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Supermoon - Not All That Special

by Eric Hall

June 22, 2013

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Donate Note: This is a repost from 2011 from my personal blog. Because of another " Supermoon" this weekend, I thought it made sense to repost it here. I made minor edits, including updating Phil Plait's links. Phil Plait also wrote an updated post about it here. Also note I referenced everything to the Central Time Zone in the US since that is where I am located. You can adjust the times from there.

The "science" being reported on the "supermoon" makes me really sad sbout science education and science reporting in the United States. If one takes a look at a lunar calendar, you will see the moon has a perigee at this distance about every 2 1/2 years or so. In fact, the last one in December 2008 was also at a full moon. The minor difference is this year the close perigee happens right about the exact time of the full moon, where as the in 2008 it happened about 4 hours after the full moon. The irony is in 2011, the true full moon happened at about 4 pm in the Central Time Zone (when the moon was still hidden) whereas in 2008 the full moon happened about 7:30 pm in the Central Time Zone. That means the perigee happened about 11:30pm in the Central Time Zone. So during that night in 2008, you could actually see the moon be closer than you could in 2011.

Some want to call this the supermoon because it is happening right at the time of the full moon (within the hour the moon is truly "full"). Well, if you want to go by that, then 1993 doesn't work either. In 1993 (which is when most are reporting the last time this happened), the perigee of the moon happened over an hour before the full moon. Now in 1992, the perigee happened within the hour of the full moon, but it happened in January. By my math, that is over 19 years ago. It should also be noted it was even closer than it was this year by 29 km.

Before that, the new moon in 2005 was that close - so it would have been bigger during the day when the moon was visible due to earth shine. In 2003, 2001, 1998, 1994 all had large full moons as well (within 200 km of this year's supermoon). There is no way that 200 km would be noticeable when the moon is still over 350,000 km away. In 2008, the perigee was actually 10 km closer and would have actually been visible in our time zone when it occurred.


I checked the calendar for the next few years, and May 6, 2012 the full moon falls right on the perigee, although about 400 km farther than this year. June 23, 2013, the same thing happens. August 10, 2014, another full moon perigee and only 300 km farther than this year. September 28, 2015 is yet another. November 14, 2016 is a perigee even closer than this year, at 356,511 km (compared to 356,577 this year) . The perigeeprecedes the full moon by 2 hours in this instance, but again it isunnoticeable compared to the "true" full moon. Maybe we can call this one the ultra-moon?

While it is neat to think the moon is a couple percent larger visibly than average and a good 8% or so larger [Update] can appear up to 14% larger (thanks NASA) than when it is farthest away, it is hardly a uniqueoccurrence. Every full moon perigee is roughly 14% larger than a full moon at apogee, give or take 1%.

One other thing to note: people are often fooled to thinking the moon is very much larger by how it looks when it first rises. This is simply an optical illusion as explained here. If you want proof it is an illusion, when the moon is on the horizon, bend over and view it between your legs. It will appear normal size. When viewing it normally, our brains fool us because of buildings, trees, etc. in the foreground.

For all of you interested in astronomy, I recommend followingPhil Plait orNeil deGrasse Tyson on Twitter, read Phil Plait'sBad Astronomy blog . These guys are experts and some of the best personalities in science.

by Eric Hall

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