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Earthsky Tonight – Dec 23 2009 Star-hop to Mercury from the Summer Triangle

Courtesy of EarthSky
A Clear Voice for Science

December 23, 2009
December 23, 2009

The first three stars to pop out into your western sky after sunset are Vega, Altair and Deneb. These brilliant stars make up the huge asterism known as the Summer Triangle. An asterism is a star formation that is not an officially recognized constellation.

Don’t mistake the planet Jupiter (outside of our sky chart) for a Summer Triangle star. Jupiter beams in the southwestern sky, to the left of the Summer Triangle. Jupiter is far and away the brightest point of light in the December evening sky.

Although the planet Mercury is brighter than any Summer Triangle star, it probably won’t look brighter. Mercury hovers close to the horizon, where the twilight glare tends to obscure the planet’s luster. Some 45 to 60 minutes after sunset, draw a line from the star Demeb through the star Altair to locate Mercury near the horizon. Mercury sets less than 1.5 hours after the sun. If you can’t see Mercury with the unaided eye, try binoculars.

Mercury, the innermost planet, has the shortest year yet the longest day of any solar system planet. A year on Mercury is 88 Earth-days long, whereas a day on Mercury is twice that long: 176 Earth-days.

Written by Bruce McClure