Human Journey

Sizing Up the Supermoon

by Robert J. Vanderbei

The moon’s orbit about the Earth is not a perfect circle—it is slightly eccentric. As a result, during part of its orbit it is a little closer to us than at other times.

The closest approach is called perigee, while the greatest separation is called apogee.

On average, the moon’s distance from Earth is 239,228 miles (385,000 kilometers).

At perigee it is a bit closer at 221,643 miles (356,700 kilometers), whereas at apogee it is somewhat farther away at 252,463 miles (406,300 kilometers).

Saturday’s full moon has been called a supermoon, because the moon was closer to us than it had been at any time in the last 18 years, making it appear unusually large in the night sky.

After the event many pictures were posted of this super full moon. But many of my visual observer friends tell me it didn’t look much different from any other full moon. So, how much different was it?

It just so happens that I took a picture of the almost full moon on December 19, 2010—just a day and a few hours before the famous “solstice lunar eclipse” (pictures).

Fortunately, I used the exact same camera and telescope to take a picture of the March 19 supermoon.

Hence, a side-by-side comparison, such as the one shown above, of the two pictures gives a good idea of the relative apparent sizes of these two full moons.

My planetarium program tells me that on December 19 the center of the moon was 233,523 miles (375,820 kilometers) from my home in New Jersey. The same program tells me that on March 19 it was 220,084 miles (354,192 kilometers) away.

Measuring the height of each moon in the picture and dividing, I get that the diameter of the moon on March 19 was 6 percent larger than the December 19 moon, making it 12.4 percent larger in area.

Robert VanderbeiRobert J. Vanderbei is chair of the Operations Research and Financial Engineering department at Princeton University and co-author of the National Geographic book Sizing Up the Universe. Vanderbei has been an astrophotographer since 1999, and he regularly posts new images on his astro gallery website.

Robert J. Vanderbei is chair of the Operations Research and Financial Engineering department at Princeton University and co-author of the National Geographic book Sizing Up the Universe. Vanderbei has been an astrophotographer since 1999, and he regularly posts new images on his astro gallery website.
  • Awinash Kumar

    Dear Sir,
    What are the adverse affect of this SUPERMOON…as these days this news is spreading in air like anything…so please make me clear about it…..and if it so then do clear me why there will be so….why it is going to affect it…..

  • chanel

    This looks really very special.

  • julian

    the effects of the supermoon; floods, ie… memphis 2011/ 1927, katrina, japan ect ect.

  • There are no adverse effects of the Supermoon. Only the joy of witnessing something unusual.

  • Richard

    Well, since it was 12% closer it would make sense that it appeard 12% larger

  • Robert Vanderbei

    Indeed. And, every month it varies over this range of distances because its orbit is slightly elliptical. The only thing special about the Supermoon was that it passed by the closest part of its orbit at the same time that it was a full moon.

    It is reasonable to think that a 12 percent variation in distance could have a geological effect—after all, the tides are a result of the pull of the Moon (and the Sun). But, the effect caused by the Moon’s elliptical orbit, whatever it is, happens every month.

    In fact, during the Supermoon, the Sun and the Moon were on opposite sides of the Earth and therefore any effect would be minimized. If someone wants to worry about geological effects, they should pay attention to SuperNewMoons, not SuperFullMoons.

  • katie

    Do I have to wait 18 years for another one? I missed this one. I grew up in the desert and the moon always looked far more impressive than it does now. Do you think that’s because I was little and more easily awed or because I was in the desert?

  • You might have to wait several years to see a Supermoon as big as the one a few months ago. But, you can see one almost as good every year in March/April. This is because the Moon actually gets closest to us at one particular time in each of its monthly orbits. That particular time coincides with a full moon if it happens in late March. So, any March (or early April) full moon will be pretty close in size to the recent Supermoon. By the way, full moons in September are at the opposite extreme—they are about as small as you will see. Of course, part of the message of my posting was that the difference between a Supermoon and any other full moon is not very great.

  • katie

    Thank you!

  • ryan

    Are you sure there are no adverse effects of the SUPERMOON? I am very concerned for the next SUPERMOON event occuring as my home is prone to flooding as well as landslides……I need to know how to stop the SUPERMOON from destroying my town…….

  • There are no adverse effects. In terms of the gravitational pull of the Moon, the Moon gets “super” close once every month, month after month, year after year and so we experience this small additional gravitational pull monthly. We call it a “Supermoon” when the moment of getting close happens to coincide with the Moon being fully illuminated; i.e., a full Moon, which occurs when the Sun and Moon are on opposite sides of the Earth. If there were any “gravitational” concerns, I’d worry more if the Sun and the Moon were on the same side—that is, during a “Supereclipse” of the Sun—than during the “Supermoon” event when the Sun and the Moon are on opposite sides of the Earth and partially cancel each other out. And, because the effect is small, I’m not worried about Supereclipses either.

  • Madison

    Thanks for the info this helped with a report at school 🙂

  • buk lua

    i like scienc

  • manguai gaichu muthengi

    aaai the difference is soooo small that i cannot notice it

  • Robert Walker

    Your news article is about the present orbit of the moon.

    The moon and the Earth are separating about an inch and a half further apart each year. I do not know if this separation has been increasing or decreasing.

    Has the eccentricity of the orbit been increasing or decreasing? How high were the tides when the Earth and the Moon were closest? How much greater were the tides when vertebrates appeared? Can the effects of a closer Earth Moon pair be detected by geologists in rocks? Did a larger tidal zone increase the niche available to creatures who could navigate with limbs during low tide?

    • These are very interesting questions. I’m not a geologist, so I don’t feel confident to comment on the geological record. But, this wikipedia page might help… However, if we just look at the orbital mechanics, I do have some tools to assess the effect, say, of Jupiter on the Moon’s orbit. On my local copy of this Java applet, I added some print statements to monitor the eccentricity over time and I am currently running the simulator backwards in time. So far, I’ve only gone back 522,000 years. Over this time, the eccentricity has not changed in any significant way.

  • Devin

    this helped me with school to. THANK YOU ^-^

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