SECOND LUNAR ECLIPSE OF 2007: UPCOMING ON AUGUST 28TH:

BASIC EXPLANATION OF AN ECLIPSE:

An eclipse is basically a celestial event when one body moves into the shadow of another. While the term is used mostly to describe either solar or lunar eclipses on Earth it could just as well be used to describe other events such as a planet moving into the shadow cast by one of its moons (or vice versa) or the transit of a moon on another planet as it moves into the shadow of its home planet or when it crosses between that planet and its parent star. In our solar system the relatively large size of our Moon means that Earth is the only planet where "total" solar eclipses can occur- if you exclude the Pluto/Charon system of course. On other planets, however, there would be any number of lunar eclipses as their moons passed into the shadow of the planet. The larger term for such events is "syzygy" which refers to the alignment of three or more celestial bodies in the same gravitational field along a straight line. A "transit" is a syzygy where the nearer object appears considerably smaller than the further one. Examples would be the transit of Venus or Mercury across the Sun or the transit of Earth across the Sun as seen from Mars. The term transit is also used loosely to describe the passage of an extrasolar planet across its star as seen from Earth. Such transits are useful in the discovery of extrasolar planets. An "occultation" is where a celestial body is hidden by the passage of another body which passes between it and the observer. Unlike an eclipse the further body doesn't necessarily enter a shadow.

THE ORBIT OF THE MOON

The Earth/Moon system is actually quite "privileged" in terms of eclipses because of the relatively large size of the Moon as compared to the Earth. The Sun is about 1.4 million kilometers in diameter and is generally about 150 million kilometers away from Earth. The Moon is about 3,500 kilometers in diameter and lies about 380,000 kilometers away from the Earth (on average). The ratio of the Sun's distance to its size is about 110/1. So is the ratio of the Moon's distance to its size. This means that the two objects appear to be about the same size in the sky. In terms of a solar eclipse when the Moon moves between the Earth and the Sun the Moon appears to just exactly fit the disc of the Sun. Well mostly. The orbit of the Moon is an ellipse rather than a circle. this means that sometimes the Moon is further away from the Earth than at other times. If the Moon is closer to the Earth, its perigee, the shadow of the Moon will be larger than the apparent diameter of the Sun and the eclipse will be "total". If the Moon is near its far point from the Earth, its apogee, the Moon will not entirely cover the disc of the Sun and a halo of the Sun will appear around the Moon. This is called an "annular eclipse".

The Moon is actually receding gradually from the Earth, mostly due to "tidal friction" whereby some of the Earth's angular momentum is being transferred to the Moon's orbital momentum. This also results in a gradual slowing of the Earth's orbit which means that the days get longer and longer. Sort of a cosmic daylight savings time. The Moon pulls away from the Earth at a rate of about 38 millimeters per year, and the length of a day increases by about 17 microseconds per year. This means that our day is longer than it was in the past. Sixty five million years ago when the cometary hit man bumped off the dinosaurs the day was only about 23 hours in length. It also means that the apparent diameter of the Moon has varied. Hundreds of millions of years ago it would have seemed considerably larger than the Sun. In the future all eclipses will be annular eclipses. Presently slightly more solar eclipses are annular rather than total.

A lunar eclipse happens when the Moon travels through the shadow of the Earth in its orbit. This can only happen when the Moon is on the opposite side of the Earth from the Sun, hence only when the Moon is in its Full Moon phase. But, you may say, the Moon obits the Earth every month, about 29.531 days from one phase of the Moon to the recurrence of the same phase. This is known as the "synodic month". There are other ways to describe a "lunar month". The "sidereal month" is the time it takes the Moon to return to its previous position in respect to the fixed stars. This "month" is about 27.322 days.This is shorter than the sinodic month because of the motion of the Earth/Moon system around the Sun. It is also the explanation for why each full Moon is in a different sign of the zodiac. There are three other "lunar months". The "anomalistic month" is the time it takes the Moon to go from perigee to perigee (or apogee to apogee) . This "month" is about 27.555 days. The "tropical" month is the time it takes the Moon to go from two passes of the same ecliptic longitude (about 27.322 days-about the same as the sidereal month). The "draconic month" is the time it takes the Moon to go from one ascending mode to another (or from one descending mode to another). See below for more on this. If this confuses you, as it should, look into the Wikipedia article on 'Orbit of the Moon.

