Which term denotes the interval between two consecutive upper meridian transits of the mean equinox?

• A. Sidereal Day
• B. Tropical Day
• C. Mean Solar Day
• D. Solar Day

Answer: (A)

Time is measured by how long it takes for the Earth to rotate relative to fixed celestial references. The mean equinox serves as such a reference point on the celestial sphere. When you look at how long it takes for this reference point to cross the local meridian twice in a row, you’re tracking the Earth’s rotation relative to distant stars, not the Sun. That interval is the sidereal day, about 23 hours 56 minutes 4 seconds. The Sun’s position isn’t what’s being used here, so the solar day (about 24 hours) or tropical/mean solar-day concepts, which hinge on the Sun’s motion, don’t apply to this measure.