On our way back to Chicago from Michigan, we were talking about the phrase “Chasing daylight” as a way of saying going west and we started to wonder what the heck the groundspeed of the Sun is. I want to know if how we figured out to figure it out is right or if there is a more accurate way to do it.
Find out Sunrise time at point A.
Find out Sunrise time at point B.
Point A and Point B have to be at same Latitude.
Find out mileage between A and B.
Divide time difference into mileage and voila. Groundspeed of sun.
Is that right? I used New York and Chicago (not sure if they’re on same latitude, but thought they were close) as points of reference and figured it to be 814 miles an hour. Is that about right? (for this latitude naturally, it would be faster at equator…right )
You are correct but why bother calculating sunrise which varies day to day when you can just base it on a nominal 24 hour day and the diameter of the earth? You can make some assumptions like 8,000 mile (statute miles I think) diameter and a roughly spherical earth which won’t throw things off too badly for an estimate.
For the equator it’s diameter * pi / 24 or 8,000 * 3.141 / 24 = 1047 miles per hour.
For another elevation multiply the diameter by the cosine of the latitude. For 32º in the southen states it’s 8,000 * 0.848 * 3.141 /24 = 888mph
Or you can simply calculate the velocity of a point on the equator, then multiply that by the cosine of the degrees of latitude at the point in question. There are some localized differences due to the slightly less than precisely spherical nature of the Earth, but you’ll be close enough.
Roughly, the speed of the terminator at the equator is 1000 mph. At 45 degrees latitude (for example, Grand Traverse Bay, Michigan), the speed drops to just over 700 mph.
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