Expert: Tom Whiting - 1/3/2010

Question
I am a sculptor and I have this idea for a kinetic sculpture that the top needs to be under the sun at all time so the effect that I am trying to achieve takes place. so my idea is to create a base with a mechanism that will rotate at the same speed of the sun so the sculpture is under the sun at all times. Now since this sculpture will be taken to several shows around the country I need to adjust the position of this base according to the time of the day when the sculpture is put on the exhibition. So, my question to you what is the speed or how lon does the sun take to describe the arch between east and west. I hope I make sense with this description.
Thank you
Alfonso

Answer
Hi Alfredo,
Do you mean rotate (motion around an axis) or apparent travel across our sky, based on
the Earth's rotation?
No matter... the sun rotates on it's axis about once every 25 days, but being a gas, these
values are slightly different up near the solar poles.

But what I really think you want is:
The sun appears to move 15 degrees westward per hour across our sky, because the Earth rotates
(on it's axis) at exactly 15 degrees eastward per hour. (15 degrees x 24 hours = 360 degrees,
a complete circle).
The arch (as you call it) or arc across the sky... it's length, and height in the sky, is determined by one's latitude on the Earth and the season (summer or winter).  But it's movement across the sky is always at 15 degrees per hour (or 90 degrees in a 6 hour timeframe, to make it easier on you).  And of course in the polar regions, right now above the Arctic circle, there is no sunlight in Northern winters, and Antarctica has 24 hours of sunlight, the sun describing a big circle all around the sky... but still moving at a velocity of 15 degrees per hour (or about one full handspan at arm's length.)
So set your base to advance westward at 15 degrees per hour, and you'll exactly match the Earth's rotation...this is also the normal speed of a standard 24 hour clock, which makes one complete sequence in one day, so that's why we in astronomy call it "clock drive" so our scopes will continue pointing at the same object all the time.
(Basically, you're just counteracting the Earth's eastward rotation).
A normal 12 hour clock motor won't work because it makes 2 secquences every 24 hours; one for AM
and one for PM).  So you simply go out and purchase one of our 'telescope clockdrive motors'
and that will work for your purpose too, keeping your sculptor right under the sun.
(There will be a very slight error because solar motion is slightly different than stellar
motion, as the sun advances eastward about 1 degree every 24 hours, but this is a minor correction that you can make every day... 1 degree is equal to the apparent diameter of the
sun times 2, or 2 solar diameters since the sun's disk is = 0.5 degree.)
So if you want it, the clock drive, to stay exactly on the sun, just set your clock motor to run 10 seconds slow every hour. Then it will track exactly on the sun's position.
Hope this helps,
Clear Skies,
Tom Whiting
Erie, PA USA