Acamar (Theta Eridani)
Acamar (Theta Eridani) is a star in the constellation Eridanus, magnitude 2.9.
The old end of the celestial river — a fine double whose inner pair may once have blazed far brighter.
Ptolemy’s Almagest, in the 2nd century, listed Acamar among the very brightest stars in the sky, and al-Sufi did the same in 964. Today it is a modest third-magnitude star. One suggestion is that its inner pair went through a long outburst, lasting perhaps a thousand years, as one star spilled gas onto the other.
That inner pair is remarkable in itself: two white stars, each more than twice the Sun’s mass, whirling round each other every four days. They are so close that tides pull them out of round, and as they turn, the light we see rises and falls. W. H. Wright detected the pair in their spectrum in 1905; in 2025 the GRAVITY instrument at the Very Large Telescope split them directly. A third white star, Theta² Eridani, completes this multiple star.
The name is a copyist’s slip. Arabic Ākhir an-nahr, ‘the end of the river’, became Acamar when ‘rn’ was misread as ‘m’. For the Greeks the river really did end here: Achernar, which carries the same Arabic name, never rose above their horizon.
Acamar (Theta Eridani) is 160 light-years away: the light we see from it set off around 1865, 13 years before Edison lit his first practical light bulb — the start of light pollution.
Did you know?
- Due for a swap. As the larger star of the inner pair keeps swelling, it is expected to begin pouring gas onto its partner within a few tens of millions of years.
- The Celestial Orchard. Chinese astronomers placed Acamar in Tiān Yuán, the Celestial Orchard, an asterism of twelve stars in Eridanus. It is the Orchard’s sixth star.
A fine double star for small telescopes: two white stars of 3rd and 4th magnitude, about 8 arcseconds apart. It lies in the southern reaches of Eridanus, roughly 19 degrees north-east of brilliant Achernar.
When and where to see it
It is best placed in the evening sky around December, when it crosses the meridian at about 10 pm local time (around midnight in November). It can be seen from everywhere south of 50°N and never sets south of 50°S.
At its highest it stands below the horizon from 50°N, 15° up from 35°N, 50° up from the equator, 85° up from 35°S.
Facts
- Type
- Star
- Constellation
- Eridanus
- Designations
- Theta Eridani, θ Eri, HIP 13847
- Magnitude
- 2.9
- Position
- RA 2h 58.3m, Dec −40° 18′ (J2000)
- Spectral type
- A2V
- Distance
- 160 light-years
- Discovered
- Known since antiquity
Open AstroPilot to see when Acamar (Theta Eridani) is highest tonight from your location, how easy it is with your telescope or binoculars, and an eyepiece view.