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Brightest Moon In Decades Saturday Evening, March 19, 2011

Brightest Moon In Decades Saturday Evening, March 19, 2011



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Dan,

Wondering what time the moon is going to be rising over the mountains tomorrow night? I understand it is going to be full and very big. Is the weather going to be clear enough to view the moon at its biggest?

Mark Francis

Mark,

Moonrise in Salt Lake City Saturday Evening is 8:07 PM. The Moon will be at 99.5% fullness. 100% fullness occurs at 12:20 PM in the afternoon Saturday.

There is only a 20% chance we will be able to see the Moon through mostly cloudy skies and lingering rain and snow showers. Improving chances for viewing the Moon will happen later in the night; as skies partially clear.

I had to do a bunch of looking around to truly understand why this full Moon will appear 14% larger and 30% brighter than "average". At first, I was skeptical. Usually, at the closest approach of the Earth and Moon, called "Perigee" is 8% different (closer) than average or normal distance. (This normal distance is only the average of Perigee and Apogee; the distance is always changing as the moon circles around Earth). Brightness at full moon changes as this distance changes. Average distance rarely occurs because of the elliptical orbit, which is what makes Saturday evening's event so interesting (answer to your question about when this I going to happen).

The thing that happens this Saturday evening is the Moon and Earth are at "Perigee" when we are at approximately 100% full Moon. When this Perigee happens at three quarter phase, or 80 percent full, or 20% full, then you don't get the advantage of full moon and "closest approach" or Perigee. Almost all of the time, you don't get the benefit of the "full Moon" and Perigee.

I think it should be noted that we are comparing this Moon to what would be a normal "full Moon". And, due to the elliptical orbit of the Earth and Moon, it is rare to have the combination of Perigee and a full Moon together. It hasn't happened for two decades and won't happen again until 2029. So, it is rare and worth seeing (if you can) through the clouds.

Respectfully,

Certified Broadcast Meteorologist Dan Pope KSL-TV & KSL.com

Addendum: I know most of us in Utah could not see this moon, but a nice shot of the Moon was taken in Chile...

You can see it here: http://ds.cm/post/3975860827/supermoon-chile-1-wide-field-20-mar

Dan Pope

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