The average year is, in fact, 365.242 days long – just so long as you’re not talking about the sidereal year. So what’s that? Simply put, a sidereal year – it’s pronounced sy-dear-ee-al ...

Slightly less than the 24 hours of a solar day (23 hours, 56 minutes, 4.1 seconds). Used in astronomy, sidereal means "when a star crosses the meridian" (the meridian is an imaginary circle around ...