By Lance Ulanoff From Mashable

The moon is about to block our view of the sun. It’s known as a total solar eclipse and on Friday millions of people in Europe, Northern Africa and Asia will have a rare opportunity to see, indirectly of course, the sun’s brilliant corona. If you’re gearing up to enjoy it, though, there are some things you should know.

Yes, a total solar eclipse is rare, in that it only appears for a short period of time, for a relatively narrow band of terrestrial viewers once every few years (and in the same place once every few hundred years), but eclipses in general are relatively common.

“Essentially you have eclipses, both solar and lunar… partial or full, twice a year, and that’s because the moon is orbiting Earth and we’re orbiting the sun,” said Summer Ash, director of outreach for Columbia University’s Astronomy Department, in an interview. That twice-yearly alignment of the Earth, moon and sun is not always a perfectly straight line, Ash explains, which is why most of the eclipses are partial.

A solar eclipse is basically the result of a very large body –- the moon — moving between you and the sun. Instead of the sun’s bright light, you see only the moon’s shadow. It gets so dark that the sky looks like dusk. One might assume that the whole world is cast into darkness, but the reality is that solar eclipses only affect part of the Earth, and not all at the same time.

If you’re experiencing a total eclipse of the sun, then you are standing in what’s known as the moon’s umbra, a sort of sharply cast and extremely dark shadow that sits at the center of the moon’s larger shadow. It gets smaller as it heads toward Earth, which means that only a very small section of those experiencing an eclipse will get the full experience.

Friday’s solar eclipse, for example, will be visible between 8 a.m. and 10 a.m. GMT throughout Europe, northern Africa, northern Asia and Greenland. North and South America as well as Australia will miss it entirely.

There is a larger and more diffuse shadow around the umbra called, naturally, the penumbra. This shadow grows larger on the way to Earth and anyone who sees or experiences it is in a partial solar eclipse.

There is another factor in today’s eclipse, though, that makes it noteworthy.

Supermoon
A moon that appears unusually large in the night sky is both a real thing and an illusion. When the moon is low on the horizon, it can appear quite large, but it’s really just our eyes and brains playing tricks on us.

However, a supermoon is actually that moment when the moon orbits more closely to the Earth and is full. So the moon looks somewhat bigger because it really is closer.

As it happens, a supermoon does correlate to a total solar eclipse.

Since the moon usually orbits further away from the Earth, even when it passes between the Earth and the sun, it doesn’t entirely block the sun’s light and leaves a bright ring around it. A supermoon, Ash says, puts the moon closer to the Earth, allowing it to completely block the sun so only the sun’s corona, or plasma ring, is visible.

In other words, a supermoon is a requirement for a total solar eclipse where the corona is visible. On the other hand, those in the eclipse path won’t actually see the supermoon, at least not how they normally would.

“ People seeing the supermoon will be seeing the supermoon in shadow People seeing the supermoon will be seeing the supermoon in shadow,” said Columbia University’s Ash.

Sadly, those not seeing the eclipse won’t even see the supermoon because, in the U.S., at least, it will be a new moon, which is not illuminated by the sun at all.

Ash does have some good news for U.S. sky watchers feeling a little celestially left out. “I’m getting more excited by the [total eclipse] in 2017 that will go straight across the continental U.S. I’m saving my energy for that.”

As for Friday’s third celestial element, the vernal equinox, that’s a bit of a red herring. The vernal, or spring equinox occurs when the sun crosses the plane of the equator and day and night are of roughly equal length. It is not, however, related to the total solar eclipse or the supermoon. “It doesn’t really have an impact,” said Ash.

Even without the vernal equinox’s help, Friday’s total solar eclipse will be a spectacular event for millions of people watching from the ground and those tracking it online.

“It’s always cool when you see something go in front of the sun. It’s an amazing reminder that we’re living on this rock orbiting this big ball of fire and another rock can occasionally block our view,” Ash said.

Note: Last image. This sketch by Spanish astronomer Jose Joaquin de Ferrer, depicts the solar atmosphere, or corona, during a June 16, 1806, total solar eclipse. Before astronomical photography, observers depended on sketches of eclipses to study the sun’s corona. – IMAGE: NASA/JOSE JOAQUIN DE FERRER

Top image. On Jan. 30, 2014, beginning at 8:31 a.m EST, the moon moved between the NASA Solar Dynamics Observatory, or SDO, and the sun, giving the observatory a view of a partial solar eclipse from space. – NASA Image

Second image. This illustration by Gary Osborn shows the orbit of the moon around the earth and how its tilted orbit impacts when and where it passes between the earth sun. MAGE: GARY OSBORN

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