How Long is a Day on Earth?
Written by Fraser Cain
How long is a day on Earth? It's actually a trick question. You'd think that a day on Earth lasts 24 hours… but it doesn't. Oh sure, the Sun returns to the same position once every 24 hours, but there are two things happening at the same time.
For starters, the Earth actually takes 23 hours, 56 minutes and 4 seconds to complete one rotation on its axis. In other words, a specific location on Earth takes this long to come back to the same position relative to the background stars. Astronomers call this length of time a "sidereal day".
Obviously you'd expect us to lose 4 minutes a day at this rate. Eventually day and night would flip, and it would be madness. So why doesn't that happen?
Once you take into account the fact that the Earth is also orbiting around the Sun, everything adds up to 24 hours. The location of the background stars move about 1°/day (or a Sun or Moon diameter every 12 hours) eastward. Since a circle has 360°, this works out almost perfectly for the 365 days of the year.
A complete rotation of the Earth on its axis, so that the Sun comes back to the meridian, takes 24 hours. Astronomers call this a "solar day".
So now you can see why the question, how long is a day on Earth, is actually pretty complicated. It depends on the kind of day, solar or sidereal.
Filed under: Astronomy
