|home | alphabetical index|
A day is any of several different units of time. The word refers either to the period of light when the Sun is above the local horizon or to the full day covering a dark and a light period.
Ancient custom has a new day start at either the rise or set of the Sun on the local horizon. The exact moment, and the interval beween two sunrises or two sunsets, depends on the geographical position (longitude as well as latitude), and the time of year.
A more constant day can be defined by the Sun passing through the local meridian, which happens at local noon (upper culmination) or midnight (lower culmination). The exact moment is dependent on the geographical longitude, and to a lesser extent on the time of the year. The length of a such a day is nearly constant. This is the time as indicated by sundials.
A further improvement defines a fictituous mean Sun that moves with constant speed over the equator; the speed is the same as the average speed of the real Sun, but this removes the variation over a year as the Earth runs its orbit around the Sun.
The present common convention has the civil day start at midnight, which is near the time of the lower culmination of the mean Sun on the central meridian of the time zone. A day is commonly divided into 24 hours of 60 minutes of 60 seconds each.
|copyright © 2004 FactsAbout.com|