I was outside in Tucson one May moonless evening well after sunset, armed with a digital watch, preparing to see if I could do some magic. In the northeast, Pontatoc Ridge stood silent, an oblique black line sloping downward from the lower part of the Santa Catalina Mountains to the east, as if to tell me there was nothing to see. Well, not yet. I looked at the time: one minute to go. The spring stars of Arcturus and Spica were well above the horizon.
I looked again at my watch with my red light, Ten seconds to go, then as I looked towards the ridge, I said aloud, “Four, three, two, one, RISE!” The star Vega suddenly appeared.
Wow, like magic. It wasn’t, of course, but still pretty cool.
I like Vega. It’s bright, high overhead in summer, and in the beautiful constellation Lyra the Harp, which contains a globular cluster M56 and the famous Ring Nebula, M57, a picture of which Trekkies saw on the USS Enterprise. Vega conveniently rose over an sharply defined horizon, rather than the flat horizon, where obstructions, refraction, and thickness of looking through more atmosphere make exact timing difficult.
Two nights earlier, I noted Vega shortly after its rising and decided the following night to look earlier to get the exact time. I did just that. The stars rise about 4 minutes earlier each night or two hours earlier each month. Multiply 2 hours by 12 months, and the cycle begins anew. With the exact time of Vega’s rising, I went out the third night to see if the rising was 3 minutes 56 seconds earlier, a more exact time. It was. May in Arizona often allowed three consecutive clear nights.
The stars rise and set because of the Earth’s rotation, but the Earth, with its tilted axis, revolves about the Sun, which changes where and when the sun rises. The stars are so much further away they are essentially fixed in position, no matter where we are in our orbit. Vega is 1.6 million times further away than the Sun. From one rising to another of Vega—or any other nighttime star—is the 360 degree rotational period of the Earth, the sidereal day. It’s 3 minutes and 56.1 seconds less than our 24 hour solar day, our clock time, 366 sidereal days in a 365 day year.
Do the math: There are 1440 minutes in our solar day. Three minutes 56.1 seconds earlier a day over 366 days is 1440.01 minutes.
Do it yourself: find a star near a building or some other fixed terrestrial reference. Find when it appears or disappears, and prove it does that a little fewer than four minutes earlier each day. Command it to rise, like I did, if you wish.
Boreal B[l]og 
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