Alan White wrote:

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.

I can remember the joyful suprise at suddenly understanding that during
an OU early Saturday morning broadcast after a damn good party. Everyone
else asleep, me still awake with "Understanding Space & Time" on the
telly... Keppler's equal area one and the one with Einstein on a train (and
in the station as the train swept past) with a light beam and two mirrors...
time dilation made easy!
Whatever happened to those OU programmes?

Les


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!

--
Remove Frontal Lobes to reply direct.


By all means let's be open-minded, but not so open-minded that our
brains drop out. (Richard Dawkins)

http://armsofmorpheus.blogspot.com/

http://www.richarddawkins.net/index.php


Les Hemmings a.a #2251 SA