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NightSky Friday: The Goat Star: A Strange Light in the Northeast

By Joe Rao
SPACE.com's Night Sky Columnist
posted: 07:00 am ET
19 September 2003

SEPTEMBER 19

Recently I received an interesting inquiry from Professor Rob Eisenson, Assistant Professor in the Dept. of Physics, Astronomy, and Meteorology at Western Connecticut State University.

"In recent SPACE.com columns you have been advising people to look for Mars low in the southeast sky after nightfall," Eisenson writes. "Yet, I have also been seeing another unusually bright star-like object, but low in the north-northeast sky soon after it gets fully dark. It isnt so much that it is bright, it is just that I dont recall ever seeing such a bright star located so far to the north. It also sometimes seems to twinkle with the same kind of yellowish-orange light that Mars shows now. Can you identify what I am seeing?"

What Professor Eisenson was looking at is indeed a brilliant star with a distinct yellowish hue. In fact, its the sixth brightest in the sky (magnitude 0.08) and as seen from mid-northern latitudes, ranks number four behind Sirius, Arcturus and Vega. able -->


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SKY MAP: Capella as seen at around 10:30 p.m. from mid-northern latitudes on Sept. 19.

* Graphic made with Starry Night Software
 

CLOSE-UP: Find Capella's three kids.

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It is Capella, in the constellation of Auriga, the Charioteer.

Auriga is one of those star patterns whose exact origin is a hopeless mix of antique conceptions. The Greek and Roman legends made Auriga a famed trainer of horses and the inventor of the four-horse chariot. But the most ancient legends also had Auriga as a goatherd and a patron of shepherds. The brilliant golden-yellow Capella was known as the "Goat Star," with a nearby triangle of fainter stars representing her kids.

The confusion in concepts is reflected in the ancient allegorical pictures and star names. Auriga is usually represented holding a whip in one hand in deference to the Charioteer story, but in his other arm he is holding a she-goat (Capella) and her three kids.

In his classic guidebook, "The Stars, A New Way to See Them" (Houghton Mifflin Company, Boston), Hans A. Rey (1898-1977) drew Auriga looking like a man with a tough expression, a jutting chin and a pug nose, " . . . as befits the driver of a war cart."

Capella measures 16 times as large in diameter as our Sun, 174 times as luminous, and is located 42 light-years away. It is part of a multiple star system, interestingly containing at least four stellar components.

As was noticed by Professor Eisenson, Capella appears to rise well to the north of due east. In fact, it is the nearest to the North Pole of the sky of all the first-magnitude stars, and across much of the 48-contiguous United States it is visible at some hour of the night throughout the year.

From western Connecticut, for example, Capella is below the horizon for only about 3 hours out of a 24-hour day. Lying 46 degrees north of the celestial equator, Capella can pass directly overhead for anyone living at that latitude north of the terrestrial Equator (say, Houlton, Maine or Geneva, Switzerland). And for anyone at points north of latitude 44 degrees (for example, Minneapolis, Minnesota or Bologna, Italy), Capella will appear to graze the northern horizon, but will not go below it.

Interestingly, the brilliant blue-white star Vega (which we discussed last week) is only a trifle brighter. If you look toward Polaris, the North Star, Vega and Capella appear almost on opposite sides of the sky from it. Vega, at about 39-degrees from the celestial equator, affords a similar rising reference for northern sky watchers in the early spring sky

Rise times

In the table below, we have prepared the rise times for Capella for three dates this week (Sept. 19, 22 and 25) as seen from 10 different latitudes, each separated by 2-degree increments. Also provided for each latitude is the azimuth the direction on the horizon where Capella will first appear. As already noted, from latitude 44 degrees and all points north, Capella is always above the horizon.

Latitude Azimuth
Sept. 19
Sept. 22

Sept. 25

44 degrees N.

Always above

Always above

Always above

Always above

42 degrees N.

13 E. of due N.

7:39 p.m.

7:27 p.m.

7:11 p.m.

40 degrees N.

19 E. of due N.

8:15 p.m.

8:03 p.m.

7:48 p.m.

38 degrees N.

23 E. of due N.

8:42 p.m.

8:30 p.m.

8:14 p.m.

36 degrees N.

27 E. of due N.

9:03 p.m.

8:51 p.m.

8:35 p.m.

34 degrees N.

29 E. of due N.

9:21 p.m.

9:09 p.m.

8:54 p.m.

32 degrees N.

31 E. of due N.

9:37 p.m.

9:26 p.m.

9:10 p.m.

30 degrees N.

33 E. of due N.

9:52 p.m.

9:40 p.m.

9:25 p.m.

28 degrees N.

35 E. of due N.

10:05 p.m.

9:53 p.m.

9:38 p.m.

26 degrees N.

37 E. of due N.

10:18 p.m.

10:06 p.m.

9:50 p.m.

All times are given in civil or local daylight time (LDT), which differs from ordinary clock time by many minutes at most locations. Most civil time zones worldwide have been standardized on particular longitudes at increments of 15. As an example, across Europe, 0 (the Greenwich Meridian); 15 east; 30 east, etc. Across North America, there is 60 west (Atlantic Time), 75 west (Eastern Time), 90 west (Central Time), etc. If your longitude is very close to one of the standard meridians, luck is with you and your correction is zero.

To get local standard time, add four minutes to the times listed for each degree of longitude that you are west of your time zone meridian. Or subtract four minutes for each degree you are east of it.

Your clinched fist, held at arms length will measure roughly 10 degrees. So 20 degrees would measure roughly "two fists" when making an estimate of azimuth.

EXAMPLE: From Durham, North Carolina, on September 22, when and where will Capella appear to rise? Durham is located near latitude 36 north and longitude 79 west. Looking at the column under September 22, we see a rise time of 8:51 p.m. for latitude 36. But since Durham is located 4 west of the Standard meridian, we must add 16 minutes to 8:51 p.m. So Capella will actually rise at 9:07 p.m. From the latitude of Durham, Capella will appear to rise 27 degrees east of due north, or less than "three fists" to the east (right) of due north.

For other dates and latitudes, you can interpolate.

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Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.

DEFINITIONS

Degrees measure apparent sizes of objects or distances in the sky, as seen from our vantage point. The Moon is one-half degree in width. The width of your fist held at arm's length is about 10 degrees.

1 AU, or astronomical unit, is the distance from the Sun to Earth, or about 93 million miles.

Magnitude is the standard by which astronomers measure the apparent brightness of objects that appear in the sky. The lower the number, the brighter the object. The brightest stars in the sky are categorized as zero or first magnitude. Negative magnitudes are reserved for the most brilliant objects: the brightest star is Sirius (-1.4); the full Moon is -12.7; the Sun is -26.7. The faintest stars visible under dark skies are around +6.

 

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