But, you say, after all that yabba yabba you still haven't answered the question, "Why isn't there an eclipse of the Moon every month when the Moon is on the opposite side of the Earth from the Sun?". The answer is that the Moon doesn't revolve about the Earth on 'exactly' the same plane as the orbit of the Earth about the Sun- the ecliptic. It is inclined about 5 degrees away from this plane. See the diagram above. This means that the path of the Moon is such that it crosses the ecliptic only about twice a year at points called the "nodes". The "ascending node" is when it crosses the ecliptic in the "up" direction and the "descending node" is when it crosses this path in the "down" direction. Only when the Moon is at or near one of its nodes is it in direct line with the Earth/Sun, and only then can there be an eclipse. How close it is to an exact nodal position determines the type of eclipse that can be seen.
THE TYPES OF LUNAR ECLIPSES:

Lunar eclipses are divided into three basic "types", 'penumbral', 'partial' and 'total'. A total eclipse has three phases that include partial and penumbral along with the period of totality. The Moon will actually only be covered by a full shadow when it crosses the 'umbra' of the Earth. The 'penumbra' is an area of lesser illumination that still receives some direct sunlight. When the Moon experiences a "penumbral eclipse" the difference in illumination can be quite difficult to perceive with the naked eye. Some argue that the surface of the Moon nearest the umbra seems a bit darker when there is a "total penumbral eclipse", a rare event because the penumbra is narrow as compared to the umbra. Unlike solar eclipses where the area of totality is quite narrow, at the best, about 250 kilometers wide and lasting only 7 minutes, 40 seconds, a total lunar eclipse can be viewed anywhere on Earth where the Moon is above the horizon for the duration of the eclipse , and the totality phase of the eclipse may last up to 107 minutes (one hour,47 minutes). A total lunar eclipse is when the Earth's shadow totally obscures the Moon. A partial lunar eclipse is when only a portion of the Moon is covered by the shadow. A penumbral eclipse is when the Moon enters the area of only partial shadow. Previous posts on this blog have given times for when the August 28th eclipse will enter and leave these phases of the eclipse (see below). When the Moon is in total eclipse its luminosity may be only 1/10,000 of its regular brilliance. The actual darkening of the Moon is measured by a scale called the 'Danjon Scale' (also see below). There are always at least two, and sometimes more, lunar eclipses per year, but not all will be total eclipses. This year has two total eclipses. 2008 will have one partial and one total eclipse. In 2009 there will be 3 penumbral eclipses and only one partial eclipse and so on. See the SunEarth page from NASA for more information. Thus we are quite privileged to see two total lunar eclipses this year. This won't happen again until 2011. The next total eclipse will be February 21st, 2008, and it will be visible not just in the America but also in Europe, Africa and the Central Pacific.

PREVIOUS POSTS ON THIS BLOG ABOUT THE LUNAR ECLIPSE:

AUGUST 24: 'DOUBLE MOONS AND DOUBLE ECLIPSES:THE REST OF THE STORY'

AUGUST 24:'WHAT'S ALL THIS 'DOUBLE MOON STUFF ?'

AUGUST 21: 'WHEN WILL IT OCCUR ?'

AUGUST 18: 'HOW DARK WILL IT BE ?:THE DANJON SCALE'

AUGUST 16: 'WILL YOU BE ABLE TO SEE IT?'

Also on Molly's Blog the following posts were put up during the last Lunar eclipse on March 3rd:

FEB 16:'LUNAR ECLIPSE, MARCH 3RD'

MARCH 2: 'LUNAR ECLIPSE FACTOIDS'

MARCH 2: 'MORE ON TOMORROW'S LUNAR ECLIPSE'

MARCH 3: 'THE MOON AND THE LAST MINUTE'