Big Moon tonight?

Contributed by
Jun 30, 2007

A lot of websites are saying that the Full Moon will look huge tonight (Saturday), and urging people to go out and look. This is true, but I want to make sure people understand what's going on. It's not that the Moon will look any bigger tonight than it does at any other time; it's that it stays near the horizon longer.

Any time you see the Moon rising, especially when it's full, you can get the Moon Illusion: the appearance that the Moon is huge, looming over the Earth. I won't go into the details here; go to that link where I wrote up a brief intro to the phenomenon.

You only get the illusion when the Moon is near the horizon; when it's overhead the illusion vanished. Because this month is the summer solstice, tonight the Moon stays close to the horizon for northern hemisphere observers. When the Moon is full, it's opposite the Sun in the sky. And because the Sun is high in the sky during the day during the summer, the full Moon will be low in

the sky at night. For me, in Boulder Colorado (latitude almost exactly 40 degree north), the Moon only gets to an altitude of about 23 degrees above the horizon tonight -- about the width of two fists outstretched.

The angle at which the Moon rises is also small, so it takes a while to rise, prolonging the illusion.

What's funny is that physically, the Moon should look smaller tonight than usual! It orbits the Earth in an ellipse, so sometimes it is farther from the Earth, and sometimes closer. I checked the good ol' Naval Observatory website, and looked up the distance to the Moon for a couple of weeks centered on tonight. Here are the results:

Date   Distance (km)  
Jun 24   404320  
Jun 25   404445  
Jun 26   403418  
Jun 27   401396  
Jun 28   398592  
Jun 29   395246  
Jun 30   391604 <--tonight
Jul 01   387893  
Jul 02   384299  
Jul 03   380954  
Jul 04   377939  
Jul 05   375291  
Jul 06   373024  

The Moon was farthest from the Earth about a week ago (June 25), and will be closest around July 10. Right now, the Moon is a little bit farther then average, so if you were to measure its apparent size very carefully, you'd see it's smaller than usual!

Anyway, whatever reason works for you is good enough for me tonight. Go out and take a look. And when you do, take a look to the "upper right" of the Moon -- the bright "star" there is the mighty planet Jupiter, fully 40 times the diameter of the Moon, but well over 1600 times as distant. That's where distance really matters!

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