Total Lunar Eclipse
By Dr Noorali T Jiwaji
The total lunar eclipse at dawn on Thursday 21st February 2008 is unique in many ways. It is the last such eclipse for more than two years, as the next total lunar eclipse will not occur until December 2010. More important, it is a two-in-one event in which you will even be able to even understand why the eclipses occur.
On eclipse day at dawn, as the sun rises in the east at 6:30 am, you will be able to see the fully eclipsed moon set in the west. Go out on the terrace or on open ground, having rubbed the sleep out of your eyes. Now as you look at the rising sun on your right, imagine a 1.5 million kilometer huge orb boiling with nuclear fire rising in the east. This huge glowy ball is 150 million kilometers away from you on your right.
When you look left, you will see an orangey glow of the eclipsed moon sinking under the horizon. The moon is just a 3,500 kilometer ball of sand and stone and is only 300,000 kilometers away to your left. With yourself (i.e. the earth) right in the middle, you can now picture the dazzling sun casting a huge shadow (of earth) onto the tiny moon, covering it up completely and causing the eclipse as shown in the diagram below (courtesy Fred Espenak, 2000, http://www.mreclipse.com/Special/LEprimer.html).
Lunar eclipses occur only at Full Moon because that is the only time when the positions of the sun, earth and moon can be aligned in an exact straight line for the earth’s shadow to fall on the moon.
At dawn on 21st, the moon first enters the penumbra (see above diagram) at 3:35 am. This is a very light shadow so we cannot notice any change in appearance of the moon. However at 4:43 am the moon enters the umbra which is a dark shadow and a crescent shape from the moon will be seen to be eaten up. By 6:00 am the moon is entirely in the umbral shadow which will cover it up fully.
Due to contrast with the bright reflected light from the moon, the shadow looks dark when it is partially covered. However, once the bright moon is completely covered, the shadow suddenly takes on a vivid glow. Usually this is red coppery coloured due to light that has filtered through the earth’s atmosphere and refracted into the umbral part of the shadow. The dark golden colour indicates that the atmosphere does not contain pollutants. However, if there is any significant dust, smoke or other particles in the earth’s atmosphere, the moon can take a ghostly grey glow. For us, since the total eclipse occurs so close to sunrise, you can expect the colour to be less strong in the dawn sky. But the sky in the west should be dark enough for you to enjoy the colourful sight.
Observers in the coastal areas will not be able to watch the rest of the eclipse. But those in the western border areas such as Kigoma will be able to see the end of the total phase of the eclipse at 6:52. After that the moon will begin to move into the penumbra and a bright crescent will grow until the whole moon is uncovered by 8:10 am. We in Tanzania will obviously not see this last part because the sun will blanket the sky with bright light. The whole eclipse finally ends at 10:18 while we are all busy with the day’s duties.
Lunar eclipses are completely safe to observe since the light that comes off the moon is reflected light. Take time to view this eclipse even if you have to lose some sleep. It will help you understand what is happening in parts of our Universe that you can never reach. The next time you get this chance you will have grown three years older.
Dr N T Jiwaji
ntjiwaji@yahoo.com
17th February 2008