The Whole of the Moon

Tonight as the Full Moon rises it will appear huge on the horizon. This is true any Full Moon, but is especially true around the solstices, and this effect will be visible for the next one or two moonrises as well. For fun, go outside with a piece of paper and a nice new pencil with a nice new eraser. Observe the Moon as it rises. Turn away and write down how big the moon appears to you - "Nickel", "Quarter", "Dinner Plate", whatever. Maybe even sketch the Moon approximately the size that you remember it being. (No peeking! This is from memory!)

Now, turn back to the Moon and hold out the pencil at arm's length. Move the eraser so it covers the Moon. How did you cover something so big with something so tiny?

It's called the "Moon Illusion" and has to do with our inability to estimate the distance to faraway objects, as well as our distorted assumptions about the size and "shape" of the sky. For a pretty good article, check out this article on MSN:

Don’t miss a huge June moon illusion

And for a howler, read this article, which is linked from the other:

Is the ‘full moon’ merely a fallacy?

It's a bit of pedantry of the sort that really bugs me. The upshot: the Moon is never truly "Full" for an observer on Earth, because most of the time it is not directly in a line with the Sun and the Earth, and is therefore slightly off to one side, and therefore has a teeny tiny slice of shadowed side showing; in fact, the only time the Moon could be Full is when it is directly opposite the Sun from the point of view of an observer on Earth - which would place it dead center in our shadow for a total lunar eclipse. Of course, the percentage of the visible Moon that is in shadow when we say that it is "Full" is often vanishingly small. So let the pedants have their fun; for the rest of us practical, reasonable folk, we have a pretty good working definition of what it means for the Moon to be "Full."

But that's not the funny part. The funny part is this photo and accompanying caption:

Like the article said, the only time the Moon could be truly "Full" is during a lunar eclipse - and lunar eclipses always occur during Full Moons. The Moon in that picture ain't full. Not even close. Probably at least three days before Full, in a "Waxing Gibbous" phase. No Full Moon = No Lunar Eclipse. You would think that for an article being pedantic about a term used to refer to the Moon, they'd be a bit more careful with the choice of the illustration?

Anyway. As is always the case in these situations, you can get a much better explanation from M.G.J. Minnaert's excellent book Light and Color in the Outdoors (item 131, "Apparent enlargement of the sun and moon at the horizon," and item 132, "The connection between the apparent enlargement of heavenly bodies at or near the horizon and the shape of the celestial sphere.")

Title reference: This song by The Waterboys, shamelessly stolen from Gort's site. I probably heard this song once on the radio, twenty-some years ago. Thanks to Gort I have now heard it a dozen times or more.

I saw the crescent
you saw the whole of the Moon

Update, 10:20 PM:

Overexposed Moon rising over my neighbor's shed

Fast shutter (sports mode)

We were nearly clouded out, but the Moon finally broke through the clouds. I didn't have a clear view of the horizon, anyway.

Phil Plait had a bit on this last year on Bad Astronomy. I forgot about that, but it showed up on my SiteMeter!