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!
This
article was provided to SPACE.com by Starry Night Education, the
leader in space science curriculum solutions.
advertisement
