A local sidereal day is the interval of the time between the two successive upper transits of the First Point of Aries on the observer's meridian. Which of the following transit also occurs at intervals of 24 sidereal hours?

• A. Aldebaran
• B. Polaris
• C. Betelgeuse
• D. Vega

Answer: (A)

The key idea is that the meridian crossing of a fixed star repeats with the Earth's rotation relative to the stars. A sidereal day is defined by the time between successive upper transits of the First Point of Aries, meaning the sky has returned to the same orientation with respect to distant stars after one sidereal day. Because a star has a fixed position on the celestial sphere (its right ascension is constant over short times), it crosses the observer’s meridian again after exactly one sidereal day. The upper transit—when the star is on the meridian above the horizon—thus recurs every 24 sidereal hours.

Aldebaran, like any fixed star, will have its upper meridian transit repeat on each successive sidereal day, so its transit also occurs at intervals of 24 sidereal hours. (This is true in general for stars with fixed positions; the exact local clock time of transit drifts relative to solar time due to the difference between sidereal and solar days.)