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The Honolulu Advertiser
Posted on: Sunday, April 29, 2007

Earth at prime tilt to view Mercury at apex

By Carolyn Kaichi
Bishop Museum

Mercury makes another regular appearance in the evening sky, as it bounces from eastern sky to western sky every other month and a half or so. We see the little planet best when it is at the maximum angle away from the sun (called greatest elongation), no higher than 27 degrees from the horizon. In May, Mercury will not only be at greatest elongation, but the tilt of our planet will position us at the best angle for the year to see it at its highest.

With Mercury in the spotlight, it's a good time to highlight a mission now on its way to the closest planet to the sun. The Messenger spacecraft launched in August 2004 and is one-third of its way toward its goal of orbiting Mercury by 2011.

Messenger will deliver new information about the smallest "official" planet in more than 30 years. Mariner 10 was the last spacecraft to study Mercury in 1975, when only 45 percent of the planet was imaged.

Mercury is the smallest planet (due to Pluto's reclassification), not much larger than our moon. The length of a day on Mercury is almost as long as its year, not so unusual since its orbit is so tight and fast around the sun that it only takes about 88 Earth-days to complete its revolution, as opposed to the 59 Earth-days it takes to rotate once. Its temperature ranges from 800 degrees on the sunny side to minus 280 degrees on the night side, and its atmosphere is negligible, similar to our moon.

Look for Mercury as a small, bright yellow "dot" shortly after sunset starting around May 11. You need a clear view of the west horizon until the planet gets higher in the sky by the following week or two. A much brighter and larger Venus shines above Mercury throughout the month.


It's Lahaina Noon season again, that time when the sun passes directly on top of our sky's zenith point. Because we live in the tropics the area defined from the Tropic of Cancer to the Tropic of Capricorn the sun's path to and from these boundaries goes over us twice a year. These periods occur in May and July, and as the sun reaches the zenith, it causes shadows to "disappear" as sunlight shines vertically down upon objects.

For the date and time of Lahaina Noon in your area, check our Web site: www.bishopmuseum.org/planetarium.


Another note of interest for May: We have a blue moon. A blue moon is a second full moon in a month. Since the moon's cycle is 29.5 days, it is possible to have a full moon at the beginning of the month and again at the end.


Lastly, the Eta Aquarid meteor shower peaks in the early morning hours on May 5, but conditions are not favorable this year. The moon is a waning gibbous, just three days past full and rises around 10:30 p.m. This keeps the night sky too bright to see a lot of meteors during the peak times from midnight to dawn. The radiant, or the "center" of the shower, rises at 2:30 a.m., making this a meteor event for the very motivated only.


Mercury: Mercury rises low and bright in the evening sky by May 11 and will continue to climb higher in the sky through the end of the month while losing some of its brilliance. On May 17, you may try to catch a peek of a very new moon next to the planet, though it will be challenging through the fading sunlight.

Venus: Although Venus dominates the sky in planetary brightness, it shares the spotlight with three other planets in May. But by far it remains the brightest object in the night sky aside from the moon. Like Mercury, this inner planet goes through phases similar to our moon that can be observed with a small telescope. Throughout May, we see less of the planet illuminated (like a crescent) though it becomes larger as Venus comes toward us in its orbit. Venus makes a quick path from Taurus to Gemini during May, and on May 19 can be seen very close to a crescent moon.

Mars: Mars is still quite distant from us and appears small and unassuming in the morning sky. It rises around 3:30 a.m. in the early half of the month and by 2:45 a.m. at the end of May. It's in the constellation of Pisces a large, sprawling group of stars that isn't easy to identify unless you are fairly familiar with the night sky. On May 12, however, you can find the reddish-orange planet by looking in the eastern sky before dawn, around 4:30 a.m. next to a crescent moon.

Jupiter: Jupiter is up in the sky by 9:30 p.m. from early May to the east of the brightest star in Scorpius, orange Antares. By the end of the month, it will rise two hours earlier as it heads toward opposition in June, when it will be in the sky all night. Jupiter far outshines the supergiant Antares, being a mere 34 light minutes away as opposed to Antares' 620 light years. (A light year is the distance light travels in an Earth-year, almost 6 trillion miles.)

Saturn: Saturn is straight above at zenith by the time darkness falls in the early half of the month. By the later half of May, it is in the western sky at dusk. During May, Venus and Saturn start to creep closer together, heading toward a rendezvous in June. Don't forget Saturn is still a nice sight in telescopes, and if you don't have one, come to our evening planetarium program at 7 p.m. May 4, when we open the observatory for night viewing. Call 848-4168 for reservations.

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