Deep Penumbral Lunar Eclipse

Deep Penumbral Lunar Eclipse

 This Friday midnight, going into Saturday 19, we will experience a Lunar eclipse when the Moon will enter the light shadow of Earth, more than 300,000 km in space.  This is the distance that Moon orbits Earth, so the shadow has to extend into deep space, 25 times the diameter of our planet and reaches the Moon.

As seen in the diagram of the geometry of  the eclipse, a lunar eclipse can occur only when Sun, Earth and Moon are aligned in an almost exact straight line, with Earth between the Sun and Moon.  During this alignment, the Moon is in Full Moon phase because sunlight strikes the full face of the Moon and we see it as a full, brightly lit circle.  If the alignment is exactly straight, a total eclipse occurs when the dark, umbral shadow of Earth covers the whole Moon darkening the bright face of the Full Moon.

However, as happened  on  April 25 this  year, the Moon is just slightly misaligned from exact straight line, so only a penumbral eclipse will occur this Friday/Saturday.  However, while the  April 25  event saw 100 percent of the Moon covered in light penumbral shadow, this weekend, 76 percent (three quarters)  of  the Moon will be immersed in the faint penumbral shadow, producing a deep purely penumbral eclipse. 

This event will be interesting to follow as a challenge to see if your eyes can detect the slight decrease in the brightness of the Full Moon, just after midnight of Friday October 18 going into Saturday October 19.

This weekend, opposite to what happened during the April 25 penumbral eclipse, the Moon moves through the northern part of Earth's shadow, so the Moon's southern edge will be darkened while the its northern edge will be completely free from shadow.

The diagram shows the movement in the sky, of the Moon through the Earth's shadow during the eclipse.

Faint penumbral shadows are normally difficult to see, but since a large part (75%) of the Moon will be deep inside the penumbra we expect to see a gradual but significant darkening of the Moon between 1 am and 5 am, with maximum effect around 3 am.  You will need to give up on your beauty sleep that night, but the Saturday timing will be attractive to weekend revelers.

The best way to detect difference in the brightness of the Moon is to mark in your mind the brightness of the Full Moon just before the eclipse begins and to come out to watch the Moon again near full eclipse at around 3 am.  Maximum eclipse occurs at 02:50 am.  I will be tweeting the progress of the eclipse from @ntjiwaji and you can get more viewing  details at this  website.

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 east to west movement of celestial objects that we are used to (e.g. daily movement of Sun  from east to west from morning to evening) 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.

This October penumbral eclipse is also well centered above Africa, as was the April 25 penumbral lunar eclipse, so we are perfectly positioned to see the full eclipse.

This eclipse is a reminder of the feast that awaits us on November 3 when a total solar eclipse will occur.  In  Tanzania we will see 70 percent  of the Sun covered up by the Moon.  During the two weeks after the October 19 eclipse the Moon will have shifted to the next alignment position when it will be between the Earth and the Sun.  At this  time the phase of the Moon is New and only the dark side of the Moon faces us and so is not seen.  On November 3, this  dark face of the Moon is what will be hiding the Sun from our view.  The total eclipse passes through northern Uganda and Kenya where the Sun will be fully covered up for about a minute.  To prepare yourself to watch this eclipse, look up this the Astronomy in Tanzania website