Sunday, March 20, 2011

Vernal Equinox

This post should appear at 7:21 pm EDT on March 20, 2011 - the exact time of the Vernal Equinox. What exactly is the Vernal Equinox?

The Earth is rotating on its axis (counterclockwise when viewed from above the North Pole or west-to-east as viewed from above the Equator).  Each rotation takes 24 hours - a day.  The axis is tilted, however, by 23.5°.  Tilted with respect to what, you might ask?  There is no "up" in space.  Well the Earth is also revolving about the Sun.  The time it takes to complete each orbit is roughly 365.25 days - a year.  The plane of the Earth's rotation around the Sun is called the plane of the ecliptic.  Perpendicular to that is up and down and the Earth's axis is tilted with respect to that perpendicular.

As the Earth rotates on its axis and revolves around the Sun, its axis is always pointing in the same direction in space.  Right now, it happens to be pointed toward a star called Polaris (the North Star) in the constellation of Ursa Minor.  Because the Earth's axis wobbles a bit, it hasn't always pointed in that same direction - it was different a few thousand years ago (the ancient Egyptians used the star Thuban in the constellation of Draco as a pole star) and will be different a few thousand years from now.

So, as planet Earth orbit the star that is our sun, its Northern and Southern Poles are sometimes tilted toward the Sun and somtimes tilted away from the Sun.  Here in the Northern Hemisphere this year, we're tilted the most toward the Sun on June 21 - the Summer Solstice - and tilted the most away from the Sun on December 22 (Eastern time) - the Winter Solstice.  The equinoxes are the points between the solstices on March 20 (Vernal or Spring Equinox) and September 23 (Autumnal or Fall Equinox) when we're neither pointed toward nor away from the Sun.

From our perspective here on Earth, we see the Sun rise in the east, move across the sky, and set in the west.  At my latitude (42° N), around the time of the Winter Solstice, when we're tilted away from the Sun, it rises in the southeast (about 32° south of east), doesn't get very high in the sky (it barely makes it to 25° above the horizon), and sets in the southwest (about 32° south of west).  On the Summer Solstice, on the other hand, the Sun rises in the northeast (about 32° north of east), gets up to around 71° (at my latitude, the Sun never gets up to 90° or the zenith) above the horizon, and sets in the northwest (about 32° north of west).  Today, and on the Autumnal Equinox is September, the Sun rises in the due east, gets up to about 48° above the horizon, and sets in the due west.

Again, at my latitude, on the Summer Solstice, since we're tilted towards the Sun, the sunlight can wrap more than halfway around the Earth and days are around 15 hours long, on the Winter Solstice, since we're titlted away from the Sun, the sunlight can't even make it halfway around the Earth and days are only 9 hours long.  Today, on the Equinox, the Sun wraps halfway around the Earth and days are 12 hours long (sunrise is around 7 am and sunset around 7 pm EDT).

For winter's rains and ruins are over,
And all the season of snows and sins;
The days dividing lover and lover,
The light that loses, the night that wins;
And time remembered is grief forgotten,
And frosts are slain and flowers begotten,
And in green underwood and cover
Blossom by blossom the spring begins.

Atalanta in Calydon
Algernon Charles Swinburne (1837-1909)

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