Astronomy News
Dr. Neal Sumerlin keeps us abreast of happenings in the night sky and the progress of the new Belk Astronomical Observatory.
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Other Posts:
Describing the Indescribable (07/22/2009)
Total Solar Eclipse in July (06/07/2009)
Other Earths (05/20/2009)
Save Those Old Computers! (04/13/2009)
Play With Pictures from Mars! (04/13/2009)
Saturn in 2009 (04/13/2009)
The New Worlds (02/04/2009)
Christmas at the Moon (12/10/2008)
Potpourri of Space News (12/10/2008)
Night Sky Happenings (11/17/2008)
Power Sources for Space Probes (11/17/2008)
R.I.P., Mars Phoenix Lander (11/17/2008)
Pictures of Planets (11/17/2008)
Ice Geysers of Enceladus (09/22/2008)
Constellations (09/22/2008)
More News from Mars (06/04/2008)
Search (but no rescue) on Mars (05/20/2008)
We lose a friend (05/03/2008)
Quiz winner! (04/29/2008)
Seeing and Patience (04/22/2008)
The World at Night (03/31/2008)
New Stars that are Really Old (03/14/2008)
Latest From Planetary Spacecraft (03/14/2008)
Lunar Eclipse Update (02/18/2008)
Aiming a Telescope (02/18/2008)
Observatory Update (02/04/2008)
Venus and Jupiter in the Morning (02/04/2008)
Total Lunar Eclipse (02/04/2008)
Messenger Mission to Mercury (02/04/2008)
Happy Equinox Day!
Posted on 09/22/2008Today, September 22nd, 2008, is the last day of summer and the first day of autumn. This is the day that the sun appears overhead at the equator at local noon, on it way to the southern hemisphere. The exact moment was at 11:44 AM EDT.
The earth' north-south spin axis (an imaginary line from the north pole to the south pole, around which the earth rotates) is tilted, something you probably absorbed at some point in the dim past of your pre-college education. Two questions may come to your mind:
1. Tilted relative to what?
2. So what?
Easy question first. The earth orbits around the sun, and its orbital path describes a plane. Imagine a marble rolling in a closed path on a tabletop, and that gives you an idea of what we mean by the earth's orbital plane. Now imagine that marble, at the same that it is moving, somehow spinning like a top at the same time. The marble is the earth, of course, and if its spin axis points straight up and down from the tabletop, then its equator is always lined up with its orbital plane. To move from marbles to planets, the sun would always be overhead at noon on the equator.
But it isn't. The spin axis is tilted relative to this orbital plane, pointing to the same place in the sky relative to the distant background stars. This means that at local noon, the sun will appear farther north or farther south in the sky at a given location depending on the time of year.
Here is a great interactive tutorial to see this in animation:http://astro.unl.edu/classaction/animations/coordsmotion/eclipticsimulator.swf, and a screenshot from the program to give you a taste.
A great real-time map showing the areas of day and night on the earth's surface is here:http://www.die.net/earth/. Note that today the light/dark boundary is vertical (north-south); at other times of the year it will be tilted, reflecting the earth's greater tilt relative to the sun.
As for the "So what?" question, it is this axial tilt that gives rise to the seasons. More direct sunlight in the summer (sun is higher in the sky and in the sky longer) is the reason for higher temperatures, not proximity to the sun. We are actually moving slightly closer to sun as autumn proceeds, and reach our closest point in January.
AND-no, this is not the only day of the year during which you can balance an egg on its end. Due to very small bumps on the surface of an eggshell, you can do this on any day of the year if you are more sure-handed than I.