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About the cover

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


Constellation of the Month: Indus, the Indian

The middle part of the last millennium, with the Dark Ages at an end, was a time of great discovery. European ships constantly sailed off to the west and south. Their navigators, especially those going south, got to see a whole new sky, and the heavens became another field of discovery. On voyages to the East Indies, Dutch navigators Pieter Dirkszoon Keyser and Frederick de Houtman uncovered many new stars previously below the southern horizon and formed 12 new constellations.

Click the small map above for a larger one


Just coming over the southern horizon, Indus, the Indian, has few interesting objects. The only really interesting object is Epsilon Indi, the 11.83 light-year-distant star that will just clear the horizon between 8:30 p.m. and 10:30 p.m. This star is not only close to us, but has two companions in the form of a pair of brown dwarves. Even though it does clear the southern horizon, Epsilon Indi is hard to see since it is only magnitude 4.7.

One of these is cresting just above our southern horizon during October evenings. Indus, the Indian, is a faint group of stars that first appeared on a globe of the heavens created by Petrus Plancius in 1598. Indus is depicted as a man holding spears or arrow as through hunting game. It's not clear if Indus is supposed to be a native of the East Indies or the Americas.

Indus is a small, faint constellation, 49th in size out of 88. The brightest star is Alpha Indi, also known as "The Persian," only magnitude 3.11. Belonging to spectral class K0, it is a reddish-orange star about a hundred light-years away.

The real discovery in this constellation is the star Epsilon Indi. This is a K4 dwarf star, even redder than "The Persian," but 10 times closer, only 11.83 light-years away. To give you an idea how much fainter Epsilon Indi is than our Sun, if you were at Epsilon Indi looking back at the Sun, you would see a yellow-white star shining at magnitude 2.6, some six times brighter than the magnitude 4.5 that Epsilon Indi appears from Earth.

Watch the Skies
(all times MDT)


Oct. 6, evening — Moon 2.3 degrees south of Jupiter
Oct. 7, 3:04 a.m. — First Quarter Moon
Oct. 15, 2:02 p.m. — Full Moon
Oct. 21, 5:55 a.m. — Last Quarter Moon
Oct. 22, 4 a.m. — Mercury greatest elongation west (18 degrees)
Oct. 25, evening — Venus near Antares
Oct. 28, 5:14 a.m. — New Moon
Oct. 31, evening — Antares near the Moon

Epsilon's proximity to us came to light in 1847 when Heinrich Louis d'Arrest was comparing positions of stars from multiple catalogs going as far back as 1750. He noticed that Epsilon Indi's current position was different than in older catalogs. This proper motion is the movement of the star in the plane of the sky. Epsilon moves 3.961 seconds of arc across the sky each year, the third fastest moving naked-eye star in our sky after Groombridge 1830 and 61 Cygni.

In 1883, astronomers David Gill and William L. Elkin at the Cape of Good Hope measured a parallax of 0.22 seconds of arc for Epsilon Indi — how much its position changes as our viewing location on the Earth moves from one side of the Sun to the other. This is similar to what happens when you move your head from side-to-side and a nearer object appears to move relative to the background objects. Modern astronomers have derived a parallax of 0.275 seconds-of-arc, allowing us to compute Epsilon's 11.83 light-year distance.

But this was not the end of the discoveries. In January 2003, astronomers discovered a brown dwarf orbiting Epsilon. A brown dwarf is in-between a planet and a star — too large and hot to have different chemical layers as you would find moving out from the center of a planet, but lacking enough mass to start nuclear fusion in its core. Its feeble glow comes from the slow cool of its core. By August 2003, astronomers discovered this brown dwarf was actually a binary pair spinning around each other. The heavier dwarf is 47 times the mass of Jupiter, while the lighter is 28 Jupiters. They orbit Epsilon Indi at a distance of 195 million miles.

There is also evidence that a planet orbits Epsilon Indi, but searches with the Hubble Space Telescope have not found anything yet.

The Planets for October 2008


Mars is too close to the Sun to be seen in October, but Venus is low in our southwestern sky all month, shining at magnitude -4.0. Venus starts the month in Libra, passes through the northern panhandle of Scorpius and into Ophiuchus by month's end. At mid-month, Venus's disc is 12.8 seconds-of-arc across, and it is 82 percent illuminated. Venus sets around 8:15 p.m.

Jupiter continues to move slowly eastward among the stars of Sagittarius. It starts the evening as the brightest object in our southern sky, unless the Moon is up, above and left of the Teapot asterism. At magnitude -2.6, Jupiter is 37.9 seconds-of-arc across and slowly shrinking. It sets around 11:30 p.m.

Saturn rises into our morning sky around 4:30 a.m. and is moving eastward through eastern Leo. The Ringed Planet has just come out of the morning twilight, and the Earth is racing to catch up with it. At mid-month, Saturn's disc is 16.3 seconds-of-arc across while the rings are 37.0 seconds-of-arc across. The southern face is still showing, but the rings are tilted up only 2.8 degrees. Saturn's rings will continue to become more edge-on during the rest of the year.

Hanging low in the west last month, Mercury swings past the Sun on Oct. 6 and into our morning sky. Even so, it spends the entire month in Virgo. Starting around Oct. 12, Mercury will be visible low in the east around 6:30 a.m. Each day thereafter, Mercury will be higher in the sky until Oct. 22, when it will be farthest from the Sun. Mercury will then start to sink back toward the Sun, but will continue to be visible for the rest of the month. Mercury swings around the Sun in only 88 days, so it is pulling away from the Earth, making it appear to shrink and become more full. On Oct. 22, it will be 56 percent illuminated and 6.9 seconds-of-arc across at magnitude -0.5. By month's end, it will be 84 percent illuminated,5.6 seconds-of-arc across at magnitude -0.9.

Remember that Daylight Savings Time will not end until next month. Also next month will be the Leonid Meteor Shower, so break out your long-johns 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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