SKY REPORTER: October 2011


Sky Data

Jupiter and its four largest moons, discovered by Galileo. Credit: Jan Sandberg

Jupiter and its four largest moons, discovered by Galileo.

Credit: Jan Sandberg

Saturday October first civil twilight begins at 6:25 a.m. and sunrise is at 6:52. That evening the Sun, located in the constellation Virgo, sets at 6:38 p.m. Civil twilight concludes at 7:05.

Civil twilight starts at 6:39 a.m. and sunrise is at 7:07 a.m. on Saturday, October 15th. That day sunset occurs at 6:16, with civil twilight concluding at 6:45 p.m.

On the last day of the month, Monday October 31th, civil twilight begins at 6:56 with sunrise at 7:25 a.m. sunset is at 5:54 p.m. and civil twilight ends at 6:22.

During October, average overnight temperatures recorded in Central Park drop from 55 degrees Fahrenheit on the first of the month, to 45 degrees on the 31st. On Halloween, the Sun is located at the border between constellations Virgo and Libra.

Principal Phases of the Moon during this month are:

First Quarter October 3
Full Moon October 11
Last Quarter October 19
New Moon October 26

On the first of the month, Mercury, Venus, and Saturn are very low in the western sky at sunset, too close to the sun for convenient viewing. Saturn is at conjunction with the Sun on October 3rd then emerges in the early morning sky during subsequent weeks. As the month progresses both Venus and Mercury gradually move further east and set an increasing number of minutes after the Sun. Although Venus becomes obvious during evening twilight, Mercury as usual remains an elusive object.

At the start of this month Mars in the constellation of the Crab and with a magnitude of 1.29 is rather bright. The red planet rises at 1:43 a.m. on Saturday October first. During the following week it is about halfway between the twin stars of Gemini and Regulus in Leo. After locating Mars use it as a guide for viewing nearby M44, the Beehive star cluster, also known as the Praesepe. This cluster is visible with binoculars from New York City and can be seen with unaided eyes from locations far from artificial lights. By the end of the month Mars will have moved into Leo and rises just after midnight.

Jupiter is above the horizon at sunset in the constellation Aries and is a brilliant eye catcher all night. It is at opposition October 28th, when its magnitude is minus 2.93. Opposition means a planet in a larger orbit than Earth’s such as Mars, Jupiter, or Saturn is directly opposite the Sun in our sky. As a consequence, at times of opposition a planet is located at the celestial meridian around midnight (one a.m. when Daylight Saving time is in effect). At opposition a planet’s distance from earth is at or very close to its minimum, its disk appears circular, and its brightness is at maximum for the year.

On the night of Sunday October 9th, the waxing gibbous Moon is about two-thirds of the way along a 90 degree line extending from Vega to the star Fomalhaut.

A bright waning gibbous moon is near the direction of Jupiter on the night of Thursday October 13th. During this month when Jupiter is near opposition, a good pair of binoculars may show the planet’s four largest moons. They are a wonderful sight in even small telescopes.

On Saturday October 15th Fomalhaut, brightest star in the constellation Piscis Austrinus, is at the meridian 10:15 p.m. This position represents a celestial object’s highest altitude of the night. Fomalhaut shines with visual magnitude 1.15, at a distance from Earth of 25 light years. It is a main sequence object, which means it is in the lengthy middle stage of its existence and produces light and heat entirely by the nuclear fusion of hydrogen into helium in its core. Fomalhaut’s color index is 0.14 which represents a color in the white to blue-white part of the visual spectrum. Color index is determined by measuring a star’s magnitude of brightness at narrow wavelengths in both blue and yellow portions of its spectrum. The yellow magnitude value is subtracted from the blue measurement and the difference becomes the color index number. Because Fomalhaut’s distance is rather close to the standard 32.6 light years used to compute absolute magnitude, Fomalhaut’s absolute magnitude 1.72 is in a range similar to its apparent magnitude. Apparent magnitude represents a measure of how bright an object appears to an observer on earth. Absolute magnitudes provide an indication of a star’s intrinsic brightness when the effect of distance is factored out. Individuals with exceptional eyesight, viewing stars near the zenith, under a moonless sky hundreds of miles from cities and towns may be able to see stars of magnitude seven. In Manhattan we usually can see stars as faint as second magnitude, if we shield our eyes from direct artificial lights and observe on cool, haze free nights.

