• TechNotes
• Sun Study Accuracy

Our clients often ask about the accuracy of the Sun Study.

So we have compared sun data between ArchiCAD and Interactive Computer Ephemeris by the U.S. Naval Observatory Astronomical Applications Department. (They are said to be experts in this field.) Their DOS application is available from: http://riemann.usno.navy.mil/AA/software/docs/ice.html
We found that the biggest difference in 15 tests of various locations and times between their data and ArchiCAD's was:!!! 0.09 degrees !!!

Please see an example of the (in)significance of this error:
In Madrid (Spain), on March 22, 9:00 in the morning, the Sun's altitude is about 20 degrees.

The shadow of a 100 meter (300 ft) tall high rise building is 275 meter long, and the error of 0.09 degrees makes a 1.35 m (4 ft) difference in the length of the shadow. The speed of altitude change at this time is 0.18 degree/minute. So 0.09 degree makes a difference of 30 seconds...
At 13:20 the Sun is at its zenith, and altitude changes slowly, but shadows are much shorter. So it takes the Sun 13 minutes to change its altitude by 0.09 degrees, but the shadow length difference is 27 cm (1 ft).

NOTE: The City locations in ArchiCAD were taken from the Map control panel of Mac OS version 6. Be circumspect, because in case of big cities there could be differences in the latitude value! The given locations are precise only in an appropriate point of the city.

So if you use a city as it is defined in the control panel in ArchiCAD, use the results as an approximation.

The local standards must always be observed, and if needed, the accurate location of the building site should be purchased at the local responsible authority.
If you enter the accurate location of the building site, you can expect the accuracy mentioned above.

Another discrepancy can derive from the leap year adjustment because ArchiCAD does not count with it.

Leap years (almost every fourth year) correct the difference that is caused by the fact that Earth orbits the Sun in cca. 365.25 days instead of exactly 365 days. This isn't taken into consideration by ArchiCAD so there can be a difference of maximum +/- 0.15 degrees that derives from this discrepancy.
ArchiCAD stands in between the extremes, so plus 0.15 degrees to the 0.1 degree error mentioned above. That gives 0.25 degrees.
Note: altitude above sea level is NOT a factor in our algorithms, but in reality it makes a very small difference. Please consider that the radius of Earth is more than 6000 km, so one or two extra km's make a reasonable difference.

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