Today is All About the Fall

In a funny coincidence, the UARS satellite will fall to Earth on, you guessed it, the first day of fall.

Today, September 23, 2011, is in the news for two big, astronomical reasons: the fall of the Upper Atmospheric Research (UARS) satellite as well as the arrival of the Autumnal Equinox, which officially heralds the start of fall.




UARS Takes the Plunge
As for the UARS satellite fall, the official word from NASA is that they still do not know exactly where the fall will take place, other than it is expected to occur sometime between afternoon and evening, United States time. As for what can expected to be seen, optimistic estimates have the UARS producing a fireball as bright as the Full Moon.

Want to learn more about the UARS plunge? Well, here's complete UARS coverage from both my Examiner columns as well as other places.


:Official NASA updates
Q&A with space junk expert
NASA begins to estimate fall zone
Re-entry fireball could be as bright as Full Moon
FEMA braces for the worst
Amateur's amazing UARS videoTrack the UARS on your Android

Fall Arrives

For anyone not keeping track, fall arrives (in fact, it has already arrived at my EST location) today, September 23, specifically at 5:05am. So, with the seasonal change, why do we have seasons at all?

Answer: it all has to do with the Earth’s 23 degree tilt. If the Earth were spinning on its axis with no tilt at all, everyone would be treated to days of identical length every day of the year, with latitudes nearer the equator having longer days than those nearer the poles. However, with the tilt, the angle of the Earth relative to the Sun changes as or planet moves about its orbit.

On the Autumnal Equinox, the Sun will rise/set exactly due East/West. The Sun will climb about 50 degrees high and the day and the night will be exactly 12 hours long (Equinox means 'equal night'). After the equinox, the Sun will never leave the Southern celestial hemisphere until the next Vernal (spring) Equinox.

After the Autumnal Equinox, the shortening of the days will continue until the Sun finally reaches its most Southerly rise/set on the Winter Solstice, the shortest day of the year., which is around December 20. On this day, the Sun will rise/set low in the Southeast, get only about 25 degrees high (at Cleveland's latitude) at local noon (at about 12:30 thanks to a return to Standard Time). The final result: a day that is only 9 hours long.

From that point on, the Sun will only get stronger, once again having an Equinox, the Vernal, around the 20th of March before culminating in its most Northerly rise of the year, the Summer Solstice, around June 20, at which point the Sun will peak at a height of about 72 degrees (Cleveland, again) at local noon (about 1:30pm). Result of the high-flying Sun, a 15+ hour long day.

So there it is, the mechanics of why we have the seasons.



Year-Long Photo Shoot
For anyone looking to have some fun with a camera, go out tonight and photograph the Sun just as the solar disc starts to dip below the horizon in order to minimize the glare. Now, take note of what lens you are using (use an ultrawide if you have one) and exactly where you are standing as, in the following months, you will return to the exact same spot to shoot the solstices in December and next June, thus giving yourself a complete picture of just how much the Sun's rise/set point moves along the horizon during the year.

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