Friday, September 19, 2008

Sumer is igoin out

Lhude sing cuccu!

On Monday, September 22, 2008, at 15:44 UTC (more on this below), the autumnal or fall equinox will occur in Earth’s northern hemisphere. Strange as it may seem, the vernal or spring equinox does not occur simultaneously in Earth’s southern hemisphere, although spring will begin at that time in Earth’s southern hemisphere. No, the terms “autumn” and “spring” in relation to equinoxes is based on whether the sun is crossing the equator from north to south or from south to north. According to Wikipedia, the online encyclopedia, the equinox that will occur on September 22 is known as the fall or September or autumnal or Libra or Virgo or southward equinox, as opposed to the other one (there are two per year) that is known as the spring or March or vernal or Aries or Pisces or northward equinox.

If you have a burning need to know even more about an equinox, here is another interesting paragraph from the Wikipedia article:

“On a day that has an equinox, the center of the Sun will spend a nearly equal amount of time above and below the horizon at every location on Earth and night and day will be of nearly the same length. The word equinox derives from the Latin words aequus (equal) and nox (night). In reality, the day is longer than the night at an equinox. Commonly, the day is defined as the period that sunlight reaches the ground in the absence of local obstacles. From Earth, the Sun appears as a disc and not a single point of light; so, when the center of the Sun is below the horizon, the upper edge is visible. Furthermore, the atmosphere refracts light; so, even when the upper limb of the Sun is below the horizon, its rays reach over the horizon to the ground. In sunrise/sunset tables, the assumed semidiameter (apparent radius) of the sun is 16 minutes of arc and the atmospheric refraction is assumed to be 34 minutes of arc. Their combination means that when the upper limb of Sun is on the visible horizon its center is 50 minutes of arc below the geometric horizon, which is the intersection with the celestial sphere of a horizontal plane through the eye of the observer. These effects together make the day about 14 minutes longer than the night at the equator, and longer still at sites toward the poles. The real equality of day and night only happens at places far enough from the equator to have at least a seasonal difference in daylength of 7 minutes, and occurs a few days towards the winter side of each equinox.”

All righty, then. Let's push on.

In honor of Ruth Hull Chatlien’s new Bulova watch, which her husband Michael gave her as a fiftieth birthday present (hers, not his), I would like to turn now to the subject of Coordinated Universal Time (which is abbreviated UTC even though it looks as though it ought to be abbreviated CUT and which in French is known as Temps Universel Coordonné which looks as though it ought to be abbreviated TUC but is still referred to as UTC), which Wikipedia says is International Atomic Time (TAI, not IAT) with leap seconds added at irregular intervals “to compensate for the Earth's slowing rotation” which sounds a bit alarming because a slowing implies an eventual stop but apparently is of no great consequence. I would like to, but I dare not, as the Wikipedia article on the subject is confusing enough to make your head spin and have an equinox of its own, depending on your axial tilt.

It must suffice to report to you the fascinating information that the Earth’s rotational speed is very slowly decreasing due to tidal deceleration, causing the mean solar day to increase in length, which requires the occasional insertion of the aforementioned leap second by people who know exactly how and when to do it.

Perhaps Ruth should link to them so that she will know when to adjust her new Bulova watch.

Update: I left out a really important fact about UTC or CUT or TUC or TCU (no, that's Texas Christian University): It's the same as Greenwich Mean Time (GMT), the time at the Royal Observatory in England, the time along the Prime Meridian, zero degrees longitude. So the autumnal equinox will occur in the United States at 11:44 Eastern/10:44 Central, as they say on television, on Monday.


  1. I fear you lost me after the date and time !

  2. Oh dear. I think I'll stick with my usual practice of setting my watch to the U.S. Naval Observatory.

  3. UTC is (almost)as bad as hexadecimal mathematics! :) I now know why I'm aging so fast; the leap seconds have caught up with me!

  4. Wow, what a post-full of information! I've been watching the sun lately as it heads back south for its rising and setting. I like to imagine I'm an ancient person watching the idiosyncracies of the sun like we modern people watch TV. The sun would have been quite an amazing show for them! No wonder they figured out when solstices and equinoxes were, and thought those times were very special.

    A friend of mine visited Ecuador this summer.....near the equator, of course. She said the days and nights are always equally balanced in length there, of course. I was thinking, wouldn't it be odd to have it that way ALL the long-lighted summer days and no short winter ones!

  5. Thanks, everyone for commenting! On my honor, I will do my best to make shorter, less brain-rattling posts in the future!

  6. You are so full of information and I always learn something. Not to say I understand it all but I feel the stretching of my wee little mind when you write.