An eclipse from space with a two-way Moon

By Phil Plait | March 6, 2011 7:00 am

This is pretty wild: a partial solar eclipse on March 4, 2011 as seen from NASA’s Solar Dynamics Observatory! Watch as the Moon slips in front of the face of the Sun in this video:

Cool! In this far-ultraviolet view, you can see magnetic activity on the Sun’s surface, like arcing gas eruptions and sunspots (which are dark in visible light, but very bright in the UV), and the roiling, bubbling motion as pockets of hot solar gas rise and fall. Then, suddenly, the Moon makes its appearance!

When I first saw this, I was surprised the Moon appeared to reverse direction. But after a moment’s thought I figured it out: parallax! Here’s how it works:

On the right is a diagram (not to scale, duh) I put together to illustrate the situation. SDO orbits the Earth, and I drew it so that it orbits counterclockwise. When SDO is at the bottom of the diagram, it’s moving to the right (position 1). A little while later (position 2) it’s moving to the left.

Now imagine the Sun is way off the top of the diagram. SDO stares at the Sun, locked onto it, so it doesn’t appear to move as SDO orbits the Earth. But the Moon is much closer, and is greatly affected by SDO’s orbital motion. When SDO is at position 1, to it the Moon appears to be moving to the left. When SDO is at position 2, the Moon looks like it’s moving to the right!

So at different points in its orbit, SDO sees the Moon moving in opposite directions. The point where the Moon’s direction just switches direction is about when the Earth, SDO, and the Moon make a right angle, at about the 3:00 position in its orbit as I’ve drawn it here (and again at 9:00). When SDO saw the partial eclipse, it must have just been at the point in its orbit where the Moon switched directions. That’s why in the video you see it moving one way, slow, stop, then reverse.

Pretty cool, huh? It goes to show you that sometimes things don’t happen the way you expect, but if you’re familiar with the situation a logical explanation can be found.


Image credit: NASA/SDO. Animation made using a very cool solar viewer called Helioviewer.

Related posts:

Incredible solar flare video
KaBLAM! Footage of the X-class solar flare
First Earthward-heading solar flare of the cycle
SDO lunar transit: now with video

MORE ABOUT: Moon, SDO, solar eclipse, Sun

Comments (23)

  1. Messier Tidy Upper

    Strange and wonderful clip, BA, thanks. :-)

    The same or a very similiar phenomenon here – with SDO and the Moon motion reversal – also explains why certain places on Mercury experience two sun-rises – ie. the Sun rising, setting and rising again, correct? Or am I too tired and not thinking clearly thus confused on that?

  2. Blaise Pascal

    Is that parallax or retrograde motion? As far as I can tell, it’s the same geometry which accounts for the retrograde motion of the planets in a heliocentric model.

  3. Grand Lunar

    Very weird and cool Phil!

  4. You didn’t finish your last line sir. It’s supposed to be:

    Science, it works b!tch3$!


  5. Bob P

    I’m with M. Pascal. Earth overtakes an outer planet; SDO earth-orbiting satelllite overtakes Moon. That generates the appearance of retrograde motion. It’s the change of direction in the relative motion here (from your diagram, SDO starts moving L-R, then changes to R-L, in reference to the moon) that best explains the observation.

    When you say parallax, I think of an object’s apparent movement against a background when viewed from two distinct positions. Both assume a static object to observe, and an observation platform in motion, but relative motion captures a sequence in time, and parallax compares two events at widely separated times and places. Or at least, that’s how I handle it. Probably comes from having mainlined PBS’s “The Mechanical Universe” in my youth.

  6. Aaron

    @ Blaise Pascal, et al.:


    (The apparent retrograde motion of the planets as seen from Earth is caused by parallax.)

  7. As Aaron (6) says, retrograde motion is caused by parallax. Parallax is the change in the apparent position of an object due to a change in the position of the observer. That can produce several effects, including retrograde motion.

  8. “This is pretty wild … parallax!”