A 2008 report in the journal “Science” told of the discovery of a possible extra solar planet designated Fomalhaut b having an orbit within a disk of cold dust surrounding the star. A team led by Paul Kalas of the University of California, Berkeley reported on observations with the Hubble Space Telescope that showed evidence consistent with the presence of a planet having no more than three times the mass of Jupiter orbiting 119 astronomical units from Fomalhaut. This represents a distance 119 times that between Earth and Sun. The following year D. Nero and J.E. Bjorkman of the University of Toledo in a paper published in the “Astrophysical Journal” described Fomalhaut b as having mass and orbital characteristics suggesting it was formed as a result of fragmentation in parts of the star’s circumstellar dust disk, rather than the planet having been more slowly assembled by the process of accretion.

During early mornings of Friday and Saturday October 21st and 22nd, the wide waning crescent moon is in the eastern sky near Mars. Currently the magnitude of Mars is 1.17, about the same as several of the sky’s brighter stars such as nearby Regulus, currently 10 degrees of arc to the east of the Red Planet.

At the end of October Mercury and Venus remain low in the western evening sky during evening twilight. On Halloween both set within an hour after sunset.

Sky Lore

During October the Great Square of Pegasus is near the celestial meridian during evening hours. At the first of this month the center of the Square is at the meridian about 11 p.m. By month’s end this alignment will be seen around nine p.m.

Pegasus, the Flying Horse, from Johannes Bayer’s Uranometria (1603). © Linda Hall Library of Science, Engineering, & Technology 2005 From Digital Services & History of Science Department

Pegasus, the Flying Horse, from Johannes Bayer’s Uranometria (1603).

© Linda Hall Library of Science, Engineering, & Technology 2005 From Digital Services & History of Science Department

Three hours after sunset in the middle of October zodiac constellations span the sky from Sagittarius low in the southwest to Taurus in the east. The array includes Capricornus, the Sea Goat; Aquarius, the Water Carrier; and Pisces, the fish. These are part of a group of nearby constellations including Piscis Austrinus, the Southern Fish; Cetus, the Whale; and Delphinus, the Dolphin. All are associated with water and this section of sky is sometimes called the Celestial Sea. The hydrological links are believed to have originated in ancient times when these constellations were seen in the eastern predawn sky during weeks of early spring when rivers and streams ran deep with melted snow.

During autumn evenings, our sky view is generally centered on the Milky Way’s southern pole, located in the faint constellation Sculptor. This constellation was created by astronomer Nicolas Louis de Lacaille during the 18th century and represents a sculptor and sculptor’s tools. In this region we look toward a sparse field of stars occupying the thin southern section of the Galaxy’s disk. None of the zodiac constellations between Scorpius and Taurus contains any star of first magnitude or brighter. Although a number of vivid stars associated with summer evenings, including Vega, Altair, and Deneb are still seen during October, as we move towards Halloween those stars are seen progressively lower in the western sky.

Solitary first magnitude Fomalhaut in the constellation of the Southern Fish can be considered the “Star of Autumn”, based on its location near the meridian during evenings in the middle of the fall season. In ancient times Fomalhaut was regarded as one of four “Royal Stars” along with Aldebaran, Regulus, and Antares. With the exception of Fomalhaut, these are in the zodiac and marked the Sun’s general location during respective seasons of the year.

Another assembly of autumn constellations is associated with the “Royal Family of the Sky”. It includes Andromeda, Perseus, Cassiopeia, Cepheus, Pegasus, and Cetus. These were protagonists in classic mythological tales about Perseus and his rescue of Andromeda, a princess of Ethiopia. Andromeda’s mother, queen Cassiopeia was said to have been quite vain claiming she was far more beautiful than the Nereids, daughters of Nereus, god of the sea. As punishment Nereus sent a sea monster, represented in the sky by Cetus, to plague the Ethiopian coast. An oracle suggested that only the sacrifice of Andromeda would bring relief. Andromeda was chained to a coastal rock and as Cetus approached Perseus arrived just in time, riding the flying horse Pegasus. Perseus slayed the monster and rescued the princess. Andromeda and he then were married, ruled their own kingdom, and became great-grandparents of mighty Hercules, who is also represented by a nearby constellation.