Which statement about the lengths of solar and sidereal years is true?

• A. There are 365.25 solar days in a year.
• B. There are 366.25 sidereal days in a year.
• C. There are 364.25 sidereal days in a year.
• D. Both A and B.

Answer: (D)

Understanding how many days make up a year depends on whether you count solar days or sidereal days. A solar day is 24 hours long, defined by the Sun's position, while a sidereal day is about 23 hours 56 minutes, defined by the stars.

The tropical (solar) year, the cycle that governs our calendar, is about 365.242 days long. So the number of solar days in a year is roughly 365.24, and it’s common to round that to about 365.25 for simple estimates.

The sidereal year, tied to the stars, lasts about 365.256 days. Since a sidereal day is shorter than a solar day, you fit a bit more than 365 solar days’ worth of rotation in a year, roughly 366.24 sidereal days. It’s also common to round that to about 366.25.

Thus both statements are reasonable approximations: about 365.25 solar days in a year and about 366.25 sidereal days in a year. The other values don’t match these widely used approximations as closely.