April 25 Deep Lunar Penumbral Eclipse - A Perfect Fit

A Perfect Penumbral Eclipse on April 25

Time in Tanzania - Full eclipse from 09:03 pm to 01:12 am (after midnight of 26 April)

The first eclipse of 2013 year will occur on April 25 at night, during Full Moon, between 9 pm and 1 am.  It is a lunar eclipse, in which the faint shadow of our Earth will cover the whole Moon. Three celestial bodies, the Sun, our Earth and our Moon will be in exact line.  The Earth will be in between the Sun and the Moon, and the Earth’s huge shadow will fall on the Moon. 

Lunar eclipses can easily be seen when the main dark shadow, called the umbra, covers the whole Moon.  However, on Thursday April 25, the whole Moon will be in the faint shadow, called penumbra.  A slight edge of on the north side will be covered by the umbra (i.e. the dark shadow) from 10:54 pm to 11:21 pm.  In the eclipse diagram, the light blue area is the dark shadow (umbra), while the yellow area on the edges of the umbra is the light shadow region called the penumbra.

The shadows that we normally see are dark shadows that form when the light from the Sun or any other light is obstructed by an object.  However, around the edge of such dark shadow you may have seen a very thin band of faint shadow, especially for objects that have sharp edges such as a line or a circle.

Penumbra occur when the light source is large.  When light from one end is blocked by an object, the light from the other end will not be blocked. So penmbras are shadow areas into which some light has been able to penetrate.  A small (point) source can only have a dark umbra. 

At the distance of our Moon, which is more than 300,000 kilometers away, the Earth creates a huge shadow.  So the band of the penumbra is also very wide.  During the Thursday April 25 eclipse, the whole Moon will able to fit perfectly inside the exact width of the penumbra.  Hence we have called it a perfect penumbral eclipse.  Such an exact fit is very rare. See diagram of Earth’s shadow with penumbra band surrounded by the umbra (dark central area).  You will see the size of the Moon is almost exactly the same size as the penumbra and the two have a perfect fit. e Moon is almost exactly the same size as the penumbra and the two have a perfect fit.

Faint penumbral shadows are normally difficult to see, but since the whole Moon will be deep inside the penumbra we expect to see a gradual but significant darkening of the Moon between 9 pm and 1 am.  You will need to compare the brightness of the moon from its Full Moon brightness before and after the eclipse.  Within this look carefully at the northern edge from 10:54 pm to 11:21 pm to see the edge of the Moon being cut off as it enters the umbra of the Earth’s shadow.

Note that the Moon moves west to east through the Earth’s shadow as the eclipse progresses.  This is the natural movement in our sky for all celestial bodies due to anticlockwise rotation of our solar system.  This is opposite to what we are used to seeing in the sky daily; for example the Sun’s movement from east to west or the movement of stars during hour by hour as the night progresses.  The opposite direction comes about because we are looking at the sky from the Earth, which is itself rotating in the natural direction from west to east.  Hence celestial bodies will appear to move in the opposite direction, from east to west.

The eclipse is well centered above Africa as seen in the map, so we are perfectly positioned to see the full eclipse during suitable hours from 9 pm to past midnight.  Make sure you take the challenge of looking for a distinct decrease in brightness during this rare perfect penumbral eclipse.  It will also be interesting to a thin edge of the Moon being cut off beginning just before 11 pm up to 11:20 pm.