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Keep Your Eye on the Sky

The Moon will became full on February 27th and this full Moon is special because it?s the biggest and brightest of the year. ?Not all full Moons are alike,? says astronomy professor George Lebo. ?Sometimes pollution or volcanic ash shades them with interesting colors. Sometimes haloes form around them -- a result of ice crystals in the air. This full Moon is unique in another way. It will be closer to Earth than usual.

?The moon's orbit around our planet is not a perfect circle,? says Lebo. ?It?s an ellipse.? At one end of the ellipse (called apogee) the Moon is farthest away from Earth, and at the other end (called perigee) the Moon is closest to us.

Wednesday?s full moon was near perigee and appeared 9% wider than normal and shone 20% brighter.

The extra moonlight is caused, in part, by the Moon?s nearness to Earth. But the Sun is closer to Earth, as well. Lebo says, ?Every year during northern winter, Earth is about 1.6% closer to the Sun than normal. The Moon reflects sunlight, so the Moon is brighter during that time.?

This effect is not the same as the ?Moon Illusion,? a trick of the eye that makes Moon rising near the horizon appear swollen. The Moon this week really is bigger and brighter.

The first three full Moons of 2002 will all be brighter-than-average. All three happen when the Moon is near perigee, and when Earth is relatively close to the Sun. Full Moons later this year will be smaller and dimmer by comparison. August?s full Moon will be about one-third dimmer than February?s.

?The human eye can easily discern a 20 or 30% difference in the brightness of two similar light sources,? says eye doctor Stuart Hiroyasu, so a sky watcher can certainly tell the difference.

Many northern landscapes in February remain covered with snow. Snow reflects about two-thirds of the light that hits it, while bare ground reflects only about 15%, so a snowy moonlit landscape always seems remarkably bright.

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A newly discovered comet, now approaching the Sun and Earth, could develop into a bright object can will be able to be seen with the naked eye in coming weeks. Even city dwellers should be able to spot it.

Kaoru Ikeya of Japan and Daqing Zhang from China first sighted the comet in the constellation Cetus, the Whale, on February 1st. Both described it as a weak glow in their telescopes with no mention of a tail.

The comet has been named Ikeya-Zhang. The latest orbit calculation indicates it will pass closest to the Sun on March 18 at a distance from Earth of 47.1 million miles. After rounding the Sun, the comet will continue moving toward Earth, making its closest approach to our planet on April 28, when it will be 37.6 million miles away. The best chance we will get to see Ikeya-Zhang will be in late April, when it is expected to become bright enough to spot easily with the naked eye, perhaps even in daytime.

Ikeya-Zhang?s expected path across the sky in the coming weeks means it will be easier to see in the Northern Hemisphere. During most of March and early April, the comet will be visible near to the north-northwest horizon about an hour after sundown. Bright moonlight may hinder observations during the last week of March.

After the first week of April, with the Moon no longer a factor, the comet will also be visible in the morning sky, rising earlier and getting progressively higher above the northeast horizon each night.

The first few bits of data suggest that Earthlings may have seen this comet before. Initial calculations place this comet?s orbit in a path similar to comets that passed the Earth in 1532 and 1661, but more detailed calculations place it in line with the comet of 1532. Further study will determine whether comet Ikeya-Zhang and the famous comet of 1532 are the same.

The 1532 comet?s claim to fame is that it might have been the brilliant daytime comet seen by the Aztecs and Incas just as the Spaniards arrived in the New World. Such astronomical events were considered ominous, and at the time it was seen as a prediction of the collapse of the Aztec and Inca empires. The 1532 comet was apparently a bright comet according to Oriental records.

Brian Marsden of the Smithsonian Astrophysical Observatory in Massachusetts says ?a revolution period of 400-500 years (for Ikeya-Zhang) is likely,? meaning this may be a return of the 1532 comet.

?In recent days, several observers have made their own independent calculations suggesting that Ikeya-Zhang might have an orbital period of roughly 500 years, making for a strong argument that there may indeed be a direct connection with the comet of 1532,? says John Bortle of Sky & Telescope magazine.

Bortle believes the new comet will have a magnitude of around 3.0. The 1986 appearance of Halley?s comet, considered disappointing by many, also peaked at around magnitude 3.0. In contrast, comet Hale-Bopp, in April 1997, attained a brightness close to magnitude -1, or about 60 times brighter than Halley.

Binoculars or a small telescope will allow most skywatchers to see Ikeya-Zhang?s fuzzy head, called a coma, and its tail.

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