By Jim Haklar
I love eclipses. I mean, I really love eclipses! I love eclipses so much that two years ago, I flew to Albuquerque for the weekend to see a solar eclipse. But more about that later…
An eclipse happens when the Earth, Moon and Sun all line up. Technically, this is called syzygy (try to form that word in a game of Scrabble). A lunar eclipse happens when the earth is between the Moon and the Sun. When the Moon is between the Sun and the Earth you get a solar eclipse. Lunar eclipses happen when the Moon is full and solar eclipses occur when the Moon is new. So, why don’t we have an eclipse twice a month? Well, the Moon’s orbit is tilted by a little over five degrees. So most of the time there isn’t perfect alignment and you don’t have syzygy.
From any given location on the Earth, lunar eclipses are more frequently seen than solar eclipses. That’s because the Earth casts a bigger shadow on the Moon than the Moon does on the Earth. The shadows consist of two parts. There is a smaller, darker umbra surrounded by the lighter penumbra. If you are in a location where the Moon’s umbra passes through, you will see a total solar eclipse. Otherwise the solar eclipse will be partial (since you will be in the penumbra). For lunar eclipses, the situation is a little different. The Moon can completely or partially pass through the Earth’s umbra (resulting in a total or partial lunar eclipse) or just pass though the penumbra (called a penumbral eclipse).
Now for my Albuquerque story. Two years ago the Moon’s umbra passed directly in front of the Sun and this was visible in many cities including Albuquerque (I went to Albuquerque because of the clear weather). But since the Moon was at a point in its orbit when it was farther away from the Earth, it didn’t completely block out the Sun. Instead, an annulus or ring of light from the Sun’s disk encircled the Moon. This is called an annular solar eclipse.
On August 21, 2017 there will be the first a total solar eclipse visible from the contiguous United States in over 30 years. Information on the best places to see the eclipse is already on the Web, so start think about taking an eclipse vacation!
About the Author: Jim is an environmental engineer at the EPA’s Edison, New Jersey Environmental Center. In his 28 years with the agency he has worked in a variety of programs including Superfund, Water Management, Public Affairs, and Toxic Substances. He has been an amateur astronomer since he was a teenager, and can often be found after work in the back of the Edison facility with his telescope.