Achernar in the birth chart: the star at the end of the river

Star data

sefstars catalogue, Swiss Ephemeris 2.10.03.

Achernar is the bright star at the southern end of Eridanus, the river constellation, and in the tropical zodiac it stands in the middle of Pisces. It lies very far south of the ecliptic and cannot be seen from mid-northern latitudes, Ukraine included. It also has no entry in Ptolemy, which makes it unusual among the stars in this section: its traditional nature rests on a later confusion with another star.

In the sky

The name comes from Arabic and means "end of the river". The river is the constellation Eridanus, and Achernar marks the point where it ends in the southern sky.

Achernar's latitude from the ecliptic is about 59° south. Only Vega, about 62° north, lies further from the ecliptic among the stars covered here. At that distance the zodiac degree is a pure projection: a planet "conjunct Achernar" in longitude shares the star's degree, but in the sky the two are separated by most of a quarter circle.

What kind of star it is

Achernar is a hot blue star of spectral class B, about 140 light-years away, with a surface temperature of about 20,000 °C and a mass about six times the Sun's. What sets it apart is its spin. Its equator turns at more than 200 km per second, not far below the speed at which the star would start to shed its own outer layers.

The spin shows in its shape. In 2003 astronomers at the European Southern Observatory measured Achernar with the VLT Interferometer in Chile and found it far flatter than models predicted: the flattest star then measured, much wider across the equator than from pole to pole. Later measurements gave a somewhat smaller flattening, but the star remains a standard example of a fast rotator. It also has a close companion, a smaller star that circles it in roughly seven years.

Why Ptolemy does not describe it

In the first book of the Tetrabiblos Ptolemy assigns planetary natures to the bright stars he could observe, and Achernar is not among them. From the latitude of Alexandria, where he worked, it was not visible, so it is absent from his star lists. Ptolemy does speak of "the last star of the River", but that was a different star, Acamar.

Precession explains why he could not see it. In Ptolemy's time Achernar stood further south than today, about 67° south of the celestial equator, too low ever to clear the horizon at Alexandria. Today it rises there, but only just.

The Arabic name Akhir al-Nahr, "end of the river", first belonged to Theta Eridani, the last star of the river known to the early Arab astronomers. Allen's Star Names and Their Meanings (1899) records that the Alfonsine Tables gave that star the separate name Acamar. When the name "end of the river" later passed to Achernar, the description attached to the end of the river came with it. That is how Achernar acquired the nature of Jupiter that some later lists give it: the attribution goes back to this mix-up rather than to anyone describing Achernar itself. The overview explains how Ptolemy's planetary descriptions are read, but for Achernar there is simply no original description to go back to.

A nature quoted for a star is only as reliable as the identification behind it. The confusion has at least been settled on the astronomical side: in 2016 the International Astronomical Union fixed Achernar and Acamar as the names of two separate stars.

Reading a conjunction

Achernar is read, like other stars, by conjunction with planets and angles within an orb of about 1°. Two things make it a weaker reference point than most stars here. Its nature is borrowed, and its distance from the ecliptic turns a longitude conjunction into a projection across a large part of the sky.

Those who use the later Jupiter attribution read it as follows.

• Jupiter on Achernar. A Jupiterian star on Jupiter itself doubles the planet's themes: growth, trust, generosity, teaching, the wish to widen one's horizons. Read this way, Jupiter speaks louder in the chart, for better and for worse, since Jupiter in excess also means overreaching.
• Achernar on the MC. On the Midheaven the star's themes move into work and reputation. Read as Jupiter, this suggests a public role in teaching or advising.

Parans and visibility

For stars this far from the ecliptic some astrologers prefer parans, and Achernar shows clearly why parans depend on latitude. Achernar never rises anywhere north of about 33° north, which leaves out all of Europe; at 50° north, at its highest, it would still be some 18° below the horizon. North of that line it never rises or sets, so for a birth there no paran with Achernar on the Ascendant or Descendant is possible at all. Further south, where the star does rise, those parans exist. To see Achernar with one's own eyes, someone in Ukraine or anywhere in Europe has to travel at least as far as Egypt. In the southern hemisphere it never sets south of the 33rd parallel, and it stands highest at midnight in mid-October and in the evening in early December.

Precession and sign

Achernar moves through the zodiac at the usual rate, about 1.4° a century. In the middle of Pisces it is far from either edge of the sign and will stay there for many centuries. Its nearest neighbour in the zodiac among the stars here is Fomalhaut, at the start of Pisces, which unlike Achernar has a nature from Ptolemy.

In celebrity charts

Conjunctions within 1° to the planets from the Sun to Saturn and to the chart angles.

• Bernard Arnault: Sun, orb 0°11′
• Marion Cotillard: Descendant, orb 0°50′

Find fixed stars in your chart