A __________ is the interval between two consecutive upper meridian transits of the Mean Sun.

A __________ is the interval between two consecutive upper meridian transits of the Mean Sun.

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

Answer: (A)

The time period being described is the mean solar day. Here, the Mean Sun is a hypothetical Sun that moves at a constant, uniform rate along the sky. Because it is defined to advance at a steady pace, the interval between two successive upper meridian transits of this Mean Sun is exactly 24 hours. That steady interval is what we call the mean solar day, and it underpins civil time. In contrast, the actual Sun’s transit (the apparent solar day) varies slightly from day to day due to the Earth’s elliptical orbit and orbital speed, which is captured by the equation of time. The sidereal day, based on distant stars rather than the Sun, is about 23 hours 56 minutes, since Earth must rotate a little more to align with the stars than with the Sun. A tropical day isn’t a standard term used for this concept.

The time period being described is the mean solar day. Here, the Mean Sun is a hypothetical Sun that moves at a constant, uniform rate along the sky. Because it is defined to advance at a steady pace, the interval between two successive upper meridian transits of this Mean Sun is exactly 24 hours. That steady interval is what we call the mean solar day, and it underpins civil time.

In contrast, the actual Sun’s transit (the apparent solar day) varies slightly from day to day due to the Earth’s elliptical orbit and orbital speed, which is captured by the equation of time. The sidereal day, based on distant stars rather than the Sun, is about 23 hours 56 minutes, since Earth must rotate a little more to align with the stars than with the Sun. A tropical day isn’t a standard term used for this concept.