“And everything under the sun is in tune
But the sun is eclipsed by the moon.” -
Pink Floyd

We normally think of eclipses as rare phenomena, something that happens only occasionally. While it’s true than any particular location only experiences eclipses — particularly solar eclipses — infrequently, we actually experience these things around four times a year, typically, somewhere on Earth.

Image credit: Chaisson & McMillan, Astronomy Today.

Image credit: Chaisson & McMillan, Astronomy Today.

If you’ve been paying attention, you might have heard that we’re in store for a total lunar eclipse just next week here on Earth, and that it will be prominently visible from most of North and South America.

Image credit: timeanddate.com, via http://www.timeanddate.com/eclipse/lunar/2014-april-15.

Image credit: timeanddate.com, via http://www.timeanddate.com/eclipse/lunar/2014-april-15.

But did you know that just two week later, we’re getting an annular solar eclipse to match? Come find out about these incredibly rare back-to-back maximum eclipses, and find out when future ones are, too!

Comments

  1. #1 Markm
    Thailand.
    April 10, 2014

    Um… Did you know that two is plural?
    … two WEEKS… !

  2. #2 Chad
    April 10, 2014

    Oh my god!! I hope your typo costs you your blog about english…. er um space….

  3. #3 rob
    April 10, 2014

    I noticed that the Encyclopedia Britannica pic of the moons path around the sun (in your Medium.com post about these eclipses) is wrong.

    Since the moon-sun force is radial, and the moon is in circular motion around the sun, the moon should not have the concavity of it’s orbit change. It’s concavity should point to the sun and have no inflection points, like at the edges of the Britannica pic.

  4. #4 Sean T
    April 11, 2014

    rob,

    I don’t see a problem with the image you question. First of all, the earth/moon system is in elliptical orbit around the sun, not circular. Second, and more importantly, you are ignoring the orbital motion of the moon around the earth-moon barycenter (which is located inside the earth, so it’s a reasonable simplification to say that the moon orbits the earth). Because of this motion, the moon is nearest to the sun at new phase. While moving from new to full, the moon is actually moving away from the sun. That is why you see the concavity pointing away from the sun.

  5. #5 rob
    April 18, 2014

    Sean,
    yes, the earth-moon system is in an elliptical orbit, however, the eccentricity is about .02, so it is very nearly circular. however, even if it were highly eccentric, it would still have to have a locally convex orbit because the gravitational force from the sun is radial. the radial nature of the suns gravitational force means that over a short arc of the orbit, the sun can only change the angle of the earth-moon velocity. there is not radial component of the gravitational force to reverse the direction of travel. this results in the orbit of the earth-moon having to be locally convex at all points in the orbit–you can never have loops in the orbit, or concavity pointing away from the sun.

    that is why the encyclopedia illustration is wrong. it shows an inflection point in the moons orbit.

    Check out these sites for discussion of the moon’s orbit shape:

    http://www.math.nus.edu.sg/aslaksen/teaching/convex.html

    http://blogs.discovermagazine.com/badastronomy/2008/09/29/the-moon-that-went-up-a-hill-but-came-down-a-planet/

  6. #6 Michael Kelsey
    SLAC National Accelerator Laboratory
    April 18, 2014

    @rob: you seem to be mixing two different concepts — the orbit of the *Earth-Moon system*, vs. the path which the Moon takes around the Sun.

    You’re absolutely right that the Earth-Moon system, or more specifically the barycenter of the Earth-Moon system, follows a nearly circular path around the sun, with no concavities or inflection points.

    However, you are not correct about the path of the Moon *alone* around the Sun. It is a trivial observation (because of the existence of both lunar and solar eclipses!) that there are times when the Moon itself is closer to the Sun than the barycenter, and other times when the Moon itself is farther from the Sun than the barycenter.

    Consequently, it is an equally trivial inference, based solely on assuming that the Moon’s path in the Sun’s rest frame is continuous, that it follows a path somewhat like Ethan’s figure (c). Sinisa provided a link to a properly scaled version of the path.

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