Star Constellation Facts: Horologium

Horologium Constellation
Credit: Horologium constellation by IAU and Sky&Telescope magazine

Horologium (“the clock”) is a small, dim southern constellation that is one of fourteen invented by French astronomer Nicolas Louis de Lacaille in the mid-18th century. The brightest star in this pendulum clock-shaped constellation is Alpha Horologii, an orange giant situated 117 light years from Earth that shines with an apparent magnitude of 3.86.


Horologium is the 58th largest constellation, taking up an area of 249 square degrees of the southern celestial sphere. It can be seen by observers located between +30° and -90° of latitude, although best viewed from November to March. The constellations bordering Horologium includes Cealum and Reticulum to its east, Eridanus to its north and west, and Hydrus to its south. The constellation is very faint, but can be found close to the bright star Achernar in the constellation of Eridanus.

Lacaille Constellation Family

Horologium is a member of the Lacaille family of constellations, together with Antlia, Caelum, Circinus, Fornax, Mensa, Microscopium, Norma, Octans, Pictor, Reticulum, Sculptor, and Telescopium.


French astronomer Nicolas Louis de Lacaille studied the stars of the southern hemisphere from an observatory in South Africa from 1751–3. He subsequently created fourteen new constellations, including one he originally called Horologium Oscillitorium (“the pendulum clock”) in honor of Dutch scientist Christiaan Huygens, who invented the pendulum clock in 1656. Its name was later shortened to just Horologium.

Principal Stars

Horologium Constellation– Alpha Horologii, the constellation’s brightest star, is an orange giant (K1III) found 117 light years from our solar system with a visual magnitude of 3.86. This billion year old star is around 11 times bigger than the Sun, with double its mass, and 47 times its luminosity.

– R Horologii (HD 18242), the second brightest star in Horologium, is a red giant (M7IIIe) located 700 light years distant that is around 14 times bigger than the Sun. This Mira variable star shines with an apparent magnitude that ranges between 4.7 and 14.3 over a 404.83 day period, which is one of the biggest variations in star brightness yet discovered.

– Beta Horologii, the constellation’s third brightest star, is a blue giant (A5III) found 310 light years away of magnitude 4.98. This 600 million year old star is around 4 times bigger than the Sun, with 2.5 times its mas, and 65 times its luminosity.

Other stars of interest in Horologium includes the blue dwarf star Delta Horologii, the white dwarf Eta Horologii
the yellow dwarf stars Iota Horologii and Zeta Horologii, and the red dwarf GJ 1061.

Notable Deep-Sky Objects

Horologium may not contain any no Messier objects, but there are a number of interesting deep-sky objects to be found in the constellation.

– Horologium Supercluster is vast supercluster of galaxies spanning some 550 million light years of space, with its nearest region around 700 million light years distant from our solar system. Also known as the Horologium-Reticulum Supercluster, it contains about 5,000 galaxy groups consisting of 30,000 giant galaxies and a further 300,000 dwarf galaxies.

is a barred spiral galaxy that is 70,000 light-years wide, and situated 30 million light-years distant. Surrounding its galactic nucleus is a 2,400 light-year-wide ring of young stars, while lying further out is a circle of dimmer stars. Meanwhile, a mere 68,000 light-years away is the much smaller lenticular galaxy NGC 1510, which is 15,000 light years across, with the strong gravitational interactions taking place between the two galaxies causing one of the spiral arms of NGC1512 to warp.

Other galaxies of interest in Horologium includes the barred spiral galaxies NGC 1483 and Miltron’s Galaxy (NGC 1433), and the unbarred spiral galaxy NGC 1448.

– NGC 1261 is a globular cluster found around 53,500 light years away of magnitude 8.3. It is believed to be around 10.24 billion years old, and contains mostly very dim stars.

– Arp-Madore 1 (AM1) is found 398,000 light years away, making it one of the Milky Way‘s most distant globular clusters. It is also around three times bigger than our own galaxy, and has an apparent magnitude of 15.72.

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