When the Earth takes a bite out of the Sun

Bad Astronomy
The entire universe in blog form
April 1 2011 7:00 AM

When the Earth takes a bite out of the Sun

In a week of ridiculously gorgeous astronomy pictures hitting the 'net, I keep thinking they can't get cooler... and then this happens: a seriously cool picture of the Sun from NASA's Solar Dynamics Observatory!

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  


Yegads! [Click to solareclipsenate.]

Given that SDO orbits the Earth and sees the Sun from space, why is the bottom half of the Sun gone in this picture? It's because we're seeing a solar eclipse which is actually more like a lunar eclipse except the Moon is not involved.

Hmm, yeah, maybe I'd better explain. SDO circles the Earth in an inclined orbit*. If the orbit were directly above the Earth's equator, the Earth would block the Sun once per day, and that's not so cool for a satellite designed to continuously observe our nearest star! So the orbit was inclined a bit, giving SDO an unobstructed view of the Sun... kinda. The orbit of SDO is inclined to maximize the viewing time for the Sun and to maintain a continuous downlink for its very large data stream (it sends about 15 megabytes of data to Earth every second!). Because of the way the orbits work out, twice a year there is a period where the orbit of the Earth around the Sun and SDO around the Earth line up, and for brief times the Earth does block the Sun from SDO's view. These are called eclipse seasons, and we're currently in one right now.

This image, taken on March 29, 2011, shows what happens: the bottom half of the Sun is gone is because the Earth is in the way! Our atmosphere blurs the edge a bit in this ultraviolet image, giving the dark part a rough edge. From SDO's position, the Earth appears far larger than the Sun, which is why the dividing line is almost straight.

The curlicue you can see apparently poking into the Earth on the left is interesting. When I saw it I figured it's probably a brighter feature that happens to be brilliant enough to still be visible through the Earth's UV-opaque air... and a quick search of the SDO image archive shows that's exactly what it was. The picture inset is of the full disk of the Sun, shortly before the Earth partially blocked the view. You can see the prominences off the Sun's limb, and the twisty arcs of material seen against the Sun's disk, including the curly one on the left. If we could see this from the side, it would resemble the prominences seen on the limb.

So what do we call this kind of event? When the Moon passes between the Earth and the Sun we call it a solar eclipse (the Sun's light is being blocked). When the Earth passes between the Moon and the Sun we call it a lunar eclipse (the Moon's light is blocked). In this case, the Sun's light is being blocked like in a solar eclipse, but it's being done by the Earth, like in a lunar eclipse. So technically I'd call this a solar eclipse, just not one caused by the Moon.

Got that? Yeah, I don't think it matters much what we call it either. It's pretty nifty either way.

* It's also a geosynchronous orbit, meaning SDO orbits the Earth once every 24 hours, the time it takes the Earth to rotate once. From the ground, it appears that SDO hovers in one general spot in the sky instead of moving rapidly across it like low-Earth orbit satellites do.

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