If there is one thing you can count on in the march of life, it's the fact that there are 24 hours in every day, for better or for worse. 60 seconds in a minute, 60 of those in an hour and 24 of those in a day.
But can you really count on this? As it happens, thanks to the fact that the Earth undergoes more than one type of cyclical (that is, repeated and regular) movement, the concept of a "day" has a more than one meaning.
How can this be? If the Earth makes one complete rotation on its axis, that's a day, correct? It turns out that this isn't quite what defines our typical solar day. Keeping in mind that humans use other objects in the sky as points of reference, the story of what makes a day becomes more interesting and illuminating.
Solar Days vs. Sidereal Days
Of the two main ways astronomers determine the length of a planet's day, one of these has been mentioned: The time it takes for the sun re-appear in precisely same position in the sky, is considered a solar day. The amount of time that elapsed on an accurate clock in a solar day is 24 hours on the nose (or 1,440 minutes, or 86,400 seconds).
On the other hand, consider what's happening between the Earth and the sun at a different level. In addition to rotating around its own axis, Earth also revolves around the sun, completing one revolution every 365 days or so. That means that each day, Earth moves about 1/365 of the way along its orbit around the sun, leading us to a slightly different measurement for a day – the sidereal day – when we speak of how long it takes Earth to rotate exactly once about its own axis.
- Sideres is Latin for "stars." "Sidereal" just means "pertaining to the stars."
The Basis of the "Star Day"
Picture the Earth-sun system from above, with a sprinkling of stars that can be considered "fixed" with respect to the solar system because they are so far away. (Consider that when you ride in a car, roadside objects appear to whiz by, buildings on the horizon pass slowly and faraway objects like mountains appear to move along with you.)
Earth both rotates and revolves counterclockwise as seen from above, in the direction that has come to be known as "east" by convention. As a result, once the earth moves through one complete rotation, the background stars will appear once again in the same position in the sky. The Earth has to rotate a little bit more, however, before the sun appears in the same position again.
- A sidereal day is about 23 hours, 56 minutes in solar time.
Basis of the Difference Between Solar and Sidereal Days
The four-minute difference between a solar day and a sidereal day is not a coincidence. Because the Earth completes close to 1/365 of its orbit every day, this should represent the difference between a solar day and a sidereal day.
If you divide a day into 365 equal pieces, this works out to (24 × 60) minutes divided by 365, which is 1,440/365 or 3.95 minutes. This means that every night, the constellations you see shift to the east by a little more than 1 degree (there are 365.25 solar days in a year, which is why a "leap year" is incorporated into the calendar every four years so that things don't drift too far out of whack).
Solar Time Calculator
You can find out precisely what solar time it is anywhere on the planet by consulting the NOAA page in the Resources. You can adjust this for Daylight Savings Time and customize the view in other handy ways if you like.