Horologium is a constellation of six stars faintly visible in the southern celestial hemisphere. The name in Latin for the pendulum clock, which comes from the Greek ὡρολόγιον, which means ‘an instrument for telling the hour.’ The constellation was first described by the French astronomer Nicolas-Louis de Lacaille in 1756, which he visualized as a clock with a pendulum and a second hand. In 1922 the constellation was redefined by the International Astronomical Union as a region of the celestial sphere containing Lacaille’s stars. Horologium’s associated region is wholly visible to observers south of 23°N.

The constellation’s brightest star, and the only one brighter than an apparent magnitude of 4, is Alpha Horologii. It is an aging orange giant star that has swollen to around 11 times the diameter of the Sun. Four star systems in the constellation are known to have exoplanets, and at least one, Gliese 1061, contains an exoplanet in its habitable zone.

Horologium ranks 58th in area out of the 88 modern constellations as it covers a total of 248.9 square degrees, or 0.603% of the sky. 

	Applicable Information
Visibility In Pacific Northwest	Only Partially Visible November to February
Best Times To View	December
Right Ascension	3h
Declination	−60°
Area	249 square degrees
Main Stars	6
Brightest Object	α Hor
Meteor showers	0
Messier objects	0
Neighboring Constellations	Eridanus, Hydrus, Reticulum, Dorado, Caelum

History

The French astronomer Nicolas Louis de Lacaille described the constellation as l’Horloge à pendule & à secondes, which translates to “Clock with pendulum and seconds hand” in 1756. This was during his time studying the southern sky during a 2 year period at the Cape of Good Hope. Horologium was 1 of the 14 constellations he devised during this time. The constellation name was Latinised to Horologium in a catalogue and updated chart published posthumously in 1763. The Latin term is ultimately derived from the Ancient Greek ὡρολόγιον, for an instrument for telling the hour.

Stars

Horologium has one star brighter than apparent magnitude 4, and 41 stars brighter than or equal to magnitude 6.5. Lacaille charted and designated 11 stars in the constellation, giving them the Bayer designations Alpha through Lambda Horologii in 1756. In the mid-19th century, English astronomer Francis Baily removed the designations of two stars, Epsilon and Theta Horologii, as he held they were too faint to warrant naming.

At magnitude 3.9, Alpha Horologii is the brightest star in the constellation. German astronomer Johann Elert Bode depicted it as the pendulum of the clock, while Lacaille made it one of the weights. It is an orange giant star that has an estimated 1.55 times the mass of the Sun, but radiates 38 times the Sun’s luminosity from its photosphere.

Horologium is home to many deep-sky objects, including several globular clusters. NGC 1261 is a globular cluster of magnitude 8, located 4.7 degrees north-northeast of Mu Horologii. The globular cluster Arp-Madore 1 is the most remote known globular cluster in the Milky Way at a distance of 402,000 light-years from Earth.

NGC 1512 is a barred spiral galaxy 2.1 degrees west-southwest of Alpha Horologii with an apparent magnitude of 10.2. About five arcmin from NGC 1512 is the dwarf lenticular galaxy NGC 1510. The two objects are said to be in the process of an ongoing 400 million year merger.

The Horologium-Reticulum Supercluster is a galaxy supercluster, second in size only to the Shapley Supercluster in the local universe. It contains over 20 Abell galaxy clusters and covers more than 100 deg2 of the sky.

Make sure to check out other articles on the site, including a brief introduction to constellations, other constellation articles, and more!