analemma

analemma, a C code which evaluates the equation of time, by Brian Tung.

The program can compute and plot 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 the GNUPLOT program:

        gnuplot < analemma_commands.txt
      

Usage:

analemma options

• -e ecc, the eccentricity;
• -l lon, the perihelion longitude in degrees;
• -o obliq, the axial obliquity in degrees;
• -h ecc, print the usage information;

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 computer code and data files described and made available on this web page are distributed under the MIT license

Languages:

analemma is available in a C version and a C++ version and a FORTRAN90 version and a MATLAB version and a Python version.

Related Data and Programs:

analemma_test

calendar_rd, a C++ program which computes the representation of a given date in a number of calendrical systems, by Edward Reingold, Nachum Dershowitz.

calpak, a C++ library which makes various calendar calculations;

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

gnuplot_test, examples which illustrate the use of the gnuplot graphics program.

weekday, a C 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 C 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.h, the include file.
• analemma.c, the source code.
• analemma.sh, compiles the source code.