On Fri, 1 Dec 2006 18:18:03 -0000, "Col"
wrote:

...
A the good old Equation of Time.
Perhaps this will be the year I finally understand it......
...

Obviously time for my annual post :-)

One of Keppler's Laws says that the area swept out by a body in its
orbit rotating about another is a constant for a given time. In an
elliptical orbit the distance of the orbiting body varies with respect
to the orbited body. For the area swept in a given time to be
constant, the speed of the orbiting body will be greater when its
closer to the orbited body and slower when it's further away.

Because the orbit of the Earth is elliptical, it's orbital speed
varies, but it's rotational speed about its axis is constant. The
effect of this is to skew sunrise, noon and sunset times
with respect to Standard Time. The amount of this skew is called the
Equation of Time and tables are published giving its value throughout
the year. Hence, to labour the point, the Sun is only due south at
noon at the Greenwich Meridian on about four dates in the year. At
other times it can be as much as twenty minutes earlier or later.
However, sunrise and sunset are roughly symmetrical about the time
that the Sun crosses the meridian so that if the Equation of Time on a
particular day is ten minutes then the Sun will cross the meridian ten
minutes after noon and sunrise and sunset will also occur ten minutes
later than if the Equation of Time didn't exist, but not symmetrically
about noon.

After the Winter Solstice logic indicates that sunrise should occur
earlier. However, around the Winter Solstice the Equation of Time is
increasing at a greater rate than sunrise is decreasing. Hence,
sunrise actually increases after the Solstice until early January when
the two rates equalise and after that sunrise moves in the 'right'
direction.

The Equation of Time is zero on or about 15th April, 14th June, 1st
September and 25th December.Only on those days will the Sun cross the
Greenwich Meridian at noon.

Phew!

--
Alan White
Twenty-eight miles NW of Glasgow, overlooking Lochs Long and Goil in Argyll, Scotland.
Webcam and weather:- http://windycroft.gt-britain.co.uk/weather
Some walks and treks:- http://windycroft.gt-britain.co.uk/walks