D e s e r t E x p o s u r e
July 2009
Constellation of the Month: Norma, the Square
As it gets dark on July evenings, you can find a sparsely populated area of the sky located just above our southern horizon that harbors the constellation Norma. Knowing this is not a classical Greek or Roman designation, you might be tempted to think this constellation was named after a Queen Norma or a Countess Norma. But you would be wrong. Norma, the Square, is actually a carpenter's square.
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Click the image for a larger map As it gets dark, the constellation Norma, the Square, just peeks over our southern horizon. Since Norma is in the Milky Way, there are a number of interesting objects within the boundaries of this constellation including the Fine Ring Nebula and the Ant Nebula. |
Ptolemy did not list anything in his Almagest for this area, since it is relatively empty. Starting in the late 1500s, explorers Frederick de Houtman and Pieter Dirkszoon Keyser plotted the positions of the southern stars, including the area around Norma. In 1589, Dutch mapmaker Petrus Plancius, who had sent the two explorers, published his celestial charts that included a new constellation he called the Southern Triangle. While everyone thought that having a Southern Triangle was a great idea, exactly where to put it was in dispute. As new charts came out, the Southern Triangle's position jumped around among the stars.
Finally, in 1763, Frenchman Nicolas Louis de Lacaille fixed the location of the Southern Triangle at its current location as the constellation Triangulum Australe, which he envisioned as an A-frame with a plumb-bob hanging from it. LaCaille then took the remaining stars in the area and created two more surveyor's tools, Circinus, a pair of compasses, and Norma, the square.
The original name was L'querre et La Rgle (the set square and the ruler) known in formal Latin as Norma et Regula. While these three constellations started out as surveyor's tools, our perception of them has changed from surveyor's tools to a set of geometrical objects: the straightedge, compass and equilateral triangle. Even the name has been shortened to just Norma. With changing constellation boundaries, Norma's two brightest stars, Alpha and Beta, have disappeared across the constellation boundary with Scorpius and are now part of that constellation.
One interesting object in Norma is the planetary nebula known as the Fine Ring Nebula. This object, also known as Sp-1, is about a thousand light-years away and shines at magnitude 13. A planetary nebula is formed when an aging star ejects its outer atmosphere into space as it expands and contracts (called "pulsing"). In most non-bipolar planetary nebulas, the ejected outer atmosphere expands in a spherical shell. The shell is relatively thin, and looks to us like a ring because we see light emitted from only the thin front and rear shells.
But the Fine Ring Nebula is different. It actually is a ring, and we are looking down the hole in the middle.
The star at the center is also unusual, being a white dwarf. This star is different than the usual giant star that is pulsing as part of the aging process. As it reaches maximum size, the outer atmosphere is able to escape to add to the expanding sphere. A white dwarf star is composed of very dense matter, so dense that a ton can fit in a matchbox. This is hardly the kind of matter that would gently waft off into space even if it were pulsing, which white dwarfs do not. So astronomers have a mystery: What mechanism formed the Fine Ring Nebula around a white dwarf star?
The Planets for July 2009
The only planet in our evening sky as it gets dark is Saturn. Its rings are continuing to close, now tilted up only 2.6 degrees with the southern face still showing. The ball of the planet is 16.6 seconds-of-arc across at midmonth, girded by its 37.6 second-of-arc rings. Saturn is 30 degrees up in the western sky at dusk, still moving eastward among the stars of Leo at magnitude 1.1. It will pass 19 minutes-of-arc from the 4th-magnitude star 77 Sigma Leonis, the rearmost hind-paw of Leo, on July 27. Saturn sets by 11 p.m.
Watch the Skies (all times MDT) Full Moon (penumbral lunar eclipse) July 14, morning — Venus near Aldebaran July 15, 3:53 a.m. — Last Quarter Moon July 21, 8:35 p.m. — New Moon (total solar eclipse, Asia-Pacific) July 27, 5 a.m. — Mars 5 degrees north of Aldebaran July 28, 4 p.m. — First Quarter Moon |
When it finally does get dark, Jupiter pops over the east-southeastern horizon in Capricornus. Approaching opposition next month, the King of the Planets is almost at its maximum size for this year, 48.3 seconds-of-arc across on July 15. Jupiter starts July right at the border with Aquarius, moving slowly westward farther into Capricornus. Rising around midnight, the King of Planets is magnitude -2.6, and its slowly enlarging disc is now 43.6 seconds-of-arc across; it glows at magnitude -2.8.
The next planet up, Mars, doesn't rise until 2:30 a.m. Starting the month in Ares, Mars moves quickly into Taurus, where it continues moving eastward through the rest of the month. At midmonth, Mars is 5.1 seconds-of-arc across and glows with a reddish tint at magnitude 1.1.
The last planet up in July skies is the brilliant Venus. Rising at 3:15 a.m., Venus is in Taurus most of the month, but on July 30, it moves into the northeastern panhandle of Orion. By August 1, it is in Gemini. On July 15, Venus is magnitude -4.1 and 16.5 seconds-of-arc across. The disc is 68% illuminated, becoming fuller and growing smaller.
There are two eclipses this month. The first is a penumbral lunar eclipse on July 7, but it is so minor that you won't be able to discern it. The big event is a total solar eclipse on July 21, but it's not visible from our area. The path of totality runs from India, across China and then over a few Japanese islands. It continues out into the Pacific where it crosses a few atolls in the Marshall Islands and finally over Kiribati (Gilbert Islands) before the lunar shadow leaves the Earth's surface. This is a long eclipse with a maximum duration of 6 minutes and 39 seconds. So if you can travel to China for this event, enjoy the spectacle 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.