Stargazing Notes for August 1990 U.S. Naval Observatory Susan D. Glutting (202) 653-1541 Planets Mercury is at its greatest elongation 27 degrees east of the sun on Aug. 11. The shallow angle of the ecliptic relative to the horizon is responsible for its difficult viewing. It is visible due west, just above the horizon directly after sunset the second week of this month. The little planet is the brightest object in this area. Venus is moving through Gemini; it passes 7 degrees south of Pollux Aug. 8- 9 and is lined up with the twin stars, Castor and Pollux, mid-month. It rises about two hours before the sun and stands at 7 degrees above the horizon at sunrise on Aug. 12. Venus passes only 0.04 degrees north of Jupiter in a dazzling conjunction. It is the closest conjunction between our two brighest planets between 1949 and 2082. Best seen in East Asia and Japan, the two planets will be within a half of a degree of each other for viewers in North America. Mars moves from Aries into Taurus late in the month. It appears in the east- northeast horizon around midnight, becoming high in the south by dawn. The red planet swells in brightness from 0.0 to -0.4 magnitude this month because Earth is rapidly catching up to Mars due to our faster orbit around the sun. Jupiter in Cancer, moves rapidly west of the Sun and into the morning sky, where it can be seen about 20 degrees above the eastern horizon at sunrise. It closes in on Venus from the lower left (see Venus above). On Aug. 18, the dominant planet of the solar system disappears behind the waning cresent moon shortly before sunrise as seen in the western United States and Canada. Saturn in Sagittarius, is low and cream colored in the southeast at sunset and is brighter than most of the stars in the summer sky. It sets after midnight. August Tid-Bits August 1 Summer is half over; celebrate Lammas, the day halfway between the summer solstice and the fall equinox. August 2 U.S. Army accepted first aircraft from the Wright Brothers in 1909. August 3 First variable star, Mira (Omicron Ceti), discovered in 1596 by Fabricius. August 4 Huge Solar Flare observed in 1972. August 5 Neil Armstrong s Birthday. He was the irst man to set foot on the moon on July 20, 1969 August 11 Deimos, Martian moon, discovered in 1877 by U. S. Naval Observatory astronomer Asaph Hall. August 12 Echo 1 launched in 1960. August 13 First test glide of Space Shuttle in 1977 Asteroid 7, Iris, discovered in 1847 Asteroid 72, Miobe, discovered in 1861. August 14 Valeriy Ryumins birthday. He has spent more time in space than any other person, about 362 days. August 16 Asteroid 46, Hestia, discovered in 1857. August 17 Phobos, Martian moon, discovered in 1877 by Asaph Hall. August 18 Element helium discovered in the sun in 1868. August 20 Viking 1 launched in 1975 Voyager 2 launched in 1977. August 25 Asteroid 84, Clio, discovered in 1865. August 29 Asteroid 74, Galatea, discovered in 1862. August 31 Comet Howard-Koomur-Michels collided with the sun in 1979. Throughout the month, look for Sirius, the Dog Star, before dawn. The phrase dog days refers to this star. Meteors The Perseid meteor shower peaks on the 12th of this month. The Perseids are remnants of the passage of Comet Swift-Tuttle which returns every 120 years, give or take two years. The Perseids were first recorded by the Chinese on July 17, 36 A.D. Unfortunately, this is a poor year for observing the Perseids because the light of the waning gibbous moon interferes with viewing. Moon The moon reaches its greatest southern declination on Aug. 2nd (-27 degrees). August 3rd is an active date for the moon as it passes 2 degrees south of Uranus, 3 degrees south of Neptune and 1.6 degrees south of Saturn. Then, on the 13th, the moon travels 7 degrees north of Mars. On Aug. 15th, the moon reaches its greatest northern declination (+27 degrees). August 18th finds the moon passing 0.5 degrees south of Venus and 0.4 degrees north of Jupiter. The moon slides 0.2 degrees south of Mercury on the 22nd. Finally, toward the end of the month, the moon moves 2 degrees south of Uranus on the 30th and 3 degrees south of Neptune on the 31st. The full moon for August is called the Green Corn Moon or Grain Moon. On this full moon, Aug. 6, the earth passes between the sun and moon. The moon does not pass completely into the umbra ( the darkest part of the earth's shadow), so we can only see a partial eclipse. It will only be visible for viewers in the area around the Pacific Ocean and the Far East. Full Moon August 6 Last Quarter August 1#New Moon August 20 First Quarter August 28 Occultations: On Aug. 18th an occultation of Jupiter by the moon will be visible from central North America, southern Greenland, northern Atlantic, southwestern Europe and northern Africa. Also on Aug. 18th, an occultation of Venus by the moon will be visible from Indonesia, the Philippines, northern Australia, New Gunea, northern New Zealand and the southern Pacific. The Dog Days of August The month of August is upon us once again, and with it, come the long, hot, dog days of summer. The term dog days refers to Sirius, the Dog Star, which lies in the constellation Canis Major, or the Great Dog. Sirius has the greatest apparent brightness of any star but, it is not the most luminous. Throughout Latin literature, there are many references to the Dog Days, which followed the heliacal rising (rising with the sun) of Sirius in the summer. These hot, parched days were thought, by that time, to derive some of their ferocity and dryness from the searing of Sirius. A tradition arose of Sirius being red because it was, in fact, red at its heliacal rising; just like any other celestial body seen near the horizon is reddened. When making a rhetorical allusion to the Dog Days, the Latins would often speak of Sirius being red at the time. Numerous influences, good and evil, have been attributed to the star, and in this regard it has been the object of many superstitions. Roman farmers sacrificed fawn-colored dogs to it at festival times and tried to complete their farm work before its ascent. Chinese astrologers warned of attacks from thieves when Sirius was unusually bright. Owing to medical (as well as astronomical) ignorance, people of centuries past attributed famine and plague to its baleful influence on the weather. Even the great Hippocrates, Greek physician known as the Father of Medicine, fell prey to this delusion! These notions of SiriusÕ unhealthy association with the midsummer heat have persisted to the present time in the form of the expression, dog days. As a winter star, Sirius was also blamed for uncomfortable extremes of cold! On winter evenings we often observe its white radiance mirrored by the gleaming snow covering the countryside. Observations of Aldebaran (in Taurus), Arcturus (in Bootes), and Sirius in the eighteenth century initiated a new era in astronomy with Dr. Edmund HalleyÕs discovery of the proper motion of stars in 1718. More than one hundred years earlier Giordano Bruno correctly guessed that the stars are not fixed, but move about in space. He was later burned at the stake for heresy. Halley, by comparing his observed positions of Sirius, Aldebaran and Arcturus with HipparchusÕ data, found that the three stars had altered their positions with respect to backround stars in the same fields; thus Bruno's early hypothesis gained credence, fundamentally changing prevailing concepts and hinting at forces in the universe that act on the stars.