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...
| Joe
|
| You and I seem to share a common interest in the unimportant:-) I had
| an equally "nerdish" mate at University all those years ago. One
| evening we were queuing for the cinema and he remarked that the
| exterior light was flickering just like a "Cepheid variable" to which
| I replied that we could "work out how far away it is then!"
|
| Anyway, your query. I presume your figures of minutes of time per
| degree are the rate of the HORIZONTAL component of the sun's apparent
| motion rather than its motion along its path. Along its path, the sun
| moves 360 degrees in 24 hours or 15 degs/hour or as you put it, 4.0
| minutes of time per degree. Now in the summer, the sun moves
| upwards during the morning at quite a steep angle (and moves downwards
| in the afternoon similarly steeply). So the HORIZONTAL component of
| its path will be less than in winter when the sun never climbs (or
| descends) as steeply.
|

The sun really does cross the sky slightly faster during the summer than the
winter. The earth's rotation rate is for practical purposes constant, but
the sun's rate of progress across the sky is slowed by our movement round it
(the earth's rotation makes the stars appear to move from east to west, but
our motion around the sun makes the sun appear to move from west to east
relative to the stars). Our orbit around the sun is not circular, so when
we are nearer the sun (closest approach is early January) our faster
movement in orbit will make the sun appear to move just a touch slower than
when we are further away (in early July). This effect can also be seen by
checking the solstice and equinox dates. The intervals between the
equinoxes and the December solstice are a couple of days less than between
the equinoxes and the June solstice. For those of us in the Northern
hemisphere, winter is just a little shorter than summer because we are
closer to the sun and therefore orbiting it faster during the winter. [But
why is the cricket season not longer than the football season?]

However, I do not believe this effect is anything like as large as the
differences quoted in the original post, so there must be other effects as
well.

BTW, the different lengths of daylight between Summer and Winter has nothing
to do with this. The tilt of the earth's axis means that a greater or
lesser proportion of the sun's apparent path across the sky is above the
horizon, depending on the season. This effect is exaggerated by latitude,
so if you go far enough north or south you can get continuous daylight or
continuous night for a period of weeks or even months, while near the
equator there is pretty much the same number of daylight hours each day all
the year round.
--
- Yokel -
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