Supermoon | Just An Illusion?

It’s time again for the Supermoon. This post is for those who will have clear skies on the evening of July 11 or 12 and want to see the Moon closer and bigger than normal. Full moon is actually at about sunrise on July 12 when it is setting in the west for those of us in the U.S. Most people don’t notice it setting full. The view of it at evening moonrise, before or after full, will appear almost exactly as it does at moonset the morning of full.

First, a little sciency stuff. This won’t hurt a bit.

  • The Moon takes about a month to orbit Earth.
  • The Moon’s orbit is not a circle around Earth. It is a bit oval-shaped.
  • During the closest part of the orbit it is called Perigee.
  • During the farthest part of the orbit it is called Apogee.
  • Closer things look bigger and farther things look smaller.
  • The Moon will appear about 14% bigger at Perigee than at Apogee.
  • Supermoon will return in August and September this year.

According to NASA…”Supermoon is a situation when the moon is slightly closer to Earth in its orbit than on average, and this effect is most noticeable when it occurs at the same time as a full moon. So, the moon may seem bigger although the difference in its distance from Earth is only a few percent at such times.”


Science @ NASA


As Seen From Space

The Moon does not orbit Earth in a perfect circle. The distance from the Earth varies between 28 and 32 Earth diameters, seen to scale in the animation below. The closest distance reached during each orbit is called perigee. The farthest distance is called apogee. We see plotted below the distance between the Earth and the Moon as it makes two orbits. Imagine you are watching from a vantage-point 1/4 of the way around the Moon’s orbit. You are also in orbit around Earth in this view. The variation in the Moon’s distance is quite pronounced. Sizes and distances are to scale in this view.

As Seen From Earth

This is a chart of the distance to the Moon from Earth over the course of 2014. The axis on the left is in thousands of kilometers. Apogee distances are the high values. Perigee distances are the low values. In July, August, and September, we have 3 smaller than normal perigee distances in a row of about 358,000 km.

Three small perigee distances in July, August, and September.

This next chart plots the apparent diameter of the Moon (blue) over the distance to the Moon (red) for the year. It simply reinforces the notion that closer objects appear bigger.

July to September is 182 to 274 on the days of the year scale on the bottom axis.

The NASA/Goddard Space Flight Center Scientific Visualization Studio has built a simulated view of the Moon animation for all of 2014. I downloaded and extracted two date ranges of interest for this post to show how the Moon looks when at apogee and at perigee.

Full Moon @ Apogee

This event took place in January 2014. The main thing to notice in this animation is how the Moon appears to recede and get smaller as it nears full. Then, it grows in size as it moves to new moon phase. In the charts above, we are viewing distance and diameter through the month of January.

Other details to watch include the small distance scale on the far right between 28 and 32 Earth diameters. Dates and specifics are printed hourly in the lower right. Start and stop at needed.

Full Moon @ Perigee

This event takes place in July 2014. The main thing to notice in this animation is how the Moon appears to come nearer and get larger as it nears full. Then, it reduces in size as it recedes to new moon phase. In the charts above, we are viewing distance and diameter through the month of July.

What’s With The Wobble? Is The Moon Drunk?

Perhaps you noticed one other thing. The Moon seems to wobble and dance around slowly. There is a combined tipping up and down along with the side to side. That is known as libration. I wrote about libration in this earlier post. It is about halfway down in the Distance section. The word libration makes me think of libation. Could the Moon be drunk from too much libation?


15 thoughts on “Supermoon | Just An Illusion?

  1. Good to know it doesn’t ACTUALLY get bigger and smaller. THAT would be scary! 😉 No, really, thanks for the definition of terms.

    AND this goes along with your posts on cosmic distances. Maybe you want to link them, too?

  2. I shot the Supermoon two years ago and will give it a shot again this year. Just not sure from where. 8:30 here in the east is past my bedtime. 🙂
    Thanks for all the science. It did hurt….but I’ll survive. 🙂

  3. The word libration is based on Latin libra, which designated a balance scale. The oscillation of the beam in a balance scale has been extended to the particular kind of oscillation seen in the libration of a satellite (in this case earth’s moon). We also know Libra as the name of a constellation representing a balance scale.

  4. A lovely sight here. There were clouds in the west, which made for a lovely sunset, but it was clear in the east. If the skies continue to clear overnight, there may be a Belt of Venus in the morning — one of my favorite sky sights.

    It’s just a quirk, I suppose, but I do hate the phrase “super moon.” I much prefer “perigee moon.” It doesn’t sounds so much like a marketing slogan or something off the menu of a fast-food joint.

    • I agree about the label. I’ll bet we won’t get it changed.

      Here it is very cloudy. No perigee moon for me tonight. I need to check in the morning before it sets.

  5. Saw the big moon last night – luminous and clear. Able to catch a pic showing the light and dark areas. Our weather has been very clear lately, so we got lucky. I’ll be looking out for it tonight too. It does appear slightly larger, as described, although that’s tricky to perceive without something to compare it to. Interestingly, I’m reading a book right now that describes the competition between astronomy and engineering as the best way to determine longitude (Dava Sobel’s book). The “lunar distance” proponent tried some devious tactics to get his way, though as we know timepieces carried the day, before GPS. Cumbersome and iffy as a technique: he had to generate enormous reference tables continuously for someone to be able to use that method, not to mention the fact that any data depends on a clear sky. That happens frequently in temperate zones doesn’t it ;-). Thanks for the moon variations explanation.

    • We got a clear patch of sky at bedtime placed just right to see the moon. Tonight it is unlikely. More rain coming. This morning we got 2.47″ at sunrise.

      A fellow blogger I follow got this great shot from Mass.

      That longitude story is great. She writes well. There was a dramatization on PBS, I think, a few years ago that was good. Rømer was the one doing the tables. Is that right? Cumbersome indeed.

      Thanks for stopping by.

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