Image: Night sky
Starry Night Software
Just as the sun is setting in the west this weekend, look to the east and see the harvest moon and Jupiter rising.
updated 10/1/2009 1:28:26 PM ET 2009-10-01T17:28:26

While the moon is always something special, the harvest moon, visible all of this coming week, is the most special of all. It is the subject of everything from epic poetry to popular songs.

What makes the harvest moon so special? Mainly it's the path it's following this week. The moon always travels close to the ecliptic, the path of the sun and planets in the sky, but this week the ecliptic is at a particularly shallow angle to the horizon. The result is that the moon never gets too far above the horizon all night long for a number of nights in a row, putting it literally "in your face."

There is a well known but poorly understood optical illusion known as "the moon illusion," whereby the moon, when low in the sky, appears much larger than it does when high overhead. This really is an illusion, as you can see for yourself by blocking the moon with a finger held at arm's length: the moon is no bigger on the horizon than overhead.

When the moon is low in the sky, it is also strongly subject to appearing yellow, orange, or red due to air pollution, particularly caused by forest fires this time of year.

So we have a  low moon, in your face, artificially enlarged, and often brightly colored: the famous harvest moon.

Take a look at this illustration. It shows the harvest moon as seen from Calgary, Alberta, at 6 p.m. this Saturday evening. The sun is setting behind you in the west, and the moon is rising in the east, Jupiter to the southeast. Notice the shallow angle between the ecliptic (in green) and the horizon, typical near full moon at this time of year. The effect of this shallow angle is that, even though the Moon moves about 12 degrees along the ecliptic every night, its rising time doesn't change very much from night to night.

This means a succession of nights with a bright moon low on the eastern horizon, lighting up the land just as the sun sets opposite it. This gives a few extra hours of light to farmers out harvesting their crops. Hence the name: harvest moon.

The harvest moon, while beautiful to look at, also presents some great "\photo opportunities. However, if you just take a snapshot of the moon in the landscape, you may be disappointed. Because the "moon illusion" doesn't affect a camera lens, the moon will look small.

If your camera has automatic exposure, the scene will look too bright and the moon will be overexposed. The trick to capturing the harvest moon in a photograph is first, to zoom in with your telephoto lens to make the moon appear larger, and secondly, to underexpose the picture by a couple of stops, to darken the landscape, saturate the colors, and expose the moon properly. Good luck!

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