Constellations for December 2024 –
Eridanus and Fornax
Eridanus is an ancient constellation with a complex history. It may be connected to one of the very first cities, Eridu, which lay on the Euphrates near modern Basra, and their constellation was then taken over by the Ancient Greeks. The Romans called the River Po Eridanus and it has also been linked to the Nile. In Greek mythology, Phaethon, the son of the Sun god, borrowed his father’s chariot, but lacking any driving skills, he swerved between the Earth and the heavens, scorching both of them. Zeus struck him dead with a thunderbolt and he fell to Earth. Eridanus with its bends and turns represented the path he took.
To observers in Havering, Eridanus labours under three difficulties. Only half of the constellation is visible from the southern UK and its two brightest stars, Achernar and Acamar, are below the horizon. With two exceptions, its stars are fourth magnitude and the constellation lacks any obvious shape. One of the third magnitude stars, Cursa (meaning footstool) lies close to Orion and neighbouring Lambda Eridani is 1.5 degrees south-west of Rigel. The other third magnitude star, Zaurak (which means boat in Persian), is a red giant which lies in the middle of the constellation (as seen from the UK).
Eridanus also lacks any deep-sky objects, but does have four interesting stars. Ran (Epsilon Eridani) is only 10.5 light years away and is the second closest star we can see with the naked eye in Havering (although at magnitude 3.7 it will be touch and go). It was called Ran after a Nordic sea goddess by schoolchildren at Mountainside Middle School, Colbert, Washington State and was accepted by the IAU Working Group on Star Names. It has an exoplanet, named after Ægir, the husband of Ran and the god of the Ocean. Keid (Omicron2 Eridani, 40 Eridani) is a very nice binary system. The main star (mag. 4.4) is a K type main sequence star. The brighter companion (but only mag. 9.3) is a white dwarf and nearby is the third even dimmer (mag. 11.2) companion which is a red dwarf. This is probably the easiest way of observing a white dwarf or a red dwarf and I have been able to observe them in a five-inch Maksutov-Cassegrain. Keid means eggshells, as the name of the neighbouring star Beid is an egg in Arabic.
There are two other doubles in Eridanus which are worth observing. 32 Eridani [1] is a fairly easy golden yellow and reddish orange pair which are magnitude 4.8 and 5.9. The stars in 55 Eridani [2] are both yellow-white and almost equally bright (mag. 6.7 and 6.8) and are easily split. It is still not clear if either of these doubles are a binary system.
Fornax is one of these pointless constellations created by Abbé Nicolas Louis de Lacaille during his trip to the Cape of Good Hope in the early 1750s from stars which had been included in Eridani. He called it the Chemical Furnace (now shortened to Fornax) in honour of chemistry. When he got back to France, he became the teacher of Antoine Lavoisier who went on to revolutionise chemistry but rarely used a furnace. It is a small constellation which is not really visible from the UK, but we can see its brightest star Dalim (so-called from al-zalim for ostrich), which is a double star. The main star is magnitude 4.0 and its companion is magnitude 7.2 with an easy separation of 5.4 arcseconds. They will, however, be very badly affected by extinction (to 4.7 in the case of the main star) as it is never more than nine degrees above the horizon. The colours are probably both yellow-white. It is a binary system with a very elliptical orbit. Slightly higher in the sky (at the right time) is Omega Fornacis [3], a binary star with a magnitude 5.0 main star and a 7.7 companion, which are nicely wide (11 arcsec.) and are yellow-white and grey-blue.