When the Earth takes a bite out of the Sun

By Phil Plait | April 1, 2011 7:00 am

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!

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.

Related posts:

An eclipse from space with a two-way Moon
INSANELY awesome solar eclipse picture
SDO lunar transit, now with video
What does a lunar eclipse look like from the Moon?

CATEGORIZED UNDER: Astronomy, Cool stuff, Pretty pictures
MORE ABOUT: Earth, eclipse, Moon, SDO, Sun

Comments (36)

  1. Jack M.

    *checks the date*

    Are you sure about this, Phil? :-p

  2. Kevin

    Well, this photo was posted a few days ago, and talked about on NASA sites, so I would say this isn’t a silly 4/1 joke.

    It’s seriously cool.

  3. 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.

    Except that you said it’s on an inclined orbit. Wouldn’t that mean that it would appear to move “up and down” in a straight line during the course of the day? (ie: it’s always over the same longitude of the Earth, but its latitude changes.)

    And, speaking of the date, Mother Nature got into the act by having it snow here in New York.

  4. Diederick

    The sun looks angry.

  5. Pete

    Yep, geosychronous != geostationary – the latter is a special case of the former.

  6. Dave


    Nice photo and commentary!

    It *is* on an inclined orbit, but I disagree with your reason. Orbiting the equator would not result in one eclipse per day, since the equator is not coplanar with the ecliptic. Orbiting in the *ecliptic* would result in one eclipse per day, but orbiting above the equator would result in just 2 times per year when eclipses are likely (when the line of intersection of the ecliptic and the equatorial plane points toward the Sun).

  7. Dave (6): You’re right, and I’m kicking myself for writing that. I fixed the text above, and thanks!

  8. Ryan

    So is it the moon that got in the way mid-day on 3/4/11?

  9. Patrick

    “So what do we call this kind of event?”

    Actually, I would call it a sunrise (or sunset). That’s what we normally call it when the earth blocks a portion of the sun from our viewing angle. The cool thing is that this picture even kind of looks like a sunrise once you start thinking of it that way.

  10. Guysmiley

    A 120 megabit satellite downlink? Nice!

  11. “A 120 megabit satellite downlink? Nice!”

    Heck, yeah. I could download a lot of…uh…pictures of…heavenly bodies and…stuff…at that rate.


  12. Alvaro

    Phil, I’d say this picture is actually pretty… hot.

  13. Given that spacecraft must use hardened electronics, which usually make them about a decade out of date, 150 Mbps is even more incredible. Here on Earth, my computer and internet service struggles to reach 5, and its almost brand new.

    Then again, I didn’t spend a hundred million bucks for my computer, so ho hum…

  14. “solareclipsenate”… Phil, you card! 😀

    … 😐


  15. I love it when syzygy occurs at the nodes!

  16. Even more amazing than 120Mbps from an Earth-orbiting satellite was the Galileo spacecraft around Jupiter. With its broken main antenna, I understand that communications was done at a whopping 10bps. (Note that there is no multiplier-prefix on “bps”.)

  17. jrpowell

    So what do we call this kind of event?

    Isn’t this a transit of Earth across the Sun from the satellite’s point of view?

  18. I highly recommend looking at other wavelengths. Partially blocked photos are available at 171 Å, 193 Å, 1600 Å, 1700 Å, and probably a few others. Just do a search on the image archive link Phil posted above(here it is again) and look for photos starting 2011-03-29 6:00:00 (you can modify the time a bit…the photos in question happen at about 7:15 UT)

  19. Chris

    I think it should be called a terran eclipse, since terra is Latin for Earth.

  20. Hmmmm love the picture but I’m squinting my eyes at today’s DATE (April 1)

  21. Roger

    I’m really disappointed that the Oxford Dictionary, who added 8 new words or so to the dictionary, didn’t add any of Phil’s words. Like to solareclipsenate, or to galactinate, or to ennebulenate, or even to ensmallestplanetate (if I missed any, just add them). Only people who follow this blog know what they mean and it’s important that these words get out to the masses. I told a friend I was going to galactinate our family picture and he punched me because he was ignorant. If the people at Oxford are going to add “words” like LOL, OMG, and whatever other texting words that were added, they should add words that actually mean something and when used, people don’t want to hurt you. When someone speaks to me in “text”, I feel like my friend but not because of ignorance….but because people are actually idiots when they talk in text.
    Something to think about.

  22. RwFlynn

    You know, a few weeks ago I saw this youtube video where a bunch of conspiracy theorists who were for some reason watching archived footage of the sun saw this same kind of solar eclipse (must’ve been the previous one). They were all freaking out about how the sun could be disappearing! Or there MUST be some alien spacecraft in front of SDO. I didn’t know what it was either, but I wasn’t about to be convinced by the lunatics and just decided to wait til I heard a good, grounded reason why that happened. Right about now, I kinda want to find that video and start tossing this link around. 😛

  23. Steve D

    So basically we’re seeing a sunset. Yawn

  24. C. Robert Dimitri

    Hey, Phil: do you have an archive on your site where you keep all these great photos that you have shared with us? I’d really like to check out a full gallery of them.

  25. What Steve D. said.

    When the Earth passes in front of the Sun, we normally call that … night.

  26. psuedonymous

    SDO’s orbit is Geosynchronous, but NOT geostationary: it’s ground track is more of a figure-8 (http://www.youtube.com/watch?v=XKlEGfzNbUY). Some more info here: http://celestrak.com/columns/v04n07/

  27. Brian Rose

    An eclipse is always about the Sun’s light being blocked. A lunar eclipse happens when the sun’s light is blocked from getting to the moon.

    We call them lunar and solar because of our perspective on earth. We can’t see the moon before a solar eclipse, so we naturally associate the event with the sun. Since the sun is not visible during a lunar eclipse, our language reflects that (pun intended) as well.

  28. Erik p

    Quote:.- “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”.

    I cant understand why this is news now? SDO was launched more than a year ago, so we should have many more pictures of this regular twice a year event, shouldnt we?

  29. toasterhead

    Why would you have a sun-observing satellite in geostationary orbit? Wouldn’t it make more sense to have it at a LaGrange point or in one of those 89-degree polar orbits over the terminator line?

  30. #17 jrpowell:
    No. A transit is when a body of smaller apparent size passes in front of a bigger one. Here, the Earth’s apparent size is far bigger than that of the Sun, so it isn’t a transit. It is in fact a partial occultation of the Sun by the Earth.

    #29 Brian Rose:
    “An eclipse is always about the Sun’s light being blocked.”
    Wrong! The correct meaning of the word “eclipse” has nothing to do with the Sun’s light being blocked. It means one body passing through the shadow of another. So the term “solar eclipse”, or “eclipse of the Sun” is in fact a misnomer; it should really be called an occultation of the Sun by the Moon – or a transit of the Moon, in the case of an annular eclipse.
    Look up the definitions of “eclipse”, “occultation” and “transit” in any dictionary of astronomy, or see how they are used in the tables of the phenomena of Jupiter’s satellites in Sky and Telescope.
    Of course, the term “solar eclipse” has been in common use for centuries, since long before those formal definitions were adopted – so we’re stuck with it.

  31. Ben H.

    your footnote is not entirely correct. As others have mentioned, SDO is geosynchronous but not geostationary. As you say, SDO has an inclined orbit, therefore it appears to stay at a fixed LONGITUDE but not a single point in the sky (that would require zero inclination). From the ground, SDO would appear to move in a North-South line.

    – Ben H.
    Mission Control, Houston


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