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Constellation of the Month: Draco, the Dragon

Draco, the Dragon, is a large constellation that wraps around Ursa Minor. This dragon was originally much larger. The ancient Mesopotamians gave him large wings that wound around Ursa Major. But the Greek philosopher Thales chopped off the wings in the sixth century BC to form a few other constellations.

Click the Image for a Larger Map

Looking north in mid-September, you can see Draco, the Dragon, wrap around Ursa Minor (which contains the Little Dipper). Above and to the right of Draco is Cygnus with its bright star Deneb and Lyra with its bright star Vega. These two form the northern two stars of the Summer Triangle. Left of Draco is Ursa Major, which contains the Big Dipper. Most of the stars in Draco and all the stars of Cepheus and Ursa Minor never set as viewed from the desert southwest since they are so close to the Celestial North Pole.

Draco's mythological origin is from one of the Labors of Hercules. To atone for an accidental murder, Hercules had to perform 12 labors. One of them was to steal golden apples from a tree that was given to Hera by Gaia, the goddess of the Earth, in celebration of Hera's marriage to Zeus. This sounds easy, but the tree was guarded by a hundred-headed dragon. Hercules knew the apples were in the Garden of the Hesperides. Since Atlas was Hesperides' father, Hercules thought Atlas might be able to find the apples if anyone could.

Hercules went up to the Atlas Mountains and found Atlas holding up the Earth. He asked Atlas to find the apples in return for Hercules holding up the Earth for awhile. Atlas pointed out that there was the guardian dragon to deal with, so Hercules threw his spear, tipped with the poison from Medusa's blood, and killed the dragon.

After Hercules relieved Atlas of his burden, Atlas went in search of the tree with the golden apples. He eventually found it and collected the apples. Returning to Hercules, Atlas did not want to take back his burden. So Hercules asked Atlas to hold the Earth for a moment while he adjusted his grip—and when Atlas took back the Earth, Hercules grabbed the golden apples and fled. Hera placed the dragon in the skies to honor him for his faithful service.

The brightest star in Draco is called Thuban. Around 2250 BC, the Pharaoh Khufu died and was buried in the largest of the Giza pyramids, the Great Pyramid. Khufu's burial chamber has two skinny shafts radiating upward. For many years, they were thought to be air shafts, but in the 1960s, astronomers discovered that one pointed toward Orion, which for the Egyptians symbolized the god Osiris, Thuban, the other toward Thuban.

Why Thuban? Because at that time the pole star was not Polaris, but Thuban. The Earth spins like a top, and like a top, if you touch its side, you apply a force to it and it will start to wobble. This is exactly what the Earth is doing. The force that is being applied is not from some giant finger, but from the Moon. The pull of the Moon's gravity causes Earth's axis to wobble in a 23.5-degree circle. This wobble is called precession.

Four thousand years ago, Thuban was near the pole. Six thousand years from now the pole will be in Cepheus and around 14,000 AD, Vega will be close to the pole. It will take 26,000 years for the North Pole to wobble back to Polaris, so come back in 28,000 AD to see our North Star return to prominence.

 

The Planets for September 2006

Down near the Sun this month are Mercury, Venus and Mars. The Sun's glare will hide these three from desert southwestern observers. Mercury will be east of the Sun in the evening sky, but it is at such a poor angle that, for northern hemisphere observers, it will be too near the west-southwestern horizon to observe.

Watch the Skies
(all times MDT)

Sept. 1, 7 p.m.
—Antares near Moon

Sept. 7, 11:42 a.m
.—Full Moon (lunar eclipse)

Sept. 11, evening
—Jupiter near Zubenelgenubi

Sept. 14, 4:15 a.m.
—Last Quarter Moon

Sept. 22, 4:45 a.m.
—New Moon (annular solar eclipse)

9:03 p.m., Autumnal Equinox

Sept. 30, 4:04 a.m.,
First Quarter Moon

Hanging just 20 degrees above the southwestern horizon, Jupiter is visible as it starts to get dark. On Sept.11, Jupiter will be just a half-degree from Zubenelgenubi, the brightest star in Libra, where Jupiter spends the month. At magnitude -1.9, Jupiter is moving eastward among the stars, and has shrunk to 33.4 seconds-of-arc across.

Saturn will be spending the entire month in eastern Leo. Rising about 4:20 a.m., Saturn's disc is 16.6 seconds-of-arc across. The rings are slowly closing, now tilted only 15.9 degrees to our line of sight with the southern face still showing. The rings stretch 37.9 seconds-of-arc from edge-to-edge. Saturn is still too low for really good telescopic viewing.

There is a partial lunar eclipse visible over the eastern hemisphere at the Full Moon on Sept. 7. Also this month is an annular solar eclipse on Sept. 22. The path of this eclipse begins on the northeastern edge of South America, crossing Guyana, Surinam and French Guiana. It then enters the Atlantic Ocean and turns southeast between South America and Africa. The eclipse ends at sunset in the ocean between the Cape of Good Hope and Antarctica

Earth's equator will pass through the line connecting the center of the Earth and the center of the Sun. This will give us the northern hemisphere Autumnal Equinox on Sept. 22 at 9:03 p.m., when the length of the day will equal the length of the night, both 12 hours. From that date on, the nights will be longer than the days, giving us more time to appreciate the desert sky. So get ready for cooler weather observing, and "keep watching the sky"!

 

An amateur astronomer for more than 35 years, Bert Stevens is co-director of Desert Moon Observatory in Las Cruces.

 

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