Solar Calendar

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The artist drawing of Woodhenge in this link is based on the solar calendar at Cahokia Mounds in Collinsville, to the east of St. Louis, Missouri.

Artist Drawing of Woodhenge

At Cahokia, the Woodhenge, calendar is a circle 410 feet wide with poles around the edges. The poles mark the northeastern and southeastern angles where the rising sun casts its shadow on the circle throughout the year. During the summer solstice, the sun rises in the northeast. During the winter solstice, the sun rises in the southeast. Halfway between these two points, the sun rises from the east during the spring and autumn equinoxes.

solcal creates a PPM image of the solar calendar depending on the year and your location. The further you are from the equator, the wider the angle between the summer and winter solstice on the calendar.

Solar Calendar based on the location of Cahokia Mounds near St. Louis, Missouri, in the year 2001.

Syntax

The syntax for solcal is:

      solcal yyyy xx.x yy.y tt.t
      ppmview <solcal.ppm.gz

Where

• yyyy is the 4 digit year.
• xx.x is degrees of longitude. West is negative.
• yy.y is degrees of latitude. South is negative.
• tt.t is the time zone in hours from UT. West is negative.
• ppmview is the program to view the PPM image.

Test Parameters

The following are parameters for testing solcal.

City	State	Year	Longitude	Latitude	Time Zone
St. Louis	Misssouri	2001	-90.067	38.667	-6.0
Fairbanks	Alaska	2001	-146.284	64.838	-9.0
Honolulu	Hawaii	2001	-156.142	21.307	-10.0
Mexico City	Mexico	2001	-98.917	19.433	-6.0
Rio de Janeiro	Brazil	2001	-42.8	-22.95	-3.0
Cairo	Egypt	2001	31.35	30.033	2.0
Sydney	Australia	2001	151.0	-34.0	10.0

Parameter Limits

The sunrise and sunset calculations in solcal produce an error beyond 65.5 degrees north or south latitude, or within a few degrees of the equator. Sunrise and sunset times within 5 degrees of the equator are not accurate.

Output Files

solcal writes

• Sunrise and sunset times and azimuth to standard output.
• A PPM image file showing a drawing of the calendar.
• The PPM filename is called solcal.ppm.gz

The tokens written to standard output are:

• Solstice (06/21 or 12/21)
• Local sunrise time
• Sunrise azimuth (in degrees)
• Local sunset time
• Sunset azimuth (in degrees)

Output for the example at Cahokia Mounds:

06/21  04:35:19        59.450179   19:32:28       300.549821


12/21  07:13:03       120.552102   16:43:23       239.447898

Azimuth Graph

Click here to read about the azimuth graph.

References

Cahokia Mounds, a community dating from 700 A.D. to 1400 A.D.

Woodhenge, the solar calendar at Cahokia Mounds.

The sunrise and sunset calculations in solcal are based on:

Peter Duffet-Smith
Practical Astronomy with Your Calculator
Cambridge: Cambridge University Press, 1988
3rd edition, 185 pages, indexed
ISBN 0 521 35629 6 hardback
ISBN 0 521 35699 7 hardback

Ppmview

PPM Conversion