Expert: Philip Stahl - 4/22/2005

Question
If it takes 23.93 hours for the earth to spin around once, but we have a 24 hour clock, then how do we adjust our clock accoringly? (By my math we add an extra 1577 min every year because of the full 24 hours!)

Answer
Hello.

The answer is that we don't "adjust our clock accordingly".

The deviations to which you refer are associated with what we call "apparent solar time" as reckoned by a good sundial. One are basically looking at the changing hour angle of the Sun, as deduced from the shadow cast on
the dial face by the gnomon. This registers daily deviations depending on date, time of year.

The important thing to note about this time is that it's variable through the year, on account of the varying rate of Earth's motion as it orbits the Sun.

To solve the problem of daily deviations, irregularities, *mean solar time* was invented, based on what is
called the "mean Sun". The mean Sun is a *fictitious body* - meaning it doesn't exist in reality! This is because it moves at a *uniform rate* through the WHOLE year - always covering each Earth rotation in exactly 24 hours(which the REAL Sun does not do!)

The time interval marked out when the mean Sun completes a single total revolution is called the *mean solar day*. And there are 365.25 of these allocated each year. (Taking the extra day for leap year, which occurs every 4 years, and making the division by 4).

So, your math on its own is fine, it's just that you're applying it to a 'Sun' that doesn't exist for the actual registry of what we call 'Standard time' in standard time zones (which is really mean time allocated to an interval of longitude equal to 15 degrees or 360 deg/ 24 time zones).

Hope this helps.