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How Repeatable is the Position of a Star in the Sky From One Year to the Next?

-- Ben | December 27, 2020

Question:

How repeatable is the position of a star at a specific time? For instance, I am located in SF Bay area, and I observe the position of Betelgeuse at 9PM sharp on December 20th, could I permanently fix the angle of a telescope to train Betelgeuse in the center of the viewfinder and expect it to be there at the same time every year? Or does the motion of the solar system move such that I need to account for that in the position, and we just do that naturally when temporarily aligning a telescope?

Thank you for your time,
-Ben

-- Ben

Answer:

Since the Earth completes one orbit around the Sun every year, it returns to the same position after 365.256 days, which is one sidereal year. The stars would appear in almost the same position in the sky after one sidereal year. The “almost” part of this statement is that there is a slow wobble in the Earth’s orbit, called precession, which causes the direction of its rotational axis to change over time with a period of 25,772 years. This causes star positions to change by roughly 1 degree every 71.6 years.

-- Jeff Mangum

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