Friday, March 02, 2007

As a supplement to what I have already said about the upcoming lunar eclipse here are a few "factoids" that you may find of interest. First of all, the Wikipedia online encylopedia has an interesting article on this matter, but an much better source is the NASA Eclipse Home Page. It gives just as clear an explanation as the Wikipedia site and has links to many other things that the Wikipedia site lacks.
There is, by the way, a scale for the degree of "luminosity blocking' of lunar eclipses, devised by the French astronomer Andre Danjon. It is as follows:
L=0 Very dark eclipse. Moon almost invisible, especially in mid-totality.
L= 1 Dark eclipse. Gray or brown colour. Features of the Moon only distinguishable with difficulty.
L=2 Deep red or rust coloured eclipse with a very dark central part and the outer edge of the umbra relatively bright.
L=3 Brick red eclipse, usually with a bright or yellow rim.
L=4 Very bright copper red or orange eclipse with a very bright blueish rim.
It'll be interesting to see what tomorrow's eclipse turns out like. Most of the lunar eclipses that Molly has viewed have been in the L2 or l3 categories above.
Here's a brief list of some of the notable lunar eclipses in history and the events they have been associated with.
a)Oct 9th, 425 BCE, choice of Cleon in Athens
b)August 28th, 413 BCE, siege of Syracuse
c)April 15th, 406 BCE, fire in the temple of Athena in Athens
d)Sept 20th, 331 BCE, eclipse before the battle of Arbela of Alexander the Great
e)March 23rd BCE, death of Herod (note the problem with dating the birth of Christ!)
f)Sept 14th, 14 CE, death of Emperor Augustus
g)April 3rd, 33 CE, presumed death of Christ (the possible dates reported in the Bible accord with no reported solar eclipse)
h)May 22nd, 1453, fall of Constantinople
i)March 1st, 1504, eclipse reported by Columbus
j)July 31st,1776/Jan 23rd, 1777/July20th,1777/Dec 4th, 1778: eclipses reported by captain James Cook
k)Jan 15th, 1805 Lewis and Clark eclipse
l)July 4th, 1917 Lawrence of Arabia's eclipse
As a final factoid lunar eclipses are predictable because of a phenomenon known as the 'Saros cycle'. This means that these eclipses will reoccur in about 18 years, 11 days and 8 hours. This periodicity occurs because of the concurrence of three different cycles in the Moon's position vis-vis both the Sun and the Earth. The first is the "synodic period", about 29.53 days, the usual period of the Moon's revolution about the Earth. The Moon, however, makes a revolution about the Earth at an inclination of about 5 degrees relative to the Earth's path about the Sun-the ecliptic. twice during a period of 27.21 days the Moon passes through this plane where an eclipse can occur. This is known as the "draconic month". Finally, the Moon orbits the Earth at a variable distance, varying from apogee (its furthest point) and perigee its closest point in a period of 27.55 days known as the "anomalistic month". The coincidence of these three periods occurs about every 18 years and hence the recurrence of the lunar eclipses. The complication is that each turn of the Saros cycle results in a time difference of about 8 hours vis-a-vis any point of Earth. To learn more about this go to either the Wikipedia article or the NASA article on same.

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