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THIS WEEK IN
CALIFORNIA WILD

Skyguide

July - September 1998

July 3  Earth at aphelion, or its farthest point from the Sun. At 94,508,694 miles, it's 3,104,992 miles farther than it was at its closest point (perihelion) in January. This occurs during the Northern Hemisphere summer, demonstrating that distance from the Sun has nothing to do with seasons.

July 9 Full Moon, also called the "Thunder Moon" by the Algonquin, the "Killer Whale Moon" by the Haida, and "Red Salmon Time" by the Nez Perce.

July 14 Spring Equinox, or the start of spring in the Northern Hemisphere ...on Mars.

July 23 New Moon

August 7 Full Moon. The traditional Algonquin name is "Sturgeon Moon," and the Cherokee name "Moon of the New Ripened Corn." Also, a penumbral eclipse of the Moon occurs--the Moon's south pole skims the edge of Earth's shadow. Mid-eclipse is at 7:25 p.m. pdt, though visible effects are imperceptible.

August 12 Peak of the annual Perseid meteor shower, named after the constellation Perseus, from which the meteors appear to radiate. Averaging 50 "shooting stars" an hour, the shower is spoiled this year by a waning gibbous Moon.

August 14 Last Quarter Moon. The bright, rayed crater Copernicus is obvious, with smaller Kepler just to its west.

August 21New Moon. An annular solar eclipse is visible from Indonesia and Polynesia. Because the Moon is slightly farther away than during a total eclipse, it doesn't completely hide the Sun--a thin ring (annulus) of the solar disc is visible around the Moon's silhouette.

September 6 Full Moon. Another penumbral lunar eclipse occurs, with the Moon passing more deeply into Earth's outer shadow than last month. Some slight darkening of the Moon's northern regions might be visible photographically. For early risers on the West Coast, mid-eclipse is at 4:10 a.m. pdt.

September 8 When the Earth's north pole is most-tilted toward the Sun, we call it Summer Solstice for the northern hemisphere. What do we call it on September 8th (other than Tuesday), when the Sun's north pole is most-tilted toward Earth?

September 20 New Moon; Rosh Hoshanah, (the first day of the Jewish new year) starts at sunset, calculated from the New Moon nearest the autumnal equinox.

September 22 Autumnal Equinox, or the start of autumn, at 10:38 p.m. pdt, for the Northern Hemisphere.

The Planets

Mercury This swift-moving planet zips from evening to morning, reaching greatest eastern elongation (angular separation from the Sun) on July 16. Inferior conjunction occurs on August 13 and reaches greatest western elongation on August 31 (the best morning showing of the year). It then drops back into the Sun's glare, reaching superior conjunction (passing behind the Sun) Sepember 25. This season, Mercury and Venus perform the first two meetings of a "triple conjunction" (see box). Venus A morning object this quarter, Venus is positively dazzling in its brilliance. Rising two hours before dawn in July, gradually later in August, it's finally drowned out in the Sun's glare in September. The Moon joins Venus for some pretty pairings on July 21, August 20, and September 19. Watch Venus gradually approach and pass Mars on August 4.

Mars Identify the "Red Planet" in the morning by its yellowish- orange hue. Compare its color with that of brilliant, white Venus nearby. During the summer months, Mars creeps from the stars of Taurus through those of Gemini and Cancer, then into Leo by the end of September.

Jupiter Rising after sunset, Jupiter moves slowly from the stars of Pisces to those of Aquarius. These are both faint con- stellations, so Jupiter should stand out prominently. Jupiter reaches opposition on September 15, rising at sun- set and visible all night long. This is the best time to observe it, but wait until the planet has risen high enough above the horizon.

Saturn Of the five naked-eye planets, bright-ringed Saturn is the slowest-moving, loitering on the border between Pisces and Cetus the Sea Monster. It rises near midnight in July, gradually earlier in August, and just after sunset by September. Like Jupiter, it should be easy to pick out against the faint stars in that vicinity.  

	Sunrise	Local Noon	Sunset
July 1	5:51 a.m. pdt	1:13 p.m. pdt	8:36 p.m. pdt
August 1	6:13 a.m. pdt	1:16 p.m. pdt	8:18 p.m. pdt
September 1	6:40 a.m. pdt	1:09 p.m. pdt	7:39 p.m. pdt
October 1	7:05 a.m. pdt	12:59 p.m. pdt	6:53 p.m. pdt
	Times are for San Francisco, CA

Planetary Pas de Deux

From the end of August through mid-November, two planets will perform a slow, stately waltz during which one will appear to pass the other three times. What accounts for this motion? From the Earth, Mercury and Venus apparently switch directions due to the angle of their orbits. As seen from Earth, Mercury is never farther than 28 degrees from our star. The angular separation of Venus from the Sun, from our point of view, can be as much as 47 degrees. These two planets can be seen close together ("in conjunction") only when Venus is within Mercury's smaller range. Venus was at its greatest angular separation west of the Sun in March, and since then it has gradually been moving eastward. Mercury, moving westward, passes Venus on August 27. This is the easiest of the three conjunctions to see, because it is away from the Sun's glare. After August 31, Mercury reverses its apparent direction, catching up with the slower-moving Venus on September 11, while still to the west of the Sun and in the morning sky. Then, Mercury scoots away toward its greatest eastern elongation in October. After that, it reverses its apparent motion once again, passing Venus for the third time on November 18, east of the Sun in the evening sky. Three conjunctions between the same two planets within one year is a "triple conjunction." The next triple conjunction between these two planets will occur between March and July in 2000.

Bing F. Quock is a member of the Morrison Planetarium staff.

Summer 1998

Vol. 51:3