"Falk Tannhäuser" wrote in message
...
| Tim923 wrote:
|
| I had thought that days at the equator were exactly 12 hours long all
| year round, but charts show varying sunrise and sunset times. How is
| that?
|
| You may want to google for "Equation of Time" - a nice explanation is
| for instance found on http://www.astronomynotes.com/nakedeye/s9.htm.
| Briefly: While the average length of solar day is 24h, the actual
| solar day (the time between meridian passages of the Sun) varies
| by a few seconds in function of the season. This deviation sums
| up to the so called "Equation of Time" - the difference between
| the mean time and the time indicated by a sun dial. The latter
| is, for instance, about 14min 15s late at mid February, 3min 40s
| in advance at mid May, 6min 30s late at end of July and 16min 25s
| in advance at beginning of November.
|

This is probably the smallest contribution to this effect.

Also present a

1) Refraction by the earth's atmosphere. As the sun approaches the
horizon, this causes its disk to appear up to a degree higher than it really
is, delaying sunset and causing sunrise to be earlier. As it takes about 4
minutes for the sun to travel this amount in the sky, each day will be over
12 hours by nearly 10 minutes.

2) The angle of the sun's path in the sky. The sun only rises and sets
vertically at the equator on the equinox days. At the solstices its path is
inclined by about 23 degrees, which will also mean that it takes slightly
longer (about a minute or so) to rise or set, because of the apparent
displacement of the sun's position as in (1) above).

The equation of time effect causes the sunrise and sunset times to vary
either side of local 0600 and 1800 by about 15 minutes, but does not alter
the interval between them, the daylength, apart from the few seconds per day
the previous poster mentions when these times are shifting at their maximum
rate.
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