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Constellation of the Month: Leo Minor, the Small Lion

Johannes Hevelius was kept busy making up seven new constellations for his new atlas of the heavens, Firmamentum Sobiescianum, which was released posthumously in 1690. One of these new constellations was Leo Minor, occupying an empty space just north of Leo. Since the sky already had Canis Major and Canis Minor (Big and Small Dogs), and Ursa Major and Ursa Minor (Big and Small Bears), why not Big and Small Lions? Leo was already a well-established constellation (at least back to the Greeks), Hevelius could not very well rename it Leo Major. So he left it as Leo and just added Leo Minor.

Click the image for a more detailed map.

Facing south and looking almost overhead, you can see Leo Minor, a flat triangle of stars just above Leo, the Lion. To the right side of the map is Venus in Gemini. Castor and Pollux are marked so you can see that Castor (Alpha Geminorum) is fainter than Pollux (Beta Geminorum).

Leo Minor is a small constellation, whose brightest star is 46 Leonis Minoris (abbreviated 46 LMi) at magnitude 3.8. All the other stars are fainter, including the only star in the constellation with a Bayer Designation, Beta LMi. This star is only magnitude 4.2. The constellation consists of three stars forming a long-base triangle, nothing at all impressive.

Bayer Designations are familiar to anyone who reads science fiction, looks at star charts, or knows that Proxima Centauri is the nearest star (Alpha Centauri is the nearest bright star). It is a lower-case Greek letter in front of the genitive (or possessive) form of the name of the constellation. These designations were made by Johann Bayer in his famous star atlas Uranometria, published in 1603. It was the first atlas to cover the entire celestial sphere and was named after Urania, the Muse of Astronomy.

Bayer was an active observer who traveled the world, making his observations wherever he went, including the Southern Hemisphere. This gave him a unique data set for his time. He plotted the stars onto charts, but he also wanted to give a more scientific name to the previous, usually Arabic, star names. This also gave him an opportunity to name fainter stars that the ancients had not considered worthy of a name.

His process was to take the stars in a classical constellation (the International Astronomical Union, IAU, did not finalize the boundaries of the 88 modern constellations until 1930) and group them into their classic brightness classifications: first, second, third, fourth, fifth and sixth magnitude. Then he just began lettering them with the Greek alphabet, starting with alpha, until he ran out of first-magnitude stars in that constellation. He then continued with the second, third and so on. At the time, there was no way to measure the brightness of stars more precisely. Today, however, we know that the alpha star is not always the brightest star in the constellation. For example, in Orion, Rigel (Beta Orionis) is brighter than Betelgeuse (Alpha Orionis), even though in Bayer's time both were simply known as first-magnitude stars.

Unfortunately, too, Bayer did not always stick to his plan. He sometimes lettered east to west, north to south, and even head to toe. In the case of Gemini, he lettered Castor as alpha and Pollux as beta because he they are always referred to as "Castor and Pollux," never "Pollux and Castor." And there are only 24 Greek letters, but more than 24 stars in many constellations. Bayer decided to follow the Greek letters with lower-case Latin letters and then upper-case Latin letters. In no constellation did he go past "Q." If you see a star with the any of the capital letters R through Z in front of it ("R Lyrae," "ZB Lyra"), that is a variable star name.

When the IAU redrew the constellation boundaries in 1930, some of the Bayer stars moved from one constellation to another and were given new Bayer Designations. So Leo Minor ended up with no alpha and only a beta, a double star with two components just 0.4 seconds-of-arc apart. The brighter one is magnitude 4.5 and the fainter is magnitude 6.5. Leo Minor also has a few galaxies; the brightest is NGC 3003, an edge-on spiral galaxy 5.9 minutes-of-arc long and magnitude 11.7.

 

The Planets for May 2007

All the evening planets are on the western side of the sky this month. Mercury will be joining Venus and Saturn at the end of May. For the last third of the month, Mercury will be low in the west-northwest, below and to the right of Venus. At the end of the month, Mercury will be magnitude 0.4, a thick crescent, getting thinner as the days go by. Mercury will be in Taurus most of the month.

Venus shines out as evening twilight begins. Venus is in the gibbous phase, thinning from 68 to 54 percent illuminated during the month. Venus moves from Taurus into Gemini during the early part of May, and it stays in Gemini for the rest of the month. At midmonth, the Goddess of Love is magnitude -4.1.

Watch the Skies
(all times MDT)

May 2, 4:09 a.m. —Full Moon

May 9, 10:27 p.m. —Last Quarter Moon

May 16, 1:27 p.m. —New Moon

May 20, 7 p.m. —Venus 1.7 degrees south of Moon

May 23, 3:03 p.m. —First Quarter Moon

May 30, 1 p.m. —Venus 4 degrees south of Pollux

May 30, 2 p.m. —Mercury 1.1 degrees north of the open cluster M35

Moving very slowly eastward this month, Saturn is in western Leo. At midmonth its rings are 40.5 seconds-of-arc across, tilted up 15.1 degrees to our line of sight with the southern face showing, while the disc of the planet is 17.9 seconds-of-arc across. The Ringed Planet is magnitude 0.3, and sets around 1:30 a.m.

Ophiuchus continues to host Jupiter as it moves slowly westward. Jupiter's disc is 44.9 seconds-of-arc across, and it shines at magnitude -2.6. The King of Planets rises in the east-southeast around 9:30 p.m., and is visible the rest of the night.

Look east at a quarter to four in the morning around the middle of May and you will see the Red Planet Mars slip over the horizon. Shining at magnitude 0.9, Mars is just 5.5 seconds-of-arc across, not really large enough for good telescopic viewing. It will be better later in the year, so "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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