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The sixth largest constellation in the sky, Eridanus is a constellation in the southern hemisphere. The name comes from an ancient Greek name for the river Po in northern Italy. In Sanskrit, it is also called ‘srotaswini’ meaning ‘stream, current or torrent’. Also known as the Celestial River, it is most often related to the Nile or Euphrates Rivers as they were so important to the ancient civilizations. The Greek astronomer Ptolemy first catalogued it in the 2nd century. Eridanus belongs to the Heavenly Waters family of constellations and is best visible in the month of December. Let us discuss the constellation in detail.

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Which stars comprise the constellation?

Some of the notable stars of this constellation include:

Achernar (Alpha Eridani): Achernar is the brightest star in Eridanus and is also the ninth brightest star in the night sky. It has a visual magnitude of about 0.445 and is about 139 light years away from the earth. The star is the hottest and the bluest among the ten brightest stars in the sky. Achernar is about 3,150 times brighter than the sun.
Cursa (Beta Eridani): Cursa is the second brightest star in the constellation. It has a visual companion with a magnitude of 10.90 and is located about 120 arc seconds away. It lies about 89 light years away from the solar system. It is a fast spinner with a projected rotational velocity of 196 km/s and its visual magnitude changes from 2.72 to 2.80.
Acamar (Theta Eridani): Acamar is a binary star, which can be possibly a part of a multiple star system. Its primary component has a visual magnitude of 3.2 whereas its secondary component has a magnitude of 4.3. Both of these stars are separated by 8.3 seconds of arc. Acamar has a visual magnitude of 3.2 and is about 161 light years away from the sun.
Zaurak (Gamma Eridani): Zaurak has a visual magnitude of 2.95 and is about 150 light years away from the solar system.
Rana (Delta Eridani): It is basically a sub giant star in this constellation. With a visual magnitude of about 3.54, it is about 29.49 light years away.
T4 Eridani (Tau –4 Eridani): It is a binary star in this constellation. It has a magnitude of about 3.70 and is about 258 light years away. It also comprises of a primary component, a giant star with a magnitude of 3.66.

The other notable stars in this constellation include e Eridani (Epsilon Eridani), Keid–40 Eridani, 82 G. Eridani (HD 20794) and EF Eridani.

Does it contain any deep sky objects or galaxies?

The deep sky objects of Eridanus include:

Witch Head Nebula: It is a faint reflection nebula in this constellation. Witch Head Nebula has a visual magnitude of about 12 and is around 1,000 light years away from the solar system.
Eridanus Group (Eridanus Cloud): Eridanus Group is basically a group of galaxies, about 75 million light years away. The group comprises of 200 galaxies and about 70% of these galaxies are spiral and irregular. The other 30% are elliptical and lenticular galaxies.
NGC 1300: It is a barred spiral galaxy in this constellation, about 110,000 light years across. NGC 1300 has a visual magnitude of about 11.4 and is about 61.3 million light years away.
NGC 1332: NGC 1332 is a spiral galaxy in this constellation and has a visual magnitude of about 10.3. It was discovered by William Herschel on December 9, 1784.
NGC 1395: It is a bright, large elliptical galaxy in this constellation with a visual magnitude of 9.8. It was also discovered by William Herschel on November 17, 1784.
NGC 1232: It is an intermediate spiral galaxy in this constellation. With a magnitude of 10.9, it is about 61 million light years away.
NGC 1234: It is a barred spiral galaxy in this constellation with a visual magnitude of 15.3. NGC 1234 was catalogued by an American astronomer Francis Preserved Leavenworth in 1886.
NGC 1535: It is a planetary nebula in this constellation, which is similar in colour and structure to the Eskimo Nebula in the constellation Gemini. NGC 1535 is about 1,500 light years away.
NGC 1531 and NGC 1532: NGC 1531 is a dwarf galaxy in this constellation, whereas NGC 1532 is a barred spiral galaxy. NGC 1531 has a visual magnitude of 12.9, whereas NGC 1532 has a visual magnitude of 10.7.

The other notable deep sky objects in this constellation are Eridanus Supervoid, NGC 1427A, NGC1309, NGC 1291 and NGC 1187.

What is its position in the galaxy?

Eridanus lies in the first quadrant of the southern hemisphere and can be seen at latitudes between +32° and -90°.

How much space does it occupy in the sky?

It occupies 1,138 square degrees of space, making it the sixth largest constellation in the night sky.

What is the cultural or mythological significance of Eridanus?

The myth of Phaeton, the son of the god Helios, is associated with the constellation Eridanus. One day, Phaeton took the reins of his father’s chariot but could not control it. This act ended up burning both heaven and earth. To avoid the consequences, Zeus killed him with a thunderbolt. The path on which Phaeton drove is represented as the constellation Eridanus. It was further found that his body was extinguished in the river Po, once cast to earth.

Which are its neighbouring constellations?

Eridanus was surrounded by constellations such as Caelum, Cetus, Fornax, Horologium, Hydrus, Lepus, Orion, Phoenix, Taurus and Tucana.

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