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The length of the day (LOD), which has increased over the long term of Earth's history due to tidal effects, is also subject to fluctuations on a shorter scale of time. Exact measurements of time by atomic clocks and satellite laser ranging have revealed that the LOD is subject to a number of different changes.
A contour plot of the hours of daylight as a function of latitude and day of the year, using the most accurate models described in this article. It can be seen that the area of constant day and constant night reach up to the polar circles (here labeled "Anta. c." and "Arct. c."), which is a consequence of the earth's inclination.
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Combining these two effects, the net acceleration (actually a deceleration) of the rotation of the Earth, or the change in the length of the mean solar day (LOD), is +1.7 ms/day/cy or +62 s/cy 2 or +46.5 ns/day 2. This matches the average rate derived from astronomical records over the past 27 centuries.
Daytime length or daytime duration is the time elapsed between beginning and end of the daytime period. Given that Earth's own axis of rotation is tilted 23.44° to the line perpendicular to its orbital plane, called the ecliptic, the length of daytime varies with the seasons on the planet's surface, depending on the observer's latitude.
In terms of Earth's rotation, the average day length is about 360.9856°. A day lasts for more than 360° of rotation because of the Earth's revolution around the Sun. With a full year being slightly more than 360 days, the Earth's daily orbit around the Sun is slightly less than 1°, so the day is slightly less than 361° of rotation.
The length of the light and dark in each phase varies across the seasons due to the tilt of the earth around its axis. The photoperiod defines the length of the light, for example a summer day the length of light could be 16 hours while the dark is 8 hours, whereas a winter day the length of day could be 8 hours, whereas the dark is 16 hours.
For example, with an axial tilt is 23°, and at a latitude of 45°, then the summer's peak sun angle is 68° (giving sin(68°) = 93% insolation at the surface), while winter's least sun angle is 22° (giving sin(22°) = 37% insolation at the surface). Thus, the greater the axial tilt, the stronger the seasons' variations at a given latitude.