# How to calculate the true length of day on a celestial body?

When I only know the sidereal rotation period, how do I determine the true length of day on a body?

We know the Earth's sidereal day doesn't have full 24 hrs unlike the true length of day. It's similar on Mars where the sidereal day isn't as long as a Sol (about 24 hrs 40 min) either. So how do we calculate the true length of day on a planet?

Let's do it on the example of the planetoid Ceres: its sidereal rotation period is 9 hrs 4 min and 27 sec. How do we calculate the true length of day on Ceres?

• You mean the mean Solar day? You would have to know the period around the star Jan 29 '20 at 12:40
• Note that the "true" or apparent day length (the time from one high noon to the next) generally differs from that of the mean solar day. See en.wikipedia.org/wiki/Equation_of_time and physics.stackexchange.com/a/469884/123208 Jan 29 '20 at 17:33
• @PM2Ring I was asking on the mean solar day however, what on Earth is 24 hrs and on Mars about 24 hrs 40 min. Jan 30 '20 at 6:29 