Earth revolves around its own axis in approximately 23 hours 56 minutes 4 seconds. If we measure the length of a day by 24 hours we hour saving 180+56=234 seconds a day. In a year that is 234 * 365 = 85, 410 seconds. In 4 years that is 341,640 seconds. Now a say has 86400 seconds, so we are the end of 4 year we have 341,640/86400 = 3.95 days. But we add 1 day to February after 4 years. Why is this?
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23 hour 56 minutes and 4 seconds is what’s called a sidereal day. The amount of time it takes for the Earth to rotate 360°. This is different from a solar day.
A solar day is slightly longer because while the Earth is rotating on its axis, the Earth is also revolving around the Sun. Since this is happening, the Earth has to rotate slightly further for the Sun to be directly above the same point on the Earth.
A mean solar day is 24 hours because it’s the average amount of time it takes for the Sun to reach same spot in the sky. Since the Earth’s orbit isn’t a perfect circle (it’s an ellipse) the distance the Earth travels each day around the sun is different. The Earth is closest to the sun in January and therefore moving faster, and slightly further away in July, and moving slower (note: this is not the phenomenon that causes seasons, this is different) so if you measure a solar day in January, it will be slightly longer than 24 hours, if you measure one in July, it will be slightly less than 24 hours. If you measure every day of the year and average them together, you get exactly 24 hours.
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