August 10, 2014 just passed. It was the most recent Super moon. The term “Super moon” was coined by astrologers not astronomers and refers to a moon which is both full and also within 4 hours of its closest approach to earth.
The media gleefully report the super moon and show pictures of huge moons (many of which have been photo manipulated). Here is the straight scoop on the subject. If you’re wondering whether that photo you’ve seen of the “too big to be true moon” has been doctored, we have an article on that.
The most “Extreme” Supermoon of the Century occurred in 2012. Here it was photographed in Yosemite approximately 15 minutes after it reached perigee.
What Makes the Moon Larger or Smaller When Seen from Earth?
Because the orbit of the moon around the earth is not circular, the distance from earth to the moon varies and thus the apparent (angular size) of the moon changes. Every lunar cycle the earth-lunar distance varies between its closest approach called perigee and its farthest distance, called apogee. How big is the difference? The closest approach is 363,104 km (225,622 miles) and the farthest, 406,696 km (252,088 miles).
What is the difference in apparent size? At apogee, the moon is 22,293 km farther away or -5.8% smaller than an average moon. At perigee the moon is 5.54% larger than the average moon. Comparing apogee and perigee moons, the difference is a maximum angular size difference of about 12% The average angular size of the moon, by the way, is half of a degree or 30 minutes of arc. That angle is slightly smaller than the size of the nail on your little finger when held at arm’s length. Those of you with significantly mis-sized pinky nails or unusual arm length might want to find another object to measure with at arm’s length.
In short: You’d have to be a very keen observer to notice a 12% difference in size between a super moon and a “wimpy” (apogee) moon.
Because the moon is slowly spiraling away from earth eventually the perigee moon will grow smaller and smaller in apparent size until one day, we will no longer experience total solar eclipses. The perigee moon will be too small to cover the angular disk of the sun which also happens to be almost exactly one half of a degree. From that point on, all solar eclipses will be “annular” like this one in May, 2012. Had the moon been closer to the earth, this may have been a total solar eclipse.
How is a Full Moon Determined?
A full moon is defined as the moment in time when the sun, earth and moon are in syzygy. Syzygy is not only an interesting Scrabble(tm) word, but it defines when three bodies are in alignment. When the sun and moon are 180 degrees opposite one another relative to the Earth, we have syzygy which is the instance in which the moon is Full Moon. Many of us think of a “full moon” as that period during the month when the moon appears to be fully lit. That period lasts almost 70 hours, so we understand how reckoning a full moon as a moment in time is a bit confusing.
If you didn’t observe the August moon within 4 hours either side of when it was full, you did not see the super moon. On the United States West Coast the super moon was not visible. Why? The moon set at 6:10 AM almost 5 hours before the moon was full. Those in Hawaii could just catch the super moon setting. Those on the East Coast of the US had no chance at all. The whole super moon window occurred during the time the moon was not visible on the East Coast.
The Last (and Next) Visible Super Moons
If you missed the May, 2012 Extreme Super Moon (my term, photo above), you’ve missed the largest possible full moon for more than a century into the future. On May 5, 2012 fullness and perigee occurred within less than two minutes of one another. But don’t fret. The difference in size between the extreme super moon an the average super moon is too small to notice unless you measure carefully. If you paid close attention you probably also noticed that the May 20, 2012 annular solar eclipse followed nearly half a lunar cycle after the May 5, 2012 super moon. That is not a coincidence! The moon was closest to us on May 5th so half of a lunar cycle away it must be farthest from us!
On August 10, 2014, full moon and perigee occurred within about 1 hour of each other. The next super moon is in September 8, 2014. The moon will not be as close to perigee at the moment when it becomes full, but the moment of full moon occurs at 9:38 PM PDT, just as the moon rises. It will be a true super moon!
Catch One Yourself
We plan to schedule a “Catching the Moon” Webinar well in advance. Stay tuned. One complication is that the wonderful Photographer’s Ephemeris Tool will cease to work in desktop mode soon. It is being replaced with a browser version. While the tools is excellent, and we highly recommend it (and that you donate if you use it!) TPE still leaves some important bits of the puzzle unresolved – we will fill those in for you and give you a crack at our tool(s).
The moon caught between El Capitan and Half Dome, Yosemite National Park – Actual size, no manipulation