Last modified: Tuesday, June 29, 2004
Cassini spacecraft arrives at Saturn
BLOOMINGTON, Ind. -- Saturn will pass behind the sun during July, reappearing at the end of the month very low in the east-northeast to the lower left (north) of brilliant white Venus an hour and a half before sunrise.
The Cassini spacecraft is approaching Saturn at the end of a voyage of nearly seven years. After passing through a gap between two of the planet's rings on July 1, the spacecraft will begin a four-year tour of the planet, including its moons and famous rings. Information and images are available at http://saturn1.jpl.nasa.gov/.
If you can rise before the sun does this month, you'll see Venus as a majestic "morning star" in the east. The brilliant white planet will be paired with the bright orange star Aldebaran in the constellation Taurus the Bull low in the east-northeast at dawn. Venus will reach its peak of brightness on July 14-15.
Jupiter will be the bright "evening star" in the west after sunset during July, but the giant planet will set earlier each night. By the end of the month it will be gone less than two hours after the sun.
Mars and Mercury will be far to the lower right (north) of Jupiter in the west-northwest, and you may need binoculars to find them in the glow of twilight. The bright stars Pollux and Castor of the constellation Gemini the Twins will serve as markers at the start of the month, with pale orange Mars to the upper left (south) of Pollux and white Mercury to the lower left. Though Mercury will be the lowest of the four objects, it will maintain its position close to the horizon as the weeks pass while Mars, Pollux and Castor glide downward past it and disappear.
Mars will pass extremely close to Mercury on the evening of July 10, the closest planet-to-planet encounter of the year. Look for them with binoculars a half hour or so after sunset very low in the west-northwest. Mars will be considerably fainter than Mercury, almost 100 times dimmer than it was during its historic close approach to Earth last August.
As Mars drifts away from Mercury to the lower right after their rendezvous, the bright white star Regulus in the constellation Leo the Lion will approach Mercury from the upper left, followed by brilliant white Jupiter. By month's end, the evening sky after sundown will be rather crowded near the western horizon.
Viewing information and graphics for the planets are available at http://www.space.com/spacewatch/.
Opportunities for telescope users
At the June meeting of the American Astronomical Society, the organization's Working Group for Professional-Amateur Collaboration announced the inauguration of an online registry service. This is a searchable database that allows amateur astronomers to describe their abilities and equipment and professionals to make known their observational needs. It's an opportunity for qualified amateurs to participate in astronomical research. If you're interested, check it out at http://skyandtelescope.com/news/article_1278_1.asp.
Although the full moon on July 31 will look ordinary, it will have a special name -- "blue moon" -- because it will be the second full moon of the month, after the first one on July 2. The origin of this familiar phrase depends on which meaning is intended. 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. 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 http://www.inconstantmoon.com/cyc_blue.htm.
A beautiful phenomenon of the twilight sky called noctilucent clouds is sometimes visible in the Northern Hemisphere during summer. These wispy clouds form when water vapor condenses onto meteoric dust particles about 50 miles above Earth's surface. Noctilucent clouds are so thin that they are invisible in daylight. They become visible only when the sun is just below the horizon, either in the northwest after sunset or in the northeast before sunrise, when the background sky is dark enough but there is still some light. Look for delicate bluish-white veils above the place where the sun is below the horizon. They are easy to distinguish from regular clouds, which appear dark against the morning or evening twilight. Photographs of noctilucent clouds can be seen at http://cumulus.helsinki.fi/~tpnousia/nlcgal/nlcgal.html.
The Delta Aquarid meteor shower will peak on July 28, with meteors visible for several nights before and after the peak as well. The bright streaks will appear to come from a point in the constellation Aquarius in the southern sky during the hours just before morning twilight. In a dark sky there might be as many as 20 meteors per hour at the peak, but this time the nearly full moon will overwhelm the fainter meteors. Some other minor meteor showers will also be spoiled by moonlight during the last week of July, but that means the moon will not be around to interfere with the major Perseid shower that will peak in mid-August.
The moon will be full on July 2, at third quarter on July 9, new on July 17, at first quarter on July 25 and full again (a blue moon) on July 31.