Jack Lucentini Moon viewers in the Northern Hemisphere will get a special treat tonight.
Our little companion orb will look exceptionally big and colorful, because it will stay close to the horizon all night.
"It's an interesting challenge for students to go out and measure the size of the moon when it's close to the horizon, and when it's high up -- it's thesame size."
This creates an optical illusion that increases the disc's apparent size. The moon will also look redder, because its light must travel further through our dusty atmosphere before reaching our eyes. The thicker atmosphere scatters and reddens the light.
The reason the moon will hug the horizon so closely is that it will be just four days before the summer solstice, which will be on June 21, the longest day of the year.
Does this moon look bigger
The Northern Hemisphere is tilted closer to the sun at summer solstice than during the rest of the year -- and thus farther from the moon, if there is a full moon about then. That's because a full moon is always on the opposite side of Earth from the sun.
As a result, the moon "never gets very high in the sky," said George Lebo, an astronomy professor at the University of Florida and summer faculty fellow at NASA's Marshall Space Flight Center.
Tonight's moon will never climb higher than 30 degrees above the horizon, or one-third of the way to the top of the sky.
The event will doubtless give new fuel to a debate that has plagued philosophers and scientists for centuries. No one is sure why the moon seems so much bigger near the horizon. It is not -- it spans the same width, both in actual space and in our visual field.
or does this one?
"It's an optical illusion," Lebo said. "It's an interesting challenge for students to go out and measure the size of the moon when it's close to the horizon, and when it's high up -- it's the same size."
One theory holds that the eye interprets the moon as being bigger near the horizon because the visual cues we see around it, such as distant mountains, coax us into seeing it as farther away. When we see something at a greater apparent distance, and it takes up the same space in our visual field, it looks bigger.
That supposition received a boost earlier this year from a new experiment. The researchers showed people perceive the moon as four times more distant when it is near the horizon than when it is at the top of the sky.
In the experiment, published in the January 4, 2000 issue of the Proceedings of the National Academy Sciences, people were asked to maneuver a computerized "virtual" moon until it seemed to be half the distance to the actual moon. This half-distance was longer when the moon was at the horizon.
One way to make the illusion vanish, some experts suggest, is to view the moon through a tube so that surrounding objects are blocked from view. Then the moon will appear the same size whether it hugs the horizon or soars high in the sky.
For those who miss it tonight, next month's full moon will be almost as good, Lebo said.
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