October 7 – This Little Light Of Ours

Tomorrow’s factismal: Tomorrow morning people on the East Coast of North America will be able to see both the Sun and the lunar eclipse at the same time!

Eclipses are one of the wonders of our world. We are fortunate that the Moon is just the right size and just far away to be the same apparent size as the Sun in the sky. And we are even more fortunate that the Moon’s orbit brings it into almost the right position to fall into the Earth’s shadow once a month. (Two weeks before or after, the Moon returns the favor by bringing its shadow to the Earth’s surface.) And the most fortunate thing of all is that the Earth has an atmosphere because that is what will make tomorrow morning’s eclipse so special.

The Earth-Moon system, draw to scale. Notice how the Moon's orbit is slightly elliptic.

The Earth-Moon system, draw to scale. Notice how the Moon’s orbit is slightly elliptic.

You see, the Moon’s orbit isn’t perfectly circular and it isn’t perfectly aligned. If the Moon’s orbit were perfectly circular and if the Moon orbited in the same plane as the Earth, then we would have a lunar eclipse every month on the full moon and a solar eclipse two weeks later on the new moon. But the Moon moves slightly closer to the Earth (a mere 225,622 mi) and slightly farther away (a whopping 252,088 mi). Because the Moon is sometimes closer to the Earth and sometimes farther away, sometimes it is close enough to be in the Earth’s shadow and sometimes it isn’t. And because the Moon’s orbit is slightly inclined, sometimes it is above the Earth’s shadow and sometimes it is below it and only every once in a while is it in exactly the right place to be in eclipse. (This is also why solar eclipses are sometimes total and sometimes just partial.)

This month is not one of those times when the Moon is in exactly the right place; instead, the Moon is just slightly too far away to be in the Earth’s shadow. So, if the Earth didn’t have an atmosphere, then there would be no eclipse this month. Instead, the Moon would stay nice and bright all night long. Luckily for us, the Earth does have an atmosphere which means that we will have a breath-taking eclipse tonight. You see, that atmosphere bends some of the Sun’s light around the Earth, creating a second, larger shadow called a penumbra (from the Latin for “almost dark”; the penumbra also owes its existence to the fact that the Sun is a big, fat star and not a tiny pinpoint of light). Like the name says, this shadow isn’t completely dark. Instead, it has the light of every sunrise and every sunset on Earth shining through it which is why the Moon turns a deep, dark red color when it passes through the penumbra.

And the Earth’s atmosphere will do us one more favor tomorrow morning. Because the atmosphere refracts light, it allows us to see things that are slightly below the horizon. That’s why the Sun takes on such a funny, squashed shape as it rises and sets. And it is also why we will be able to see the Sun rising tomorrow as the full Moon sets in eclipse. The technical term for this is a selenelion (“selene” {Moon} plus “helion” {Sun}) eclipse and it happens in about one out of every forty eclipses.

The eclipse will be visible over most of North America (Image courtesy NASA)

The eclipse will be visible over most of North America
(Image courtesy NASA)

The sunrise and moonset will be most visible on North America’s East coast from 4:15 AM (when the Moon starts to go into the Earth’s penumbra) until 7:25 AM (when the Sun rises);  though folks in the Central and Western parts of the US won’t get the selenelion effect (I told you it was rare!), they will still be able to see the blood red lunar eclipse which is still pretty darn cool. So get out of bed early tomorrow and watch one of the rarest and most wonderful things that you’ll ever see!

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