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September Skies (2005)


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Highlights: Comet Journal, Martian Landers, Planet Plotting, Meteor Showers, Autumnal Equinox, September Moon

Focus Constellations: Draco, Cepheus, Lacerta, Cygnus, Vulpecula, Sagitta, Aquila, Delphinus, Equuleus, Pegasus

Comet Journal

Comet 9P/Tempel 1 survived its July 4th collision with the projectile launched by the "Deep Impact" spacecraft and proceeded onward in its orbit. When impact occurred, the comet was very close to perihelion (closest approach to Sun), slightly outside of the orbit of Mars. It is now crossing the asteroid belt and will reach aphelion (farthest distance from Sun) just inside the orbit of Jupiter in 2008. In September it will move eastward through Scorpius and at 11th magnitude may be seen by amateur astronomers using larger (10"+ diameter) telescopes.

Initial results of the data collected during and after impact reveal a surprisingly low water and high silicate content in the dust-like impact debris. The results of ongoing in depth analyses are scheduled to be released at the American Astronomical Society Meeting in England in September and in the September issue of "Science".

The other 11th magnitude comet, P/1983 V1 (Hartley-IRAS), is between Arcturus and the handle of the Big Dipper in September. It ascended through the plane of the solar system somewhat beyond the orbit of Mars on April 30th, reached perihelion on June 25th, passed closest to Earth on July 13th, and continues to climb through Canes Venatici in September. During the rest of 2005, it embarks on a descent that culminates when it again passes through the plane of the solar system near Jupiter's orbit in December 2006.

Martian Landers

After pulling away from Purgatory Dune, Opportunity circled the dune then spent 44 sols (July 6–August 19th) traversing 351 meters or 1150 feet farther through the dunefield on Meridiani Planum. On Sol 524 (July 15, 2005) the rover encountered the first rocks seen in many weeks and proceeded to find many more rocks and cobbles as it continued toward Erebus Crater about 160 meters farther south. On Sol 543 (August 3rd) the rover arrived at a cobble strewn rock outcrop adjacent to the crater. Microscopic imaging, alpha particle X-ray spectral examination, and Mössbauer spectral studies were conducted on the underlying bedrock, the soil, and some cobbles, including one called Arkansas. Then Opportunity moved to another outcrop of bedrock called Fruit Basket and conducted further examination with the same instruments during Sols 556-559. Total travel on Mars by August 19th was 5729 meters (3.56 miles).

During the last week of July and first two weeks of August, Spirit examined four rocks, Descartes, Bourgeoisie, Hausman, and Assemblee with its microscopic imager and spectrometers. By the end of the third week (Sol 578 – August 19th) the rover completed panoramic and thermal emission imaging from its perch within 70 meters of the highest summit of Husband Hill. It has traversed a total of 4,742 meters (2.95 miles) within Gusev Crater and Columbia Hills since arriving on Mars.

Planet Plotting

Venus (-4.0 magnitude) and Jupiter (-1.7 magnitude) highlight the western evening sky in September. Venus is 1.2° S.S.W. of Jupiter at 6 PM EST on the 1st, and the pair present a glorious sight after sunset. Venus moves eastward much more rapidly than Jupiter during September as Jupiter approaches conjunction with the Sun on October 22nd. The two will be separated by 30° by the end of the month.

Mercury (-1.1) will make a brief appearance in the morning sky in Leo at the beginning of September and then disappear behind the rising Sun as it approaches superior conjunction on September 18th.

Mars (-1.0) rises before 11 PM EDT in Aries at the beginning of September and will double in brightness, reaching magnitude -1.7 by month's end when it rises at dusk. Earth motion in orbit will rapidly reduce the distance to Mars as the orbital lead of the red planet diminishes from 25° to 15° during September. As a result, the apparent size of Mars will increase by 33% to 17.9 arcseconds. In comparison, Jupiter's size is over 30 arcseconds, and the size of the Moon is ~1800 arcseconds. At its peak on Oct. 30th when it is closest to Earth, Mars will be 20.2 arcseconds in diameter and have a magnitude of -2.3.

