Solstice Moon

This week's Full Moon, which takes place just four days before the summer solstice, will appear unusually big and colorful to observers in the northern hemisphere.

June 14, 2000 -- As moms and dads can testify, the lengthening days of June present some special parenting challenges. For example, have you ever tried to explain to a 3-year old how it can possibly be bedtime when the Sun is still shining outside? The tricky part is describing the tilt of the Earth's axis and the approaching summer solstice. Most kids -- even the children of astronomers -- just won't buy it.
On June 16, 2000, the problem could be even worse than usual. Instead of a dark, sleepy night sky following sunset, the blazing rays of a bright full Moon will come streaming through bedroom windows. This June's full Moon occurs just 4 days before the 2000 summer solstice -- the longest day of the year in the northern hemisphere.

Right: Duane Hilton's rendering of moonrise over Half Dome in Yosemite National Park.
"Full Moons that occur close to the summer solstice are special because they follow the lowest path across the sky of all of the year's full Moons." explains Dr. George Lebo, a NASA/Marshall Space Flight Center Summer Faculty Fellow. "Moons seen just above the horizon look much larger than normal. It's an optical illusion, of course, but it's still a pretty sight."

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Anytime the Moon is full the Sun and the Moon are on opposite sides of the Earth. If one is high in the sky, as the Sun will be near the beginning of summer, the other must be low. On June 16, the Moon will never climb more than 30 degrees above the horizon as seen from mid-latitude sites in the northern hemisphere. Why does a moon viewed close to the horizon appear bigger than one seen high in the sky? It's a question that scientists and philosophers have debated for thousands of years. The Moon is same distance away in both cases, it shines with the same brightness, and it subtends the same angular diameter (1/2 degree). Logically, there should be no difference, but most observers perceive one anyway.

According to the most popular explanation, which springs from the "apparent distance theory" offered by psychologists Kaufman and Rock in 1962, a moon viewed near the horizon seems farther away than one shining down from overhead. Curiously, this causes the horizon Moon to appear bigger (we usually think of more distant objects as appearing smaller). The effect is similar to the 'Ponzo' railroad track illusion, illustrated here.

Right: In 1913 Mario Ponzo presented the well-known railroad track illusion in which two identical bars are drawn across a pair of converging lines. The upper yellow bar appears much larger because it spans a greater apparent distance between the rails. In fact, the two bars are exactly the same width. This effect may be at work with the mysterious horizon moon illusion. Distance cues like foreground mountains and trees may cause the horizon moon to appear more distant than a moon that is high in the sky. As in the Ponzo illusion, the more distant-seeming Moon appears wider. In fact, the Moon subtends a constant 1/2 degree angle no matter how high it is above the horizon. It's all a trick of the eye.

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Editor's Note, June 15, 2000: Dr. Carl Wenning of Illinois State University notes that airline pilots flying at very high altitudes also experience the 'Moon Illusion.' This suggests that foreground objects aren't the only important factor. According to Wenning and others, the horizon Moon seems more distant because the sky appears to be a flattened dome, with the top less distant than the edges. A third and totally different explanation of the Moon Illusion may be found at Prof. Don McCready's web site at the University of Wisconsin.

The illusory nature of June's swollen full Moon won't detract from its beauty. In fact, not only will the Moon seem bigger than normal on June 16, but it's likely to appear more colorful, too. For the same reason that sunsets can be vivid red, the low-hanging moon frequently takes on a beautiful pink or orange hue as a result of scattered moonlight in Earth's dusty atmosphere.

Jack Horkheimer, of the Miami Museum of Science and Space Transit Planetarium, notes that the term honeymoon may have its origins in the fact that the full moon of June often looks honey-colored and June is traditionally a month of many marriages. [ref]

This brings us back to 3-year olds. If your children are still awake after sunset on June 16, a field trip to the back yard for a view of June's wonderful full moon may be in order. A fun activity to try is looking at the moon directly and then through an aperture (e.g., 'pinch' the moon between your thumb and forefinger or view it through a tube, which hides the foreground terrain). Can you make the optical illusion vanish? The best times to try will be during the hours just after sunset (or before sunrise) when the bright moon is as low as possible.

Above: This picture of the full Moon was captured on 22 December, 1999, by photographer Rob Gendler. The light regions are very old heavily-cratered highlands. The dark 'maria' (seas) are huge impact craters that were later flooded by molten lava. Most of the Moon's surface is covered with regolith, a mixture of fine dust and rocky debris produced by meteor impacts. [more information about the Moon from the Nine Planets web site]