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| A Solar Day is when the Earth rotates enough so that the Sun returns to the same place in the sky. Effectively, one day is from Noon to Noon. A Sidereal Day is when the Earth rotates enough so that the distant stars return to the same place in the sky. Because the Earth is also revolving around the Sun, the amount of time for the Sun to return to it's starting point is a bit longer than that needed for the stars to return to their starting point. This always results in the Earth rotating one more time than there are days in the year. The following simple video demonstrates why there is one more Sidereal Day per year than there are Solar Days. In this video, the Earth's Prime Meridian is marked in red and starts out point directly left. Each time the Prime Meridian points directly at the Sun, a point is marked on the orbit with an arrow pointing at the sun. A count number is displayed to keep track of how many 'noons' there are in one year. Whenever the Prime Meridian returns pointing directly left, a point is marked on the orbit with an arrow pointing due left. As well, a count number is displayed to count these absolute rotations. After one orbit, you can see in this idealized orbit demonstration that there are 8 Solar Days (in red) and 9 Sidereal Days (in blue). The planet rotates a total of 9 times in 8 days. |