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

Constellation of the Month: Eridanus, the River

Eridanus, the River, is a long, winding constellation that covers 1,138 square degrees, making it the sixth largest constellation in our sky. Eridanus starts at the corner of Orion and Taurus, flowing westward before turning south. It continues southeast before turning southwestward again, ending at the first magnitude star Achenar. The north end of the constellation is 53 degrees up in our sky, while Achenar just barely peeks above our southern horizon before setting again.

click the image for a larger map Looking south in the coldest month of the year, Eridanus, the River, cuts a path that runs from just over halfway up in the southern sky all the way down to the first-magnitude star Achernar. Achernar just pops up above our horizon for only an hour before setting again. This celestial river has been identified with many rivers, both actual and mythological.

The mythology behind this constellation is a little murky. This celestial river has been identified with the Nile, Tigris, Euphrates, Ganges, Po or even the Sumerian Strong River, also known as Ariadan. But the myth connected to this constellation involves another celestial object, the Sun.

The Greek god Helios, who wore a brilliant aureole or halo (the Sun), drove a chariot through the sky each day from east to west. Helios had a son, Phaethon, by the nymph Clymene. Phaethon lived with his mother and did not know that his father was a god until he was almost grown, when he undertook to find Helios and prove his birthright. Helios swore by the river Styx that he would give Phaethon whatever proof he wanted. Phaethon, young and overconfident, asked to drive the golden chariot across the sky. Reluctantly, since he'd taken an oath, Helios allowed Phaethon to drive the chariot the next morning. The horses, sensing the inexperienced driver, ran wild, burning the Earth and forming deserts. Zeus saw what was happening and decided enough was enough. He killed Phaethon with a thunderbolt, throwing him from the chariot into the river Eridanus. To console the grieving Helios, Zeus put the river into the sky, where we see it today.

Two stars in Eridanus have interesting stories. One is Achenar (Alpha Eridani), at the southern end; indeed, the name comes from the Arabic for "river's end." It is the sixth brightest star in the sky, shining a few thousand times brighter than our Sun at a distance of only 144 light-years. This blue star takes less than two days to spin once around its axis, perhaps as short a time as two hours. This causes its equator to bulge out twice as far from the center of the star as do the poles. It also causes the star to shed part of its mass at thousands of time the rate of our Sun. This mass enters a disc of gas surrounding the equator of the star.

The other star is Epsilon Eridani — even closer, at only 10.5 light-years away. A K2-class star, it is cooler and smaller than our Sun. Because it is so close to us, Epsilon Eridani often gets used in science-fiction stories. As just one example, in the TV series "Babylon 5," the space station orbits the fictional third planet (Epsilon 3) of this star. In real life, astronomers have discovered two planets around Epsilon Eridani. The first is a Jupiter-like planet in am egg-shaped orbit whose average distance from Epsilon is the same as halfway between Mars and Jupiter would be from our Sun. The other is a low-mass planet that is out in a Pluto-like orbit. There is also a disc of gas around this star.

The Planets for January 2008

The evening sky will have two planets visible, at least during the later part of the month. The first planet in the west is Mercury, visible only during the last two weeks of the month. Starting around Jan. 12, Mercury will be in our west-southwestern sky, first as an almost full disc at magnitude -0.9, then becoming more of a crescent as it swings around from the far side of the Sun. Mercury will be farthest from the Sun on Jan. 21, when it will be 56 percent illuminated at magnitude -0.6. It then heads back toward the Sun, disappearing into the evening twilight around Jan. 28.

Mars is the brilliant red star already up in the east-northeastern sky as it gets dark. Moving slowly westward among the stars, Mars is still an excellent telescopic target, almost 14 seconds-of-arc across at mid-month, magnitude -1.1 and almost 98 percent illuminated.

Nearing 9 p.m., Saturn rises on the eastern horizon in south-central Leo. The rings will become a little more open over the next five months, then start closing again; right now, they are tilted up 7.2 degrees to our line of sight with the southern face showing. The rings are 44.3 seconds-of-arc across at mid-month while the ball of the planet is only 19.5 seconds-of-arc across. It is still getting larger as we approach opposition next month.

At 4:45 a.m., a brilliant "star" rises in the east-southeast: the planet Venus. Starting the month in Libra, Venus moves eastward among the stars, passing through Scorpius, Ophiuchus and into Sagittarius by month's end. At mid-month Venus is magnitude -4.0 and 13.4 seconds-of-arc across. The Goddess of Love is shrinking and becoming more full as it starts to move behind the Sun. Venus will be next to Jupiter on Feb. 1.

Jupiter has moved into the morning sky, shining at magnitude -1.9. In Sagittarius all month, the King of the Planets sits right above the "Teapot." It rises around 6 a.m. and is 31.9 seconds-of-arc across. Jupiter is still too low for good observation, but over the next few months it will become higher up, in a better position for observing. So get ready for the King of the Planets and "keep watching the sky"!