Followup: Supereclipse

By Phil Plait | May 19, 2012 7:00 am

I wrote earlier about the annular eclipse happening this coming Sunday. It’s a solar eclipse, with the Moon blocking the Sun, but because the Moon is at apogee — the point in its orbit farthest from Earth — the Moon appears smaller in the sky, so it doesn’t completely block the Sun. We’re left with a ring of solar surface surrounding the Moon, the so-called Ring of Fire.

I got a couple of people asking me why this eclipse is happening at lunar apogee when we just had a "Supermoon", when the Moon was full at perigee (when it’s closest to Earth in its orbit). This is a good question! It’s not a coincidence. In fact, it must happen this way! Here’s why.

First, here’s a drawing of the Moon’s orbit, courtesy NASA:

The Moon orbits the Earth in an ellipse, so sometimes it’s closer to us, and sometimes farther. The ellipticity is exaggerated in the drawing; it’s actually about a 10% difference in distance between apogee and perigee. The Moon orbits the Earth once every 27.3 days, so it takes about 13.7 days for it to go from apogee to perigee — a little less than two weeks.

This is different than the phase of the Moon, which is how much of the Moon we see lit by the Sun. When the moon is between us and the Sun, it’s new: we only see the unlit side. When it’s opposite the Sun in the sky — when the Earth is between the two — the side of the Moon we see is lit, so we say it’s full. There are approximately 8 billion web pages describing how this works; here’s one I wrote. The time it takes to go from full Moon to full Moon is 29.5 days. That means to go from full Moon to the next new Moon takes half that time, or about 14.7 days — a little more than two weeks.

We can only get a solar eclipse when the Moon is between us and the Sun. This happens when the Moon is new (I’ll note in passing that it doesn’t happen every time the Moon is new, because the orbit of the Moon doesn’t align exactly with the Earth’s orbit around the Sun).

The phases of the Moon don’t line up perfectly with its position in the orbit because of the two different periods: 27.3 days to go around the Earth, but 29.5 days to go from full to full again (this video might help you). So sometimes full Moon happens at perigee, sometimes at apogee, and most of the time sometime in between.

Now let’s put this all together! The Supermoon is when the Moon is full and at perigee, right? That’s what happened on May 5th. On Sunday, a bit more than two weeks will have elapsed since then. That means the Moon will have moved halfway around its orbit — it actually reaches apogee on Saturday May 19th. But the phase has been changing, so it’s new on May 20, and it so happens that things have aligned for it to eclipse the Sun.

Since this happens the day after apogee, the Moon is farther away than usual, and from Earth it looks smaller. BOOM. Annular eclipse.

I think the confusion stems from folks not knowing the Moon orbits the Earth once per month on an ellipse, so it goes from perigee to apogee in two weeks. Once you get that, hopefully the rest of this makes more sense.

And because why not, I’ll leave you with this video showing the phase of the Moon as well as its apparent size in the sky as they change over the course of the year. If you want a detailed explanation of what you’re seeing, here ya go.

Enjoy the eclipse! And make sure if you watch it, you do so safely.

Image credits: NASA; Sancho Panza on Flickr.

Related Posts:

Ring of fire eclipse on May 20
Solar eclipse, from space!
Newest of new moons
Record breaker: newest new Moon spotted!
What does a lunar eclipse look like from the Moon?

CATEGORIZED UNDER: Astronomy, Cool stuff

Comments (24)

  1. John

    I do have a question about Earth’s orbit — right now Earth’s perigee occurs in early January and apogee in early July. My question is this, is the date these are occurring changing? That is will perigee eventually happen in July?

  2. Dan I


    What is the impact, if any, of the EARTH’S orbit on this? I’m guessing that whether the Earth (and hence the Moon as well) is at apogee or perigee with respect to the Sun has SOME effect on whether or not we get a total or annual eclipse.

    But is, say, the Earth being at perigee to the Sun enough to cancel out the effect of the Moon being at apogee to the Earth?

  3. Stu Harris
  4. Stu Harris

    Earth doesn’t have perigees and apogees — it has perihelions and aphelions.

  5. Tim

    Dan I : According to the second answer on this page :

    …the sun’s apparent diameter in the sky ranges from 31.6′ to 32.7′ (though they seem to have reversed aphelion and perihelion), and the moon ranges from 29.43′ to 33.5′. So, when the moon is at apogee, it should appear too small to totally block the sun, no matter where the Earth is in its orbit. But, yes, if the moon is at a point in its orbit where its apparent size is between 31.6′ and 32.7′, it looks like the Earth’s position wrt the sun could make the difference between a total eclipse and an annular one.

