AstronomicalData and assumed geographic position
- To: mathgroup at smc.vnet.net
- Subject: [mg94167] AstronomicalData and assumed geographic position
- From: "sjoerd.c.devries at gmail.com" <sjoerd.c.devries at gmail.com>
- Date: Sat, 6 Dec 2008 06:15:27 -0500 (EST)
Hi folks,
According to the documentation for AstronomicalData "The default
location on Earth to assume is given by the value of $GeoLocation."
This means that
AstronomicalData["Sun",{"Altitude",{2008,12,5,13,0,0}] and
AstronomicalData["Sun",{"Altitude",{2008,12,5,13,0,0},$GeoLocation}]
should yield the same results. In fact, they do not.
Moreover, plotting one daily cycle with
Plot[AstronomicalData[ "Sun", {"Altitude", {2008, 12, 5, h 0, 0}}],
{h, 0, 23}] suggests a completly static sun.
Therefore, it seems that AstronomicalData without explicit lat,long
coordinates does not work properly. Or am I doing something wrong?
By the way, it's a pity there is no built-in sun path function. Using
AstronomicalData to get the sun position is about 20 times slower than
using the VSB functions by Greg Pelletier (http://www.ecy.wa.gov/
programs/eap/models/twilight.zip) that I converted to Mathematica
code. AstronomicalData seems to be less accurate as well, but that may
be due to differences in modelling of atmospheric refraction.
Cheers -- Sjoerd