| The Magazine of the CALIFORNIA ACADEMY OF SCIENCES |
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SKYGUIDE July to September 2004
July1: NASA's Cassini Spacecraft is expected to begin orbiting around Saturn today. During its four-year tour of Saturn and its moons, Cassini will make 74 circuits and drop the ESA's Huygens atmosphere probe into the clouds of Saturn's largest moon, Titan. July 2: Full Moon at 4:10 AM. Although it's said that no shadows are visible on the Full Moon, some dedicated Moon-watchers claim that even at this time a slight terminator (the line marking the division between day and night) is visible along the edge of the Moon's disk. Occurring only 12 hours after perigee (the Moon's closest approach to Earth), this Full Moon produces higher tides than usual. July 17: New Moon at 4:24 AM. Arguably, the youngest crescent Moon observed by the unaided eye is about 15 1/2 hours past New. Look for it low in the northwest after sunset tonight. For Muslims, this sighting would mark the start of Jumada II, the sixth month of the lunar-based Islamic calendar. July 20: Thirty-five years ago on this date, humans walked on the Moon for the first time after NASA's Apollo XI module "Eagle" landed on the Sea of Tranquillity. July 31: Full Moon at 11:05 PM. This is the second Full Moon in the same calendar month, and is popularly known as a "Blue Moon" though no color change occurs. Major volcanic eruptions or forest fires can send up particles that scatter the Moon's light and give it a bluish tint. August 12: Peak of the Perseid meteor shower before dawn. The show is best observed after midnight, away from city lights. Remember that meteor-watching requires patience. August 16: New Moon at 6:23 PM. The earliest chance to see the young crescent Moon will be tomorrow night. August 27: Uranus at opposition, rising at sunset and located above the horizon all night long, is well-positioned for viewing against the faint stars of Aquarius (see box.) Observers may want to wait several nights for the bright Moon to move away. August 29: Full Moon at 7:21 AM, rising about a half-hour after sunset against the stars of Aquarius. September 14: New Moon at 7:28 AM. At sunset tonight, the Moon will be about 12 hours old - too young to be seen. September 22: Fall Equinox (for the Northern Hemisphere) at 9:30 AM. September 28: Today's Full Moon—the nearest one to the September equinox—is also called the "Harvest Moon." The pumpkin-like color of the rising Full Moon is an atmospheric effect. Its apparent large size is a psychological phenomenon known as the "Moon Illusion." The Planets Mercury Reaching eastern elongation around July 26, the most elusive of the naked-eye planets is visible only briefly before retreating into the Sun’s glare. It reappears in September, making a quick leap upward from the eastern horizon before dawn. It reaches western elongation (greatest angular separation from the Sun) around September 8, and drops back into the Sun’s glare by mid-month. Mercury is in conjunction with fainter Mars on July 10, separated by 0.16 degree—about one-third the apparent diameter of the Full Moon. Venus The brightest planet dominates the predawn sky. From the beginning of July to the end of August, it gradually doubles its altitude above the eastern horizon if seen at the same time each morning. Look nearby in early July for the red star Aldebaran (the eye of Taurus). Venus is less than 2 degrees (four times the diameter of the Full Moon) from Saturn on August 31. Mars Located on the other side of the Sun from Earth, Mars is at its most distant and faintest, making it difficult to find. The closest conjunction between two planets this year takes place when the Red Planet and (for now) brighter, swift Mercury are 0.16 degrees from each other on the evening of July 10. Mars disappears from view by the end of July, not to be visible again until October. Jupiter The King of the Planets is located against the stars of the King of Beasts—Leo—slowly making its way eastward toward the stars of Virgo. Look for it shining in the west after sunset, gradually dropping lower as the season progresses, setting about 4 minutes earlier from night to night. Saturn Not visible at the start of July, Saturn has descended into the Sun’s glow. In conjunction with the Sun on July 8, it passes into the morning sky and appears in the east before dawn late in the month. Through August, watch as Jupiter climbs toward brilliant Venus, drawing within 2 degrees of it (about 4 Moon diameters) on the morning of August 31.
(Times are for San Francisco, CA, and will vary slightly for other locations.)
Bing F. Quock is a member of the Morrison Planetarium staff at the California Academy of Sciences. bquock@calacademy.org |