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About the cover


D e s e r t   E x p o s u r e    May 2008


Constellation of the Month: Crater, the Cup


With a name like Crater, you might think that this constellation was recently invented to commemorate the craters on the Moon, or possibly the volcanic craters on the Earth. You would be wrong. Crater is an old constellation, among the 44 classic constellations listed by Ptolemy. Crater is said to be the goblet (or cup) of Apollo.

Star map, Crater

Click the image for a larger map


May nights have the three constellations in this month's myth. Crater, Corvus and Hydra all can be found low in the south. The three brightest stars in Crater are Alkes, Gamma Crateris and Delta Crateris. The brightest is only magnitude 3.56. There are only a few galaxies in this region of space and not much more.

It seems that one day Apollo was thirsty for a nice cool drink of water. He took his goblet and gave it to his pet crow, Corvus, bidding him to fly down and get fresh water. Corvus took the cup and flew from Mount Olympus to the Earth. As he was flying, he spotted a grove of fig trees whose fruits were just about ripe. He decided to wait a few days for the figs to ripen, then spent several more days feasting before he got the water.

Knowing he would need an excuse for his tardiness, Corvus found a water snake (Hydra). Grasping Hydra in its claws and the cup in his mouth, the crow flew back to Mount Olympus. Arriving before Apollo, Corvus told a story of an epic battle with the water snake that slowed his errand.

Apollo was not fooled. In anger, he grabbed the crow, goblet and snake and cast them all into the sky, where you will find them today as constellations: Hydra the snake winds around near our southern horizon, with Crater, the Cup, sitting above his back and Corvus, the Crow, just east of the Cup.

Crater is a small, unremarkable, constellation whose brightest star is barely fourth magnitude. Its stars actually do look a little like a cup, but the three brightest stars are often connected instead to form a simple triangle. This is in part due to the difference in viewing conditions between ancient times and today. When the Greeks looked at the sky, the brightest light they had to contend with was firelight. They could easily see to sixth magnitude and usually fainter, so they could imagine the constellations from all the stars, bright or dim.

Today, with all electric lighting, it is hard to see the fainter stars. Moreover, if you go from inside a well-lit home to the dark outside, your eyes need at least five minutes to become accustomed to the dark. Complete "dark adaptation" takes about 30 minutes. (Red light presents less of a problem. The parts of the eye that are most sensitive to faint light, the rods, are insensitive to red light. This is why if you watch a movie about naval activity, the inside command bridge is lit with red lights at night. This allows the bridge crew to perform their normal tasks, while ready at a moment's notice to look out over the waves to see the faint lights of another vessel.)

We also light the outside of our houses as well, where we often do a poor job. Lighting fixtures do not always shield the bulb and the glare finds its way into the sky, reflecting off the dust in the air, lighting up the sky and wasting energy. If you multiply one such fixture by the thousands in a city, you start to understand why we can no longer see the sky the way the Greeks did. This is called light pollution.

To get more information on outdoor lighting, both good and bad, see the Web site of the International Dark Sky Association (www.darksky.org). It has a great deal of information on good lighting and how bad lighting harms wildlife, wastes energy and blots our dark skies. If you are inclined, you can join the association and help reduce the light pollution in our skies.

The Planets for May 2008

Mercury makes an evening appearance low in the west-northwest for all of May, spending the month in Taurus. The Messenger of the Gods will appear low in the sky in early May, moving eastward the first half of the month until reaching its farthest distance from the Sun on May 14, when it will be 22 degrees away. Mercury then turns around and retraces its path through Taurus, disappearing into the Sun's glow by the end of the month. At mid-month, Mercury will be 32 percent illuminated at magnitude 0.7. It will start the month with an almost-full disc that is 6.7 seconds-of-arc across. By month's end, it will have grown to 11.7 seconds-of-arc across, but will be a very thin crescent only three percent illuminated.

Watch the Skies
(all times MDT)

May3, 7 a.m. — Saturn stationary
May 5, 6:18 a.m. — New Moon
May 9, 9 a.m. — Jupiter stationary
May 11, 9:47 p.m. — First Quarter Moon
May 13, 10 p.m. — Mercury farthest east of Sun
May 19, 8:11 p.m. — Full Moon
May 24, 5 a.m. — Moon two deg S of Jupiter
May 27, 8:57 p.m. — Last Quarter Moon

Mars starts the month in Gemini, but by May 5, it will enter Cancer for the remainder of the month. Mars continues to shrink, and its 91-percent sunlit disc will be only 5.3 seconds-of-arc across at magnitude 1.4. Mars will be about halfway up in the western sky as it gets dark, and will set around 12:45 a.m.

The Ringed Planet will continue to be in Leo this month. At the beginning of the month, Saturn will be moving west toward Regulus, but around mid-month, it will turn around and start moving eastward again. Shining at a slightly yellowish magnitude 0.5, Saturn starts the evening about 20 degrees south of overhead, setting around 2:30 a.m. The rings are 41.1 seconds-of-arc from side-to-side, showing the southern face. The rings are tilted up 9.8 degrees, while the ball of the planet is 18.1 seconds-of-arc across at mid-month.

Jupiter comes over the eastern horizon just before Mars sets. By sunrise, it will be 35 degrees up in our southern sky and be visible the rest of the night. Standing stationary among the stars of northeastern Sagittarius at the beginning of May, Jupiter will begin traveling slowly westward. At mid-month Jupiter's disc is 43.0 seconds-of-arc across, while it shines at magnitude -2.5.

Venus is still too close to the Sun to view this month, but will reappear in our evening skies next month. So be ready for the return of our Evening Star, 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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