Stars over Sisters

Eridanus the River is the sixth largest constellation in the sky, but it is also obscure, being comprised of faint stars, save one.

The sprawling River originates near Rigel, the bright star that marks the left foot of Orion the Hunter. From here it flows westward then turns south before meandering back toward the east. From our latitude this is about as much of the constellation that we can see.

Eridanus continues its journey southward eventually terminating at the bright star Achernar, located only about 30 degrees from the celestial south pole. The ninth-brightest star in the sky, Achernar means "the river's end."

To see the northern portion of this constellation in January, pick a clear, moonless night and look to the west and south of Orion.

The mythology associated with Eridanus involves Phaëton, the son of the sun god Helios. Phaëton begged to drive Helios' chariot until Helios finally relented. He warned Phaëton to stay on the well-traveled road, but once Phaëton was in the chariot the horses pulling it flew off the road and into the sky. He lost control of the reins and the chariot flew so close to the Earth that the land caught on fire. Zeus struck Phaëton with a thunderbolt to stop the destruction, which caused Phaëton to fall out of the chariot and into the river Eridanus. The constellation is also often said to represent the river Po in Italy.

At a distance of 10.5 light-years, Epsilon Eridani is the third-closest star from Earth that can be seen with the naked eye, after Alpha Centauri and Sirius. It is also the nearest star to the earth that is comparable to the sun and has at least one Jupiter-size planet of its own.

Omicron2 Eridani is a triple star system, meaning that there are three stars that closely orbit one another. Companion B of this stellar trio is a white dwarf, the first one discovered and the only one easily visible to observers with a small telescope. A white dwarf is an extremely small, very dense star that marks the last stage of evolution for small-to-medium-mass stars.

There are at least two deep-sky objects of note in Eridanus. The diffuse nebula IC 2118, also known as the Witchhead Nebula because of its distinctive shape, is located at a distance of about 1,000 light-years. NGC 1300 is one of the finest examples of a face-on barred spiral galaxy in the entire sky. It is some 61 million light-years distant and measures about 110,000 light-years across, slightly larger than our own Milky Way galaxy.

On January 3 at 10:37 p.m. PST, the earth will be closer to the sun than at any other time of the year. Called perihelion, earth will approach to within 91.4 million miles of the sun; 3.1 million miles nearer than it gets in early July.

During early-to-mid-January, Venus, Mercury, and Mars will be huddled together in Capricornus, visible low in the southwestern sky. While Mercury and Venus set relatively soon after the sun, Mars stays up for an additional three hours. Rising just 20 minutes after sunset by month's end, brilliant Jupiter (currently in Leo) begins to dominate the nighttime hours. Finally, Saturn will rise a few hours before the sun in the southeastern sky and appear above the red star Antares in the constellation of Scorpius.

As the month begins, a waxing moon becomes the Full Wolf Moon on January 4. Last quarter will occur on January 13, followed by the new moon on January 20 and first quarter on January 26.