July 23, 2017

21.9.9: The Autumnal Equinox.

A celestial alignment; in a neigborhood near you? (Credit: Art Explosion).

A celestial alignment; coming to a neighborhood near you? (Credit: Art Explosion).

Can you feel it? The brunt of northern hemisphere summer is about over, giving  way to our favorite season here at Astroguyz; Fall. It’s not just our collective imagination; this Tuesday marks the Autumnal Equinox, or the spring (vernal) Equinox for those down under. This marks the mid way point for the Sun’s apparent journey form north to south, and the beginning of spring and fall, respectively. Of course, we’re the ones in motion!To be technical, this is the point that the Sun rests at 180 degrees along the ecliptic, and at a right ascension of 12 hours and a declination of exactly 0. This occurs this year at precisely 21:18 hours Universal Time on Tuesday, September 22nd. The Sun will rise exactly due east from your locale and set due west, our personal favorite observation to make on this day (weather willing) to site any potential local “Stonehenge” alignments. Balance an egg if you must, but have be known that you can accomplish this feat equally well at any other time through out the year! Happy Chuseok to those on the Korean peninsula, New Year’s Day to those still on ye olde calendar of the French Republic, and a merry Mabon to those Wiccan fans out there. Bring on the darkness!

The astro term for the month is Angle of Obliquity. No, this isn’t Spanish… in our complex motion around the Sun, our planet also slowly “nods’ back and forth, in a motion known as nutation. Together, with the variation of eccentricity and precession, this leads to what is known as the Milankovitch cycle, which is thought to be a key trigger in successive Ice Ages. This also causes the equinoxes and solstices to slowly wander by date. The tilt, or obliquity of our planet varies from 22.1 degrees to 24.5 degrees, with a current value of 23 degrees, 26′ and 21.448” measured in 1976 and decreasing. The period of variation takes 42,000 years to complete, and shows up in calculations as the Greek letter Epsilon.  Other planets swing in such a fashion, some much wider than our own. For example, the planet Mars varies from 11 to 49 degrees, due to a lack of a large, stabilizing Moon such as our own..as our own Moon slowly recedes over the eons, the difference of minimal and maximal tilt will become wider, with a variation of 27 to 60 degrees in about 2 billion years!

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