Spooky Eclipse is a Bit Different This Week

By Gary A. Becker

StarWatch 895 for the week of Oct. 13

The East Coast is set for a different kind of lunar eclipse early on the evening of Oct. 18.  
If you go outdoors around 7:50 p.m. EDT and look eastward at the full moon low in the sky, you may notice that the area around the five o’clock position on the lunar disk, may not look quite as bright as it should.  
Repeating the same observation an hour later should show that this same region has brightened considerably. What you will have observed is a penumbral lunar eclipse. 
A lunar eclipse can only happen when the moon is full, in other words, opposite to the location of the sun, and in the same direction as the Earth’s shadow. But as everyone knows, lunar eclipses do not occur every month, and that is because the moon’s orbit is tilted just over five degrees to the Earth’s orbital path.  
In order for a lunar eclipse to take place, the moon must be in its full phase, and its orbit must also be crossing the plane of the Earth’s orbit so that the moon can intersect the shadow of the Earth.  The true shadow or umbra of the Earth at the moon’s distance is about 5,700 miles in diameter, but surrounding it is a larger secondary shadow called the penumbra. It's a region where—if you were on the moon looking back at the Earth—you would observe part of the Earth covering part of the sun. When the moon slips into this region, it does not receive a full blast of sunlight.  
The portion of the moon deepest into the penumbra receives the least amount of illumination and takes on a dusky appearance. In this eclipse, no astronaut anywhere on the moon would observe the Earth completely covering the sun, so the primary shadow of Earth, the umbra, would never touch the moon’s surface.  
This week's will be an eclipse of comparisons. Observe the moon when it is deepest into the penumbra at 7:51 p.m., then observe it an hour later and compare.  My guess is you will see this slight difference, an ephemeral lunar eclipse.