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.
Different definitions of the day are based on the apparent motion of the Sun across the sky (solar day). The reason for this apparent motion is the rotation of the Earth around its axis, as well as the revolution of the Earth in an orbit around the Sun.
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 between 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 earth has over time had an increasingly longer day. The original length of one day, when the earth was new, is actually closer to 23 hours. This phenomenon is due to the moon which slows the Earth's rotation slowly over time. Because of the way the second is defined, the mean length of a day is now about 86400.002 seconds, and is increasing by about 2 milliseconds per century. See tidal_acceleration for details.
For civil purposes, since the middle of the 19th century when railroads with regular schedules came into use, a common clock time has been defined for an entire region based on the mean local solar time at some central meridian. For the whole world, about 30 such time zones are defined. The main one is "world time" or UTC (Coordinated Universal Time).
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.
In astronomy also the sidereal day is used; it is ca. 3 minutes 56 seconds shorter than the solar day, and close to the actual rotation period of the Earth.
Boundaries of the day
Nights are named after the previous day, e.g. "Friday night" means the night between Friday and Saturday.
TV-guides tend to list nightly programs at the previous day, although programming a VCR requires the strict logic of starting the new day at 0:00.
Validity of tickets, passes, etc. for a day or a number of days may end at midnight, or closing time, when that is earlier. However, if a service (e.g. public transport) operates from e.g. 6:00 to 1:00, the last hour may well count as being part of the previous day (also for the arrangement of the timetable). For services depending on the day ("closed on Sundays", "does not run on Fridays", etc.) there is a risk of ambiguity.
For the Dutch Railways a day ticket is valid 28 hours, from 0:00 to 4:00 the next night.
Expressions like "today", "yesterday" and "tomorrow" tend to be ambiguous during the night.
List of famous days
See also List of commemorative days
People named Day
- times from 10 kiloseconds to 100 kiloseconds
- Calculating the Day of the Week
- season, for a discussion of daylight and darkness near the poles and the equator and places in-between
- Show where it is daytime at the moment