During the last opposition in 2003, Mars reached a diameter of 25.1 arcseconds and a magnitude of -2.9. It was closer to Earth than it has been for the last ~60,000 years because it was at its perihelion and Earth was not too far from its aphelion. However, views for those of us in the northern U.S. promise to be significantly better in 2005 than in 2003 because Mars will be more than twice as high above the southern horizon. The dense, turbulent air near the southern horizon will not degrade telescopic images of Mars nearly as much as in 2003.

Saturn (+0.3) has appeared in the morning sky in Cancer and will rise earlier each night as it moves eastward from its conjunction with the Sun on July 23rd. It will rise at 2 AM EDT by the end of the month.

Uranus (+5.7) is at opposition in Aquarius on September 1st and Neptune (+7.8) moved through opposition in Capricornus on August 8th. They are both well placed for viewing in the evening sky, and Uranus can been seen with the naked eye in very dark skies if your vision is good. Even though they could see it, the ancient Greeks did not define Uranus as a "wanderer" (planet) because it moved so slowly through the background of distant stars. It was not determined to be a planet until William Herschel first identified it as a comet in the spring of 1781 and further study of its orbit that summer confirmed that it was a planet.

Meteor Showers

On the night of August 12th/13th, North American observers were lucky if they saw 10 Perseid meteors per hour during the Meteor Shower of 2005 although some were quite bright and fast. European and Middle Eastern observers recorded higher rates of 20 – 30 per hour because they were on the leading edge of the Earth as it plunged into the stream of debris shed during recent passages Comet P Swift-Tuttle. The comet's period is 130 years and it is traveling back into the depths of the solar system after spewing debris in its wake while circuiting the Sun in 1992. As a result, the Perseids were quite spectacular during the balance of the 1990's. It will be 117 years before the comet resupplies the meteor stream with debris, so future showers may be more like that of 2005.

The meteor showers of September are minor, but during early and mid-September, the Alpha and Delta Aurigids combine to provide viewers of dark northern skies a relatively uniform meteor rate of almost 10 per hour, twice the estimated background rate of sporadic meteors.

Autumnal Equinox

On September 22, at 6:23 PM EDT, Earth will arrive at the orbital position where it leans toward the direction of orbital motion and a straight line from Earth to Sun is perpendicular to Earth's axis of rotation. This orientation results in similar periods for day and night, so it is called the time of the "equinox" (equi = equal + nox = night) and is traditionally thought to mark the start of autumn. There are more annual hours of day and fewer hours of night because sunrise and sunset are defined as when the top (north) of the Sun rises and sets. Atmospheric light refraction lengthens the day by bending sunlight so that the Sun appears to be above the horizon before it actually rises and after it actually sets. This delays the timing of equal day and night until after the Autumnal Equinox. The amount of delay varies according to latitude, from slightly over two minutes for the time of equal day and night at the equator to more than a day at the poles.

September Moon

The Full Moon of September is at 10:21 PM EDT on the 18th in Pisces. Since it is the full moon closest to the Autumnal Equinox, it is still known as the "Harvest Moon", the name used by Colonial Americans. It is also called the "Fruit" Moon during years when it is not the "Harvest Moon". It is the "Chrysanthemum Moon" in Chinese culture, the "Singing Moon" for the Celtic people, and was the "Barley Moon" in medieval England. The Anishnaabe (Chippewa and Ojibwe) call it "Manoominike-giizis" (Rice Moon).

September's tour of the planets by the Moon starts when a very thin waning crescent Moon highlights Mercury in Leo just before sunrise on Sept. 2nd. After sunset on Sept. 6th, a three day old waxing crescent Moon is 2.5° below Jupiter in Virgo and the next night it is 7.0° south of Venus in the same constellation. On the night of the 14th/15th, the setting waxing gibbous Moon approaches Neptune in Capricorn after midnight and passes it the following morning. It then visits Aquarius and Uranus before sunset on the 16th and can be found to the east of Uranus during the evening. After dusk on the 21st, Mars rises about 5° southeast of the newly risen waning gibbous Moon; and before dawn on the 28th, the waning crescent Moon showcases Saturn which is south of the Beehive Star Cluster in Cancer.


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