    Assuming, of course, that those numbers are accurate.

  6. Dragonchild

    One Ring to rule them all. . .

  7. Chris

    @1 John
    Yes the Earth’s orbit precesses, but it takes thousands of years.

  8. What I’d really like to see in that moon size video is a representation of the sun and horizon, as well. It’d be neat to see all the near-eclipses (and in the case of this year, the actual eclipse). Obviously that makes it even more latitude- and longitude-sensitive than the video currently is, though.

  9. Robert

    So is it possible to see the moon when it is in the new moon phase?

  10. Mudhooks

    I had no idea there was one due but I was just telling a friend about the boyfriend I had (aged about 25) who, when I asked if he wanted to go out and watch the liner eclipse later that evening, said “Why? We have them every month!”

    I had to sit down with him and explain the difference between phases of the moon and lunar eclipses, including a demonstration involving oranges and a flashlight.

  11. amphiox

    So is it possible to see the moon when it is in the new moon phase?

    Earthshine, I think, would do it.

  12. @amphiox:
    Earthshine will not light up the moon enough to be brighter than the sun or daylit sky. Don’t forget that the new moon occurs when it is closest to the sun, and hence the moon is in the sky during the day.

    Phil posted a photo of a very-close-to-new moon a few months back, I’ll see if I can dig it up. The photographer had to use specialized equipment.

  13. CR

    Dotan Cohen @14 (and amphiox), it’s in the Related Posts links above, I think… “Record breaker: newest new Moon spotted!”

  14. Thanks, CR! In fact, the photographer Thierry Legault comments on that post and he mentions having photographed the moon with Earthshine during a solar eclipse.

  15. Alex Murdoch

    I don’t suppose the eclipse is going to be live broadcast on Google+? If anyone knows of a site doing this, could you please post it? Living on the East Coast of Canada, we’re not going to be able to see it.

  16. Karen MH

    Since the earth’s pole change, would that explain why this super eclipse is/has happening/happened a month earlier e.g. Dec & May instead of Jan & Jun?

  17. #4 Stu Harris:
    Actually, being pedantic, it has perihelia and aphelia. :-)

  18. Nigel Depledge

    Robert (9) said:

    So is it possible to see the moon when it is in the new moon phase?


    Whether you refer to the tiniest sliver of sunlit moon at new moon, or whether you refer to the fully-dark new moon, it is possible to see it. It’s just not easy.

    I don’t know if this still happens, but there used to be a kind of informal competition among astronomers to get the earliest glimpse of a new moon (the tiny sunlit sliver). This is hard to see because the new moon is always close to the sun in the sky (IIRC, just after sunset is the best time to try and view the new moon), so it is mostly up during daytime when the sky is bright. And because the early new moon is a very thin crescent, it does not give much light to capture.

    Then again, I am sure it would be possible to get a long-ish exposure pic of the fully-dark new moon during a total solar eclipse, when Earthshine will illuminate the near side of the moon. But I would be surprised if it were possible to make out details on the moon in these circumstances by eye.

  19. Nigel Depledge

    Karen MH (18) said:

    Since the earth’s pole change, would that explain why this super eclipse is/has happening/happened a month earlier e.g. Dec & May instead of Jan & Jun?

    When did the Earth have a pole change?

  20. #20 Nigel:
    Yes, the competition still goes on. I think the record for the youngest Moon sighting is something like 14 hours. Sky and Telescope gives predictions for when record sightings might be possible.
    This is especially popular in Islamic countries, because in the Islamic lunar calendar, the start of certain events, such as Ramadan, is taken to be the time of the first sighting of the “New” Moon. I think the record is held by a group of amateur astronomers in Iran.

  21. scibuff

    Here is an APOD of Earthshine and Moon during a total solar eclipse

    That should give you a good idea about what would be involved of seeing features on the lunar surface during an eclipse

  22. Matt B.

    The Earth is definitely not at a focus of that ellipse; parts of the curve actually get closer to Earth than the near end of the major axis. It’s just disconcerting.

    An eclipse is a great occasion to talk about the 18.6-year cycle of precession of the moon’s orbital tilt.


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