Tonight (March 13) we will have the opportunity to view another eclipse. A lunar eclipse, which simply means the moon will pass through the shadow of the Earth. It begins with what is called the penumbral phase at 10:57 P.M.

A shadow has two areas. The darkest area in the middle is called the “umbra” and around that dark area is a lighter area known as the “penumbra”. When the moon is in the penumbra portion of the Earth’s shadow, it grows dimmer, but it isn’t hidden at all until it begins to enter the umbra.

Thursday the moon will enter into the umbra and be completely dark about 1:26 A.M. and it will remain that way until 2:31 A.M. This is known as “totality”.

The moon then emerges into the penumbra becoming more and more visible until the end of the eclipse at 5:00 A.M. Friday morning.

Once completely inside the umbra the moon isn’t completely invisible. It can take on a red appearance.

This is because of “earthshine”. Normally we see the moon because the sun’s rays are hitting it and we see the light reflected off its surface.

The same thing happens on the moon. If you were on the moon you would see the earth because the sun’s rays are hitting the earth and are then reflected back to the moon. Most of the wavelengths of light bouncing off the earth are scattered by our atmosphere. The red wavelengths, being longer, are not as scattered and thus the red light reaches the moon. If the sun is not shining directly on the moon, for example during a total lunar eclipse, the only light striking the moon is the red light from earthshine. This makes the moon look red and it is called a “Blood Moon”.

People have long noticed eclipses, as they are kind of hard not to notice. The oldest record of an eclipse comes from Ireland in rock paintings believed to be 5,340 years old. Sumerian clay tablets from 4,300 years ago document a lunar eclipse. In 746 B.C. an eclipse was documented in Babylon.

Possibly the most famous of all lunar eclipses occurred on Feb. 29, 1504. Christopher Columbus was on the island we now call Jamaica and the locals were tired of supplying his men with food and other supplies. They said “enough”. All ship captains carried a book called an Ephemerides, which gives locations of stars to help with navigation. It also predicts eclipses.

From the book Columbus knew a lunar eclipse was coming. He told the locals that if they didn’t do as Columbus commanded, he would ask the gods to take the moon away. He then went into his tent to wait.

Sure enough the moon started to disappear as it entered the earth’s shadow. The locals quickly agreed to Columbus’ demands if only Columbus would bring the moon back. From the Ephemerides Columbus knew that totality for the eclipse was going to be 47 minutes long, so he played it out and after 45 minutes stepped out and agreed to bring the moon back.

As we look back over the 521 years since that eclipse, we realize that people back then were every bit as intelligent as we are. I’m sure had they wanted to, astronomers in 1504 could have predicted tonight’s eclipse and that totality would last just over an hour!

Enjoy the eclipse.