    I’m not sure how “wild” this is, as Earth orbiting observation telescopes (e.g., Hubble, SDO, and others) spacecraft correct their line-of-site pointing for orbit-phase dependent parallax of (only) the prime target. In the case of HST, for example, observations of solar system targets (planets, asteroids, KBO’s, etc.) must be (and are) are pointing-corrected not only for the target orbit (and apparent geocentric position), but for the spacecraft orbit around the Earth. This, of course, was true for solar observation satellites prior to SDO. E.g., for here a couple of great image (composites) from TRACE (the Transition Region And Coronal Explorer) taken during the 2004 and 1999 transits of Venus and Mercury, respectively:
    with the “wiggling” and non-linear temporal apparent movement of the planets across the face of the Sun due to the spacecraft low-earth orbit parallax. The “north/south” component due to the near polar orbit of TRACE, e.g. see: and
    The second shows the orbits of two spacecraft (TRACE and also ACRIMSAT) as seen from the Sun during the 2004 transit f Venus. (Trace is the one in near polar orbit this is occulted by the Earth as seen from the Sun on each orbit). More pictures, and “movies” from that transit are published his month in the Astronomical Journal and its on-line supplement (vol 114, p. 1112) with some on this web site:
    Actually we did an experiment using ACRIMSAT (a radiometer designed to very accurately measure the total solar insolation over time) to detect and measure the Venus transit treating the Sun as an unresolved point source — as a pre-Kepler analog to such planetary transits in exoplanetary systems. We could see in those data the effect of (and needed to correct for) the parallactic motion of Venus in the radiometric light curve (as it was convolved with solar limb darkening). See:
    or in more detail: Schneider, Pasachoff & Willson 2006, Astrophysical Journal, 565.
    Still, with the moon “so close” its reflective low-earth orbit parallax is pretty cool to see, and maybe even “wild” as Phil suggested. Kudos to the SDO team.

  9. Oh Glenn, you professional space-based observer you! :)

    Of course it’s obvious enough to you (and to me after a moment’s thought) — we’re used to this. In fact, when Venus transited the Sun in 2004, we were watching the TRACE observations in near real-time at Goddard and it took us a few seconds to calculate that the motion was indeed due to the satellite’s orbit. Very cool.

  10. Don Q

    And, if the linked video was instead taken from a satellite with the purpose of keeping the moon image at the center of field, we would see that the sun approaches from the bottom left, passes the moon, then reverses and then leaves the field out the bottom left.

    With a video keeping the background stars in fixed positions it’s different again.

    Which is correct? It’s all relative…

  11. réalta fuar

    Thanks Glenn, neat stuff.

  12. Wissydig

    Oh yeah i love it – more pls

  13. Neal

    Oh, right, the satellite is in a circumpolar orbit. The moon’s motion nearly perpendicular to the sun’s rotational plane caught me by surprise.


    With a video keeping the background stars in fixed positions it’s different again.
    Which is correct? It’s all relative…

    It’s all an equivalence class of transformations!

    I think a video with fixed background stars would be so similar to the video in the post that it wouldn’t warrant disagreement. The video taken couldn’t have been over more than a couple of days, so the sun’s motion w.r.t. the background stars would have been only a couple of degrees at most.

  14. Brian Schlosser

    Beautiful! Just one question:

    Are there satellites that NASA has that AREN’T in space? I mean, I guess there are plenty in clean rooms and warehouses and whatnot, but usually they don’t take such clear pictures… 😀

  15. Don

    I think the even cooler part of the video is seeing the sun’s rotation.

  16. Messier Tidy Upper

    @ me at #1 : The same or a very similiar phenomenon here – with SDO and the Moon motion reversal – also explains why certain places on Mercury experience two sun-rises – ie. the Sun rising, setting and rising again, correct? Or am I too tired and not thinking clearly thus confused on that?

    I *was* just too tired as it turns out. 😉

    Mercury’s rotation rate relative to it’s orbital speed is the cause.

    This Youtube clip :

    has more on that Mercurian phenomon. :-)

  17. Alex

    Any idea on the time exposure for that video? If that’s in real time then it really drives home how fast that satellite was moving.

  18. Joseph G

    That prominence on the left-side limb of the sun early on in the video looks an awful lot like the video that was posted earlier last week. Is that by any chance the same flare?

  19. Joseph G

    @MTU: You might want to mention, too, that the eccentricity is Mercury’s orbit is the deciding factor, there. I believe it’s the same mechanism that causes the libration of the moon.

    Heh, come to think of it, I’m remembering this old show that I used to watch – the end credits showed the earth rising over the lunar horizon. I always used to think that was a ridiculous bit of hokey graphical puffery, but come to think of it, I suppose that if you’re in a certain ring-shaped zone on the moon, the Earth would appear to bob up and down over the horizon, once per (lunar) day…

  20. Joseph G

    Actually, I’m a doofus – I think there would be two zones on opposite sides of the lunar disc (seen from the earth). Folks close to the lunar poles would see the earth move back and forth laterally.


Discover's Newsletter

Sign up to get the latest science news delivered weekly right to your inbox!


See More

Collapse bottom bar