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]

Mercury Rising

On Saturday evening, June 3, a slender crescent Moon and the elusive planet Mercury will appear together for stargazers just after sunset.

June 2, 2000 -- There will be two good times to see the elusive planet Mercury this year, and next week is one of them.

Mercury is the solar system's innermost planet, so it never strays very far in the sky from the blinding glare of the Sun. Its angular separation from the Sun (or elongation) is always less than 28 degrees. Mercury approaches its maximum eastern elongation on June 9, 2000. It will be 24 degrees from the Sun, appearing as a bright zero-magnitude object above the western horizon after sunset.

Above : Artist Duane Hilton's rendition of the close encounter between the Moon and Mercury after sunset on Saturday, June 3, 2000. The setting is Yosemite National Park in north-central California. The Moon and Mercury will appear close together in the sky, but they are really very far apart. On June 3, Mercury will be 139 million km from Earth while the Moon is only 359 thousand km away.

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Although Mercury will be a little farther from the Sun on June 9, the best time to look will be six days earlier on Saturday evening, June 3. That's when the slender crescent Moon will pass less than 3 degrees from the planet. All you need to see the show is a clear view of the western horizon. Go outside just after sunset and look west-northwest. Mercury will appear to the upper right of the Moon's waxing crescent for observers at mid-latitudes in both hemispheres.

While you're enjoying the rare appearance of Mercury in plain view, don't miss another notable sight: cradled in the arms of the slim crescent Moon will appear the ghostly outline of the full Moon, a dim glow that astronomers call "Earthshine."

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Like all the planets we see in the night sky, including Mercury, the Moon does not shine by its own light. It reflects sunlight. The side of the Moon facing the Sun shines brightly; the side facing away is nearly dark. The only significant illumination on the "dark side of the Moon" is due to Earthshine -- sunlight that bounces off the Earth and falls on the lunar surface. A slender crescent Moon with Earthshine is widely regarded as one of the most delicate and beautiful sights in the night sky.

"The phase of the Earth as seen from the Moon is nearly full when the Moon is crescent," says Dr. George Lebo, a Marshall Space Flight Center 2000 Summer Faculty Fellow. "Because the Earth is four times bigger than the Moon and is a ten times better reflector, the 'Full Earth' is 160 times brighter than the 'Full Moon.' That's why earthshine is so noticeable."

Left : The western sky on June 9, 2000, just after sunset at mid-Northern latitudes. Mercury appears in the constellation Gemini about 14 degrees above the WNW horizon to mid-latitude observers in both hemispheres.

Now that you've spotted Mercury using the Moon as a finder on June 3, you can watch Mercury's progress in the sky throughout the month. In early June, Mercury will remain at about the same height above the horizon each night if you look at the same time. After the 10th, the planet will head back toward the Sun. By the third week of June, Mercury will be almost impossible to find as it becomes lost in the Sun's glare.

After June, the best time this year to spot Mercury from northern latitudes will be in mid-November when the planet is 14 degrees over the horizon before sunrise.

THE UPS AND DOWNS OF MERCURY Mercury approaches maximum elongation six times in the year 2000, but each time is not an equally good opportunity to see the planet. If the ecliptic plane (the path in the sky followed by the Sun and planets) is nearly parallel to the horizon when the Sun sets or rises, Mercury can be at a low altitude even when its elongation is large. That's what happens on October 6, 2000, when Mercury is 25 degrees from the Sun, but only about 4 degrees above the horizon at sunset for observers at mid-Northern latitudes. Dates marked in red denote the best times to spot Mercury during the remainder of 2000.

dates of maximum elongation for Mercury	elongation	morning or evening star	altitude at sunrise/sunset (as seen from mid-Northern latitudes)
Feb. 14, 2000	18o	evening	12o
Mar. 28, 2000	27.8o	morning	7o
June 9, 2000	24o	evening	14o
July 27, 2000	19o	morning	11o
Oct. 6, 2000	25o	evening	4o
Nov. 15, 2000	19o	morning	14o

Mysterious Mercury
If it's any consolation to the often-frustrated Earthbound observers of Mercury, NASA spacecraft have a hard time, too. For instance, ground controllers can't point the Hubble Space Telescope toward Mercury because small pointing errors might allow intense sunlight to damage sensitive cameras. The only spacecraft to explore Mercury close-up was Mariner 10, which executed 3 flybys of Mercury in 1974 and 1975, surveying just 45 percent of its surface.

Last week, astronomers from Boston University announced that they had captured unprecedented ground-based pictures of Mercury covering parts of the planet's surface that Mariner 10 missed. The images, taken at the Mt. Wilson Observatory in California, revealed surface markings similar to the bright craters and dark maria ("seas") found on the Moon. [Boston University Press Release]

Right : This image of a portion of Mercury's surface not photographed by Mariner 10 in 1974-75 was obtained by Boston University astronomers using observations made at the Mt. Wilson Observatory in August 1998. Hundreds of thousands of pictures taken with short time exposures (1/60th) were examined to find the 30 images with the clearest surface markings, taken during instances of "perfect seeing" through the Earth's atmosphere.

The Boston team plans to make more observations this fall. They might even succeed in detecting sodium in Mercury's wispy atmosphere, which consists of atoms blasted off its surface by the solar wind. Because the planet is so hot, these atoms quickly escape into space. In contrast to the stable atmospheres of Earth and Venus, Mercury's atmosphere is constantly being replenished.

Mercury's dynamic atmosphere is just one of the planet's many exotic aspects. Mercury's density is the higher than any planet except the Earth -- its iron core is probably bigger than Earth's entire Moon! It is the only terrestrial planet besides Earth to possess a global magnetic field. Temperatures on the surface of Mercury vary from nearly the highest in the solar system at the equator to among the coldest in permanently shadowed areas at the poles. Radar data suggest that fiery Mercury, like the Moon, actually harbors polar deposits of ice.

In 2004, scientists hope to launch a satellite called MESSENGER (MErcury: Surface, Space ENvironment, GEochemistry and Ranging) to study Mercury in greater detail. The spacecraft is slated to enter orbit around Mercury in 2009 carrying instruments to answer the following questions:

• What is the origin of Mercury's high density?
• What are the composition and structure of its crust?
• What is Mercury's tectonic history, and is its surface shaped by volcanism?
• What is the nature and origin of Mercury's magnetic field?
• What are the characteristics of the thin atmosphere and miniature magnetosphere?
• What is the nature of the mysterious polar caps?

If all goes as planned, MESSENGER will get the closest, clearest view ever of the solar system's innermost planet.

Above: This mosaic of Mariner 10 images shows that Mercury's surface looks similar to our Moon's. Each is heavily cratered and made of rock. Mercury's diameter is about 4800 km, while the Moon's is slightly less at about 3500 km (compared with about 12,700 km for the Earth). Mercury is the closest planet to the Sun, orbiting at about 1/3 the radius of the Earth's orbit. As Mercury slowly rotates, its surface temperature varies from an unbearably cold -180 degrees Celsius to an unbearably hot 400 degrees Celsius. [more information]
The MESSENGER mission is managed for NASA by the Johns Hopkins University Applied Physics Laboratory in Laurel, MD. The Principal Investigator is Dr. Sean C. Solomon of the Carnegie Institution of Washington. For more complete information on the mission, including animations of the trajectory to Mercury with flybys of Earth, Venus, and Mercury, visit the MESSENGER home page.

Editor's note: the opening line of this story was inspired by a recent episode of Jack Horkheimer's PBS program "Stargazer."