Lunar Eclipse

Just over a week ago we got to experience an amazing sight in Tanzania, which was a total Lunar Eclipse, and I hope most of you got to see this amazing phenomena. The eclipse happened on Wednesday the 15^th of June 2011, it was the longest and darkest lunar eclipse of the century and lasted for almost a full 100 minutes.

Lunar Eclipse, 15 June 2011 as seen from Arusha. You can see have the moon in shadow.

The lunar eclipse could be seen not just from Tanzania and Africa but people in Europe, South America, Asia, and Australia got to enjoy the sight.

What is a Lunar Eclipse and why does it happen? A lunar eclipse takes place when the moon passes behind the earth, and the earth blocks the sun’s rays from reaching the moon. This kind of eclipse however can only happen if the Sun, Earth and moon are exactly aligned or very close to exactly in line, and the Earth needs to be in the middle. The time the eclipse will take place or how long it will be visible for depends on where the moon is in relation to its orbital nodes.

A lunar eclipse is great as it can be seen from almost anywhere on the right side of the earth, where as a solar eclipse as impressive as it is can only be seen from a relatively small part of the world and generally only lasts a few minutes.

Types of Lunar Eclipse: (credit, and taken from Wikipedia, The Free encyclopedia)

The shadow of the Earth can be divided into two distinctive parts: the umbra and penumbra. Within the umbra, there is no direct solar radiation. However, as a result of the Sun’s large angular size, solar illumination is only partially blocked in the outer portion of the Earth’s shadow, which is given the name penumbra. A penumbral eclipse occurs when the Moon passes through the Earth’s penumbra. The penumbra causes a subtle darkening of the Moon's surface. A special type of penumbral eclipse is a total penumbral eclipse, during which the Moon lies exclusively within the Earth’s penumbra. Total penumbral eclipses are rare, and when these occur, that portion of the Moon which is closest to the umbra can appear somewhat darker than the rest of the Moon.

Lunar Eclipse as seen from Arusha on 15 June 2011, here you can see the moon approaching full eclipse

A partial lunar eclipse occurs when only a portion of the Moon enters the umbra. When the Moon travels completely into the Earth’s umbra, one observes a total lunar eclipse. The Moon’s speed through the shadow is about one kilometer per second (2,300 mph), and totality may last up to nearly 107 minutes. Nevertheless, the total time between the Moon’s first and last contact with the shadow is much longer, and could last up to 4 hours. The relative distance of the Moon from the Earth at the time of an eclipse can affect the eclipse’s duration. In particular, when the Moon is near its apogee, the farthest point from the Earth in its orbit, its orbital speed is the slowest. The diameter of the umbra does not decrease appreciably within the changes in the orbital distance of the moon. Thus, a totally eclipsed Moon occurring near apogee will lengthen the duration of totality.

The timing of total lunar eclipses are determined by its contacts:

P1 (First contact): Beginning of the penumbral eclipse. The Earth's penumbra makes contact with the outer limb of the Moon.

U1 (Second contact): Beginning of the partial eclipse. The Earth's umbra touches the outer limb of the Moon.

U2

Greatest eclipse: The peak stage of the total eclipse. The Moon is at its closest to the center of the umbra.

U3 (Fourth contact): End of the total eclipse. The Moon's outer limb exits Earth's umbra.

U4 (Fifth contact): End of the partial eclipse. The Earth's umbra leaves the surface of the Moon.

P2 (Sixth contact): End of the penumbral eclipse. The Earth’s shadow no longer makes any contact with the Moon.

A selenelion or selenehelion occurs when both the Sun and the eclipsed Moon can be observed at the same time. This can only happen just before sunset or just after sunrise, and both bodies will appear just above the horizon at nearly opposite points in the sky. This arrangement has led to the phenomenon being referred to as a horizontal eclipse. It happens during every lunar eclipse at all those places on the Earth where it is sunrise or sunset at the time. Indeed, the reddened light that reaches the Moon comes from all the simultaneous sunrises and sunsets on the Earth. Although the Moon is in the Earth’s geometrical shadow, the Sun and the eclipsed Moon can appear in the sky at the same time because the refraction of light through the Earth’s atmosphere causes objects near the horizon to appear higher in the sky than their true geometric position.

The Moon does not completely disappear as it passes through the umbra because of the refraction of sunlight by the Earth’s atmosphere into the shadow cone; if the Earth had no atmosphere, the Moon would be completely dark during an eclipse. The red coloring arises because sunlight reaching the Moon must pass through a long and dense layer of the Earth’s atmosphere, where it is scattered. Shorter wavelengths are more likely to be scattered by the air molecules and the small particles, and so by the time the light has passed through the atmosphere, the longer wavelengths dominate. This resulting light we perceive as red. This is the same effect that causes sunsets and sunrises to turn the sky a reddish color; an alternative way of considering the problem is to realize that, as viewed from the Moon, the Sun would appear to be setting (or rising) behind the Earth.

The amount of refracted light depends on the amount of dust or clouds in the atmosphere; this also controls how much light is scattered. In general, the dustier the atmosphere, the more that other wavelengths of light will be removed (compared to red light), leaving the resulting light a deeper red color. This causes the resulting coppery-red hue of the Moon to vary from one eclipse to the next. Volcanoes are notable for expelling large quantities of dust into the atmosphere, and a large eruption shortly before an eclipse can have a large effect on the resulting color.

Lunar Eclipse as seen from Arusha on 15 June 2011, here you can see the moon now in full eclipse, and later it went even darker.

So there you have it, as word goes the next full Lunar Eclipse will be on the 10 December 2011, so if you missed the one on the 15 June, then be sure to try and see the next one.