Telling the time on Google Earth

Wednesday, 22nd November 2006 by

The KT Palmer Sundial is the largest in North America, described well at The Big Waste of Space. As sundials were primarily designed for telling the time it should be easy to find out when this satellite photograph was taken.

By my measurements the shadow of the sundial is at 337 degrees or 11 hours 14 minutes1. To translate this to the local time we must adjust for the sundial’s location on the Earth by following the instructions in this article.

Arizona is Mountain Standard Time (MST), which is GMT – 7 so the centre longitude is 105 degrees West. The sundial is 111.9 degrees West, and we need to adjust by 4 minutes for every degree off centre. That means our sundial is 27 minutes and 36 seconds behind local clocks and watches.

Next, we have to adjust for the “Equation of Time”, which means we need to know the exact day this photo was taken. As it’s a Digital Globe picture, we can find it in the archive where they date the photo to March 12 2005. Looking up this table we need to subtract 9m and 42s2.

Sticking it all together I therefore conclude that this satellite picture was taken at 10:36:42 MST on March 12th 2005!

  1. It is here where the most error is introduced, firstly because I measured the angle by holding an old protractor against my laptop screen. Secondly, we have to wrongly assume that the satellite was exactly overhead the sundial and completely stationary at the time of the photograph. 

  2. And, here’s even more error. Apparently these values are only for 12 noon and we’d have to interpolate for other times. Did someone say this would be easy?