 How long is a Day? |
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In grade school we are taught that a day is based on one rotation of the Earth. So here’s a question…
Question: How long does it take for Earth to rotate once?
Answer: If you said 24 hours, then you would be wrong. What?
There’s actually two kinds of days that apply to Earth, sidereal and solar, and most people have the two mixed up.
Sidereal Day
This is the time it takes Earth to rotate once, or 360°, on its axis. This takes not 24 hours but 23 hours, 56 minutes (and 4.0905 seconds). Sidereal comes from the word sidus which means 'stars'. After a sidereal day the stars are in the exact same position in Earth's sky as before.
But there’s a complication: during Earth’s rotation, it is also traveling around the Sun at nearly 67,000 mph. This causes the position of the Sun in our sky against the backdrop of the stars to change by about 1°. So the Earth needs to spin 1° more to put the Sun back into position. How long does this take? If you are thinking 4 minutes, then you're right!
Solar Day
This is the time it takes Earth to rotate 361° so that the Sun is in the same position as before (on the meridian). This takes 24 hours.
Here’s a video to show a sidereal vs solar day.
Of course we all keep time based on the solar day — the position of the Sun in our sky. But experienced stargazers are very much aware of sidereal time and on how it impacts stargazing.
Each solar day, Earth travels 1.6 million miles in its journey around the Sun. And each night, the stars reveal this motion by appearing 1° further west than the night before -- a finger width at arm's length. This may not seem like much, but it adds up over time. Every three months, the constellations shift 90° westward revealing a very different sky.
The cool idea is that the difference between the sidereal day and the solar day explains, in a technical way, the reason we have seasons of stars throughout the year.
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