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Hal Kibbey
IU Media Relations

Last modified: Monday, April 30, 2007


May brings "blue moon"

April 30, 2007

BLOOMINGTON, Ind. -- May will offer something unusual -- a "blue moon."

The moon will be full on May 2 and then full again on May 31. The second full moon in a month is traditionally called a blue moon. This last happened on July 31, 2004.

According to the Oxford English Dictionary, the first reference to a blue moon was in a proverb recorded in 1528, and it meant an obvious absurdity. Since then other meanings have emerged, ranging from the ecclesiastical calendar's name for the 13th moon in a year to the modern phrase "once in a blue moon," meaning very rarely. Blue moons are really not that uncommon, happening on average about once every three years.

The moon actually has appeared blue at times, because of smoke or dust particles in the atmosphere from huge forest fires, for example. More information is available at


Venus will reach the peak of its splendid display as the "evening star" during May, dominating the western sky during twilight and for more than an hour after dark. Enjoy the beauty of Venus while you can this month, for it will start to drop quickly toward the horizon in June.

Venus will have a close encounter with the crescent moon on May 19 for skywatchers throughout North America. The two will meet in the constellation Gemini the Twins, whose bright stars Pollux and Castor will keep watch directly above them.

During the second half of the month, look for Mercury well to the lower right of Venus in the west-northwest a half hour after sunset. This will be Mercury's best evening appearance of the year for viewers at mid-northern latitudes, and Venus will be the best guide you could ask for.

Saturn will be high in the southwest at dusk, far to the upper left (south) of Venus. The gap between the two planets will shrink by about half during May, as Saturn moves toward Venus. Keeping company with Saturn will be the constellation Leo the Lion, including Leo's brightest star, Regulus, a short distance to the left (south) of the planet.

Jupiter will rise around 11 p.m. at the beginning of May but soon after sunset by month's end. Jupiter will be opposite the sun early in June, so it is already visible for most of the night. Unfortunately, its position low in the south as it crosses the sky will make the view through a telescope less enjoyable for observers in the Northern Hemisphere.

Mars will continue to be low in the east-southeast before sunrise, difficult to see in the brightening sky.

Meteor shower

This month Earth will encounter a stream of dust left behind in space by Comet Halley, causing the Eta Aquarid meteor shower that will peak before dawn on May 5. This year, the nearly full moon will make meteors difficult to spot, particularly for viewers in the Northern Hemisphere. The number of meteors visible depends on the altitude of the shower's radiant, the point in the sky from which the meteors appear to come. In the Northern Hemisphere, the Eta Aquarid radiant will be low in the east-southeast at 4 a.m. The radiant will be three times higher in the Southern Hemisphere, favoring observers there.

The shower will be active from April 21 through May 12, and viewing conditions may be more favorable before the peak time on May 5. The meteors will appear to come from a point in the constellation Aquarius, which will rise in the east about two hours before the start of morning twilight.

More information about the Eta Aquarid shower and other major meteor showers of 2007 is available at

Moon phases

The moon will be full on May 2, at third quarter on May 10, new on May 16, at first quarter on May 23 and full again on May 31 (a "blue moon").