Summer Isn't Over ...

[Henry]

...Quite yet in the Western Hemisphere:

Sky watch: Timing autumn's start down to the minute
Dennis Mammana,
http://www.newsday.com/services/newspaper/printedition/saturday/health/ny-hs
sky135841204sep13,0,7734194.story  
September 13, 2008 
Autumn is nearly here.

This year, the season officially begins on Sept. 22 at 11:45 a.m. Many
people think that the beginning of autumn occurs when the sun rises due east
and sets due west, and when the length of our day and night are equal. And,
as long as we don't concern ourselves with fine details, those are both
reasonably true statements.

So why, then, do we often cite the exact time for the beginning of autumn?

That's because astronomers know the beginning of autumn, the autumnal
equinox, to be not actually a day, but rather a moment in time. It's marked
by the sun's passage in the heavens from the Northern Hemisphere sky into
the Southern Hemisphere sky; in fact, it's the moment when the sun's center
crosses the celestial equator on its journey from north to south.
Confused? Well, try to imagine what's going on.

During summertime, we in the Northern Hemisphere see the sun cross our
daytime sky high overhead; in our wintertime it crosses relatively low in
the southern sky. This perspective comes from the fact that the Earth's axis
is tilted and, as we orbit the sun, our summertime sun shines more directly
onto our planet's Northern Hemisphere, while the wintertime sun shines more
directly onto the Southern Hemisphere.

At some point during the year, the sun - traveling along its orbit called
the "ecliptic" - must cross the equator on its way southward, and that
defines the moment known as the autumnal equinox. When this occurs, our sun
lies directly over the Earth's equator. 

Now try to imagine standing on the Earth's equator. On the first day of
autumn you'd see the sun pass directly overhead. A few days or weeks
earlier, the sun would have crossed your northern sky, and a few days or
weeks later it would cross your southern sky. But on that day, the sun
stands directly over the equator so you would watch it pass directly
overhead.

Another way of thinking about it is to visualize a projection of the Earth's
equator into the starry heavens. This would create in the sky what
astronomers call the "celestial equator." The moment the sun crosses this
great circle on its way south marks the autumnal equinox, and the onset of
autumn in the Earth's Northern Hemisphere.

Henry Mu
Information, Media, and Technology Management Consultant 

http://my.barackobama.com/page/community/blog/henrymu