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About the cover


Eridanus, the River

Plus the planets for February.


Norman Maclean's novella A River Runs Through It is about two boys and the men they eventually become; one focus of the story is the Blackfoot River and how fly fishing is a common thread through their lives. So, too, does a river run through our winter sky. The constellation Eridanus, the River, runs from halfway up in our sky southwestward toward to the horizon. The eastern bank of Eridanus is formed by the constellations Orion, Lepus, Caelum and Horologium. The western bank is formed by Cetus, Fornax and Phoenix. Eridanus ends at Hydrus, near declination -57 degrees, just below our southern horizon.

star map
(Click the image for a larger map)

Eridanus, the River, runs though the center of our February sky. The Eridanus Supervoid is shown relative size, some 10 degrees across. The colors indicate the relative temperature of the cosmic background microwave radiation, with blue being the coldest. It is unclear if this is really a 500-million-light-year-across area with a very low density of matter or just an unusual fluctuation (or something completely new).

At the south end of this "river" is the brightest star in Eridanus, the first-magnitude star Achernar, which means "the river's end." Achernar barely gets a half-degree above our southern horizon when it is straight south of us. About 144 light-years away from us, this star is spinning very rapidly, so its equatorial diameter is 50% larger than its polar diameter. It would appear as an extremely oval sun in the sky of any planet orbiting it!

The mythology on this constellation is uncertain. It has been associated with the river Po in Italy, but it has also been associated with other rivers at various times by various peoples. The Greek myth of Phaëton is associated with Eridanus as well. Phaëton wanted proof from his mother Clymenē, who was married to the human King Merops, that his real father was Helios, the sun god. Helios drove his fiery chariot (the Sun) through the sky each day. Clymenē convinced Helios to allow Phaëton to drive the chariot for one day.

The day came, and Phaëton took the reins. The flaming chariot ascended into the sky, but Phaëton could not control the mighty horses pulling it, and it veered too close to both heaven and Earth, scorching both. To stop the destruction, Zeus stepped in with a thunderbolt, slaying Phaëton. His flaming corpse fell to earth, landing in the river Eridanos, which put out the flames. That river is commemorated by our constellation of Eridanus.


You cannot talk about Eridanus without mentioning the nearby star Epsilon Eridani. This 3.7-magnitude star is just 10.5 light-years away. It is only about 400 million years old, a veritable youngster. It is just a little smaller than our Sun, and possibly has two planets imbedded in a large ring of dust that circles the star. With its nearness and Sun-like properties, this planetary system has often been a target for interstellar journeys in science fiction.

Perhaps the most interesting phenomena in this constellation is an area that appears colder than most of the rest of the sky. This was first detected by NASA's Wilkinson Microwave Anisotropy Probe (WMAP). Launched on June 30, 2001, this spacecraft was designed to measure the cosmic background microwave radiation, a faint echo of the Big Bang. The goal of WMAP was to measure the difference in the temperature of this microwave radiation over the entire sky. It could not be done from Earth because our atmosphere would interfere in the measurement.

In mapping the tiny temperature differences, there turned out to be a cold spot in southwestern Eridanus. This cold spot appears to be real, but there is a slight chance that it is just an unlucky fluctuation. If this is real void, it would be the biggest known structure in the universe, some 500 million light-years across. This "supervoid" would be between 6 billion and 10 billion light-years away from us.

Astronomers checking catalogs of galaxies in the sky have found that there are fewer galaxies in the cold spot than in other areas in the sky, supporting the supervoid theory. Unfortunately, targeted telescopic counts of galaxies in the cold spot do not appear much different than in surrounding areas. Still, a supervoid has not been completely ruled out, and it will certainly be interesting to see what the European Space Agency's Planck spacecraft, launched May 14, 2009, will find in this area of the sky as it refines the observations from WMAP.


The Planets for February


Mercury is back in our evening sky, after spending last month too near the Sun to be seen. Mercury appears low on the west-southwestern horizon as the month starts. Passing Mars on Feb. 7, Mercury continues moving upward, until it reaches its farthest point east of the Sun (18 degrees) on Feb. 16. The Messenger of the Gods then turns back toward the Sun, passing Mars again on Feb. 26, and disappearing back into the bright twilight by the end of the month. This track starts in Capricorn, moving rapidly into Aquarius. Continuing east, it slows and turns north, entering Pisces, where it ends the month traveling westward toward the Sun. At its farthest distance from the Sun, it will shine at magnitude 0.1 with a disc that is a 35% illuminated (and shrinking) crescent that is 7.9 seconds-of-arc across.

Watch the Skies

(all times MST)


Feb. 3, 6:56 a.m. — Last Quarter Moon

Feb. 8, 2 p.m. — Mercury 0.3° north of Mars

Feb. 10, 12:20 a.m. — New Moon

Feb. 16, 2 p.m. — Mercury greatest distance
     from the Sun (18°)

Feb. 17, 1:31 p.m. — First Quarter Moon

Feb. 19, 4 a.m. — Saturn stationary

Feb. 22, 12 p.m. — Mercury stationary

Feb. 25, 1:26 p.m. — Full Moon



Mars is a difficult object this month, glowing at magnitude 1.2, just seven degrees above our western horizon as it gets dark. Mars is moving slowly eastward along the ecliptic in Aquarius in February, from this constellation's western boundary to its northern boundary with Pisces. The God of War's disc is a tiny 4.0 seconds-of-arc across, and it sets at 6:55 p.m.

Moving eastward just north of the Hyades star cluster in Taurus, the King of the Planets is high in our south-southeastern sky as it gets dark. At magnitude -2.5, Jupiter's disc is 40.8 seconds-of-arc across. It sets around 1:45 a.m.

The Ringed Planet starts the month near the star Zubenelgenubi in Libra. Saturn moves slowly eastward until Feb. 19, when it becomes stationary and then turns back, heading westward. It shines at magnitude 0.5, rising around 11:30 p.m. Saturn's Rings are 39.5 seconds-of-arc across and are tilted down 19.3 degrees with the northern face showing. Saturn's disc is 17.4 seconds-of-arc across.

You might be able to catch a look at Venus very low in the east-southeast during the first few days of the month, but it will quickly become too close to the Sun to be visible for the rest of the month. The Goddess of Love will not be back in our sky until late April, when it will be in the evening sky for the rest of the year. Speaking of 2013, it will not be a leap year, so you will have one less night this year to "keep watching the sky"!




An amateur astronomer for more than 40 years, Bert Stevens is co-director
of Desert Moon Observatory in Las Cruces.


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