D  e  s  e  r  t     E  x  p  o  s  u  r  e   June 2006

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Constellation of the Month: Libra, the Scales

The Scales, commonly called Libra, is our constellation of the month. To the Romans, this grouping portrayed the scales of justice carried by Astraea, goddess of Justice. Astraea was the daughter of Zeus and Themis, and is frequently depicted carrying the Scales in her left hand and a sword in her right.

Since Rome was a trading culture, scales were used to measure the corn with which Astraea was crowned. Why is this particular grouping called the Scales? One explanation is that around 2000 BC, the Sun would be in this constellation on Sept. 21, the autumnal equinox in the northern hemisphere; the equal length of day and night would suggest a sense of balance, as does a pair of scales when measuring equal weights.

Click the star chart to enlarge it.

The Greeks at times considered this constellation to be part of Scorpius. At other times, this constellation was the chariot of Pluto, god of the underworld. The chariot was pulled by four jet-black horses. Pluto would seldom venture out of the underworld, and when he did, his primary goal was to seduce a nymph or two. Or at least that was the way it was until he met Persephone. (But that is another story, which has already been told in a previous column).

This is yet another constellation of faint stars, with the brightest, Zubenelgenubi (Alpha Librae) being only magnitude 2.8. This is a wide double star, with the primary being a blue-white star of magnitude 2.9 and the secondary a white star of magnitude 5.2, almost 4 minutes-of-arc from the primary.

This pair is some 446 trillion miles away. How do astronomers know this? They use a rangefinder, only this is slightly different from ones used on Earth. A terrestrial rangefinder has two objective lenses separated by seven or more inches. A dial is adjusted until the images from the two lenses are superimposed in the rangefinder, and the distance can then be read off the dial. This is a practical application of trigonometry.

Astronomers use something similar, but since they are measuring objects much farther away, they must separate the two objective lenses by a slightly larger distance—some 184 million miles, the distance from one side of the Earth's orbit to the other. To make the measurement, astronomers precisely measure the position of the target star relative to the background stars. Six months later, the measurement is made again. The shift in the position of the target star relative to the background stars is called the parallax of the star. The parallax is then used with trigonometry to compute the distance to the star, with the Earth's orbit as a baseline and simple trigonometry to compute the distance.

 

The Planets for June 2006

Mercury puts in an appearance in the evening sky this month. At the beginning of the month, Mercury will be seven degrees above the west-northwestern horizon about 45 minutes after sunset. As the month goes by, Mercury gets higher in the sky, even as it remains in Gemini, reaching maximum altitude on June 20, when it will be 14 degrees up. The Messenger of the Gods then turns around and heads back toward the Sun, disappearing into its glow in the first week of July. At maximum, Mercury will be magnitude 0.7, and be a 37-percent-sunlit disc just 8.4 seconds-of-arc across. The disc will start out almost full, and shrink during the month as it comes around from behind the Sun in May to pass between the Earth and Sun in July.

Mars is in Cancer all month, but remains only 4.1 seconds-of-arc across. Mars is about 25 degrees up as evening twilight winds down. It passes the Beehive (M44) on June 15, and passes Saturn on the June 17. Mars sets around 11 p.m.

Watch the Skies (all times MDT) June 3, 5:06 p.m.—First Quarter Moon June 5, evening—Saturn near Beehive June 7, evening—Spica near the Moon June 10, evening—Antares near the Moon June 11, 12:03 p.m.—Full Moon June 15, evening—Mars south of Beehive June 17, evening—Mars north of Saturn June 18, 8:08 a.m.—Last Quarter Moon June 21, 6:26 a.m.—Solstice, summer begins June 25, 10:05 a.m.—New Moon

Saturn is moving eastward like Mars, but much more slowly. Saturn will pass 0.8 degrees south of the Beehive on June 5. Saturn is magnitude 0.1, fading as the Earth pulls way from it. At midmonth, Saturn's ball is 16.8 seconds-of-arc across, and the rings are 38.1 seconds-of-arc across, tilted up 19.3 degrees to our line-of-sight, with the southern face showing.

Jupiter is in Libra all month. At midmonth, the King of the Planets will be at magnitude -2.4, stretching 42.3 seconds-of-arc across. As the Sun sets, Jupiter will be 40 degrees up in the southeast, setting around 3 a.m.

Venus is still in the morning sky, but not for long. It is starting to head back toward the Sun, rising around 4 a.m. Venus is magnitude -3.9 and 12.9 seconds-of-arc across. At midmonth, Venus is 80 percent illuminated and becoming fuller as it prepares to go behind the Sun.

Northern hemisphere summer begins on June 21 at 6:26 a.m. when the summer solstice occurs. This will be the shortest night of the year, so get lots of summer sleep, since the nights will start getting longer, giving you more time to "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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