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  D e s e r t   E x p o s u r e  September 2010

Constellation of the Month: Ursa Minor, the Little Bear

Among the most famous asterisms (unofficial groupings of stars) in the sky are the Big and Little Dipper.

small map

Click the image for a larger map


Looking to our north, Ursa Minor can be found about 32 degrees above the horizon. While this little constellation is well known and contains one of the most important navigation stars in the sky, the only deep-sky object to be found here is the faint dwarf elliptical galaxy known simply as the Ursa Minor Dwarf Galaxy. It is in orbit around our Milky Way galaxy and is about 200,000 light-years away from us.

They are important in our world because they can provide basic navigational information if you don't have a compass. These two asterisms are in the northern sky; the end of the handle on the Little Dipper is Polaris, the Pole Star. The Big Dipper has six first- and second-magnitude stars (and one third-), that allowing it to be easily seen. The two stars at the west end of the Big Dipper are the pointer stars, pointing to the Pole Star. Once you've found it, you know where north is and you can determine your course. Even more useful, if you measure the altitude of Polaris above the northern horizon, you'll know your latitude as well.

These two asterisms are actually part of two constellations, Ursa Major, the Great Bear, and Ursa Minor, the Little Bear. These two constellations are linked mythologically as mother and son. Ursa Major represents the beautiful Callisto. Zeus, as was his wont, was having an affair with her until his wife, Hera, found out and turned Callisto into a bear who wandered the woods near her former home.

When Callisto's son, Arcas, reached the age of 15, he was out hunting in the forest and discovered a bear. The bear did not charge, nor did it run away. It just stared into his eyes. Arcas was about to unwittingly shoot his mother with a bow and arrow when Zeus intervened. He changed Arcas into a bear and cast both into the sky. Annoyed at this honor, Hera persuaded Poseidon to forbid them to ever enter the water. So they wander our northern sky, never setting into the ocean.

From Greece, Ursa Minor never sets, but Ursa Major dips her legs below the horizon and quickly brings them up again. From the desert southwest, Ursa Minor comes closer to dipping below the horizon, while Ursa Major dips most of her body below the northern horizon before coming back up. Both are called "circumpolar constellations" since they never set.

Ursa Minor is 56th out of the 88 official constellations in size. Its stars are mostly fifth-magnitude, thus hard to see from urban areas. The brightest are the two stars at the end of the Little Dipper (Pherkad and Kochab), second-magnitude like Polaris itself (2.02). Polaris is a triple star, but only one of its two companions is visible through our telescopes. Polaris is actually 0.7 degrees from the North Celestial Pole, meaning it's adequate for rough navigation, but not for aligning a telescope.

Most telescopes are on equatorial mounts. All mounts have two (some more) axes at right angles to each other. Equatorial mounts have one axis that points toward the celestial North Pole, called the polar axis, and thus aligned parallel to the Earth's axis of rotation. The other axis is perpendicular to the polar axis. Once this kind of telescope is pointed at an object, only the polar axis needs to be turned to continue to track the object as Earth rotates. In the days before computer-controlled telescopes, this made it possible to use a single fixed-speed motor to keep the object in view. Today, a computer can easily drive two axes, so it is not as important to use an equatorial mount.



The Planets for September 2010

Saturn is making its farewell appearance in the evening sky, low in the west at the beginning of the month. Saturn shines at magnitude 1.0 and sets around 8 p.m. The Ringed Planet remains in western Virgo with its disc of 15.7 seconds-of-arc across at midmonth. The Rings span 35.7 seconds-of-arc, tipped down 5.5 degrees with the northern face showing.

Watch the Skies

(all times MDT)

 

Sept. 1, 11:22 a.m. — Last Quarter Moon
Sept. 1, 12 p.m. — Venus 1.2° S of Spica
Sept. 4, 8 a.m. — Mars 2.0° N of Spica
Sept. 8, 4:30 a.m. — New Moon
Sept. 14, 11:50 p.m. — First Quarter Moon
Sept. 19, 12 p.m. — Mercury greatest distance
                                 west of the Sun (18°)
Sept. 21, 6 a.m. — Jupiter at opposition
Sept. 22, 1 p.m. — Uranus 0.9° N of Jupiter
Sept. 22, 9:09 p.m. — Equinox, autumn begins
Sept. 23, 3:17 a.m. — Full Moon

Venus and Mars hang together most of the month. On Sept. 1, they will be on either side of the first-magnitude star Spica, the brightest star in Virgo. Both Venus and Mars are moving eastward, but Venus also has a southward drift, slowly separating it from Mars. Both will slip into Libra just before the end of the month. At midmonth, Venus will be 35.0 seconds-of-arc across and 32% illuminated. The Goddess of Love is magnitude -4.5 and sets around 8:45 p.m. Venus is heading back toward the Sun, and will pass between the Sun and the Earth late next month.

Mars's surface has become hard to see in a telescope, with the disc only 4.4 seconds-of-arc across. It is magnitude 1.5 and barely 12 degrees up in the west-southwest as it gets dark. Mars sets around 8:54 p.m.

Jupiter is at opposition on Sept. 21, so it is visible all night. It rises around 7:30 p.m. in the east and sets in the west about 7:30 a.m. Jupiter is magnitude -2.9 at midmonth, and its disc is 49.7 seconds-of-arc across, the largest this year. The King of the Planets is moving slowly westward in western Pisces.

Mercury makes an appearance in the morning sky this month, popping over the eastern horizon as it starts to get light beginning on Sept. 10. It will be in Sextans, moving westward into Leo. It will also drift northward; as Mercury turns back toward the Sun on Sept. 18, it will miss Sextans and travel down Leo until it enters Virgo just before month's end. By then, Mercury will be magnitude -0.2, its disc 7.4 seconds-of-arc across and 43% illuminated. It rises at 5:30 a.m. Mercury is coming out from between the Earth and Sun, and will be returning around the back side of the Sun by month's end.

Fall starts in the Northern Hemisphere on Sept. 22 at 9:09 p.m., when the Sun passes the celestial equator moving southward. On the same day, Uranus will be just under a degree north of Jupiter, making it easy to find in a telescope. Uranus will be magnitude 5.7 and 3.7 seconds-of-arc across. So take out your telescope and look for a pale green disc a degree north of Jupiter 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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