Saturday, September 24, 2011

Entering the dark half of the year...

A day late, but happy Autumnal Equinox.  Astronomically, it was Friday, September 23 at 0904 UTC which corresponds to 5:04 am EDT Friday morning.

The First day of autumn.  We'll see days grow increasingly shorter as the Sun rises increasingly south of east and sets increasingly south of west moving lower into the southern sky each day until it all stops and turns around at the Winter Solstice on December 22.

While I enjoy the crisp, cool days of autumn and the changing of the leaves, the progressively earlier sunset and later sunrise each day does leave me a little depressed.  It's not fun arriving for work in the dark and leaving work in the dark each weekday!

Here's a neat video I saw on Phil Plaitt's Bad Astronomy blog.  It's composed of hundreds of images of the Earth, each taken at the same time, from the geostationary METEOSAT-9 satellite.  The video runs from the 2010 Autumnal Equinox to the 2011 Autumnal Equinox.

Note how it starts with half the Earth lit.  Tthe word "equinox" comes from the Latin for "equal night" because day and night are each 12 hours long everywhere on Earth at this time (the Sun is directly over the equator).  Then watch more and more of the Southern Hemisphere lit as we move toward December.  At the Winter Solstice (winter for us here in the Northern Hemisphere, not for those south of the equator), the Southern Hemisphere has maximum light and, here in the Hudson Valley, we only have about 9 hours of daylight each day.  The Sun is directly over 23.5 degrees south latitude (the Tropic of Capricorn) at that time.

The terminator (boundary between light and dark) tilts back to the Vernal Equinox in March when we once again have 12 hours of day and night.  Then it's the Northern Hemisphere's turn to have more and more daylight.  At our Summer Solstice, in June, the Sun is directly over 23.5 degrees north latitude (the Tropic of Cancer).  Now we have about 15 hours of daylight each day.  Finally, we return to the Autumnal Equinox.

The reason for this, of course, is the 23.5 degree tilt of the Earth and the Earth's revolution around the Sun over the course of the year.  This is why we have seasons here at mid-latitude locations.

The ever-turning wheel of the year.  Read more about this in my Vernal Equinox post from March!

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