrees;
• obliq, the axial obliquity in degrees;

The program with no options uses the following default values:

• eccentricity = 0.01671;
• longitude of perihelion = 1.347 radians = 77.18 degrees;
• axial obliquity = 0.4091 radians = 23.44 degrees;

Licensing:

The information on this web page is distributed under the MIT license.

Languages:

analemma is available in a C version and a C++ version and a Fortran77 version and a Fortran90 version and a MATLAB version and an Octave version and a Python version.

Related Data and Programs:

analemma_test

calpak, a Fortran90 code which makes various calendar calculations;

dates, a dataset directory which contains lists of dates in various calendar systems.

gnuplot_test, a Fortran90 code which writes data and command files so that gnuplot() can create plots.

weekday, a Fortran90 code which determines the day of the week corresponding to a given date, such as 14 October 1066, Julian calendar, ... which was a Saturday.

Author:

Original C version by Brian Tung. This version by John Burkardt.

Reference:

1. Brian Tung,
   Figure Eight in the Sky, a new perspective on an old fascination,
   Astronomical Games, August 2002,
   https://www.astronomycorner.net/games/analemma.html.

Source Code:

• analemma.f90, the source code.
• analemma.sh, compiles the source code.