Darkest Total Lunar Eclipse 15 June 2011 Wednesday

 By Dr. Noorali Jiwaji

ntjiwaji@yahoo.com

On Wednesday night, 15^th June 2011, Tanzania experienced the darkest total lunar eclipse of this century.  The last such eclipse took place 40 years ago on 6^th August 1971. The Moon  passed  right through the centre of the Earth’s shadow, which is the deepest part of the shadow.

The dark (umbral) part of the eclipse  started  at 9:23 pm on June 15^th and ended just past midnight at 01:03 am, of 16^th June. 

It was  a Full Moon Night, with th e Moon rising at 6:03 pm 4 minutes before sunset.  Three hours later, at 9:22 pm the eastern edge of the Moon’s bright disk began to be eaten away as the Moon entered  the Earth’s shadow. This was  the partial phase of the eclipse.  After one hour at 10:22 pm, the whole Moon  was  immersed in shadow and the total lunar eclipse began.   The Moon passed diametrically through the Earth’s shadow so by mid eclipse at 11:02 pm it  was the darkest part of the shadow causing it to be the darkest eclipse of this century and not seen for the past 40 years.

The full disk of Moon was covered fully in shadow until 12:03 am, just after midnight, when the eastern edge began to come out of the dark shadow  was  lighted up.  This went  on for another hour until 1:02 am when the Moon was completely out of the dark and the dark part of the eclipse ends.

The whole eclipse normally includes its lighter (penumbral) part which is not visible to the eyes.  This begins an hour earlier than the dark part and ends an hour later after the dark part has ended.  So the full duration of the eclipse was from 8:24 pm to 2:01 am.

Eclipses occur when the sun, earth and moon are exactly aligned in a straight line so that shadow of one body can fall on the other.  Solar eclipses occur when moon’s tiny shadow falls on a small part of our huge earth.  What will happen on June 15 however is a lunareclipse, when the earth’s huge shadow will cover up completely the much smaller moon (see diagram for lunar eclipse).  During a completely total eclipse the whole moon is in the earth’s shadow while if the whole moon is not completely within the shadow, we say that the lunar eclipse is partial.  So a even this lunar eclipse will start with a partial phase as the moon slowly enters the dark portion, and also ends with a partial stage as the moon moves slowly moves out to the dark umbral part.

During a total lunar eclipse the moon does not become completely dark and disappear from view.  In fact some light enters even the darkest parts of the shadow due to bending (refraction) of sunlight by the earth’s atmosphere.  Our eyes also adapt to the darkness and the low intensity of light in the shadow, by widening (dilating) the pupils of our eyes.  So the totally eclipsed moon takes a ghostly image of a huge ball floating in the sky, a sight worth witnessing.

Pollutants such as smoke from forest fires, dust from volcanoes, and emissions from man made sources such as industries and other human activities that enter the earth's atmosphere influence the light passing through it.  Some of this light still enters the dark part of the earth’s shadow that falls on the moon.  In the outer part of the dark shadow called the penumbra (see lunar eclipse diagram), a lot of this light passing through the earth’s atmosphere leaks into the shadow on the moon.  So the colour and intensity of the totally eclipsed moon between 4:06 a.m. and 4:31 a.m. is expected to vary widely across the whole face of the moon.  A clear atmosphere makes the moon look orangey while dust and smoke will make the moon appear dirty grey.  This is your chance to observe the effect of pollutants in our atmosphere.