Reckoning Days and Hours

  1. The Day
  2. Names of the Days
  3. When Does a Day Begin?
  4. The 24-Hour System of Counting Hours
  5. How is the Day Subdivided?

The Day

The mean solar day is the average length of a day as determined by noting one passage of the sun across the meridian of an observer and calculating the time that it takes for the sun to cross the same point in the sky a second time. Because the sun's time in making such a circuit varies seasonally, the uniform length of our day is based on a fictional average rather than what is actually seen on any given day (called the apparent solar day). The mean solar day is the basis of our 24-hour calendar day. It is actually 24 hours, 3 minutes, 56.55 seconds long in sidereal time.

The mean siderial day is determined by a procedure similar to that of fixing the solar day; however, this procedure uses a star's passage across a reference point on the celestial sphere (that point now being the vernal equinox) instead of the sun's passage. The mean sidereal day is 23 hours, 56 minutes, 4.10 seconds long in solar time. That means that the solar day appears to be about 4 minutes longer than the sidereal day because Earth in its solar orbit has to move a little farther to get back to the point at which the sun crosses the same meridian.

Names of the Days

The names of the days in English derive from either ancient Latin or Saxon systems of naming days after gods or astrological planets.
English Latin Saxon
Sunday Dies Solis (Sun) Sun's Day
Monday Dies Lunae (Moon) Moon's Day
Tuesday Dies Martis (Mars) Tiw's Day
Wednesday Dies Mercurii (Mercury) Woden's Day
Thursday Dies Jovis (Jupiter) Thor's Day
Friday Dies Veneris (Venus) Frigg's Day
Saturday Dies Saturni (Saturn) Saterne's Day

The Hours

1 mean solar day = 24 mean solar hours
1 mean solar hour = 60 mean solar minutes
1 mean solar minute = 60 mean solar seconds
1 mean solar day = 86,400 mean solar seconds

When Does a Day Begin?

The standard measurement of the day is from midnight to midnight. This is accepted for civil purposes throughout most of the world, but it has not always been so. Some ancient peoples counted the day from dawn to dawn; others, for instance, certain Germanic tribes counted nights and then grouped them into units of 14 -- our fortnight; still others, such as Jews, count their days from sunset to sunset.

The 12-Hour System of Counting Hours
Midnight = 12 A.M. or 12 M, A.M. (ante meridiem) = before noon
Noon = 12 P.M. or 12 N, P.M. (post meridiem) = after noon

The 24-Hour System of Counting Hours

Because the 24-hour system does not repeat numbers and clearly distinguishes between midnight and noon, it is less confusing than the 12-hour system. It is the official system of the U.S. military and is also used generally throughout Europe. In the 24-hour system, midnight can be designated by 2400 of one day or 0000 of the day following.

How Is the Day Divided?

The length of the day is determined by the rotation of Earth. But the division of the day into hours is an arbitrary standard, as is the uniform length of the hour. Before the invention of mechanical clocks, hours were usually of unequal length. Different cultures divided their days in different ways. The Greeks, the Egyptians, and the Romans had a 24-hour day. But they divided it into 12 hours of light and 12 of dark, which meant that the length of the hours depended on the seasons. Only after the invention of mechanical clocks in the late Middle Ages did there develop a need for an hour of uniform length.
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