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Constellation in the southern celestial hemisphere

Horologium (Latin hōrologium, the pendulum clock, from Greek ὡρολόγιον, ) is a constellation of six stars faintly visible in the southern celestial hemisphere. It was first described by the French astronomer Nicolas Louis de Lacaille in 1756 and visualized by him as a clock with a pendulum and a second hand. In 1922 the constellation was redefined by the International Astronomical Union (IAU) as a region of the celestial sphere containing Lacaille's stars, and has since been an IAU designated constellation. 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 (at 3.85), an aging orange giant star that has swollen to around 11 times the diameter of the Sun. The long period variable brightness star, R Horologii (4.7 to 14.3), has one of the largest variations in brightness among all stars in the night sky visible to the unaided eye. Four star systems in the constellation are known to have exoplanets; at least one—Gliese 1061—contains an exoplanet in its habitable zone.

The French astronomer Nicolas Louis de Lacaille first described the constellation as l'Horloge à pendule & à secondes (Clock with pendulum and seconds hand) in 1756, after he had observed and catalogued almost 10,000 southern stars during a two year stay at the Cape of Good Hope. He devised fourteen new constellations in previously uncharted regions of the southern celestial hemisphere, which were not visible from Europe. All but one honoured scientific instruments, and so symbolised the Age of Enlightenment. efn|1=The exception is Mensa, named for Table Mountain. The other twelve (alongside Horologium) are Antlia, Caelum, Circinus, Fornax, Microscopium, Norma, Octans, Pictor, Pyxis, Reticulum, Sculptor and Telescopium. The Latin term is ultimately derived from the Ancient Greek ὡρολόγιον, for an instrument for telling the hour.

Covering a total of 248.9 square degrees or 0.603% of the sky, Horologium ranks 58th in area out of the 88 modern constellations. Its position in the southern celestial hemisphere means the whole constellation is visible to observers south of 23°N. The official constellation boundaries are defined by a twenty two sided polygon (illustrated in infobox). In the equatorial coordinate system, the right ascension coordinates of these borders lie between and , while the declination coordinates are between −39.64° and −67.04°.

Horologium has one star brighter than apparent magnitude 4, and 41 stars brighter than or equal to magnitude 6.5. 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 of spectral type K2III that has swollen to around 11 times the diameter of the Sun, having spent much of its life as a white main sequence star. At an estimated 1.55 times the mass of the Sun, it is radiating 38 times the Sun's luminosity from its photosphere at an effective (surface) temperature of 5,028K. At magnitude 4.93, Delta Horologii is the second brightest star in the constellation, and forms a wide optical double with Alpha. Delta itself is a true binary system composed of a white main sequence star of spectral type A5V that is 1.41 times as massive as the Sun with a magnitude of 5.15 and its fainter companion of magnitude 7.29. The system is located 179 (±4) light years from the earth. and has been little studied. It is 161 (±1) light years from Earth. that is around 1.9 times as massive as the Sun. Estimated to be around 540 million years old, it has a debris disk that appears to have two components: an inner disk is orbiting at a distance of 96, while an outer disk lies 410 from the star. The estimated mass of the disks is 0.13 the mass of the Earth. Horologium has several variable stars. R Horologii is a red giant Mira variable with one of the widest ranges in brightness known of stars in the night sky visible to the unaided eye. It is around 1,000 light years from Earth. It has a minimum magnitude of 14.3 and a maximum magnitude of 4.7, with a period of approximately 13 months. TW Horologii is a semiregular variable red giant star that is classified as a carbon star,

Iota Horologii is a yellow white dwarf star 1.23 (±0.12) times as massive and 1.16 (±0.04) times as wide as the Sun with a spectral type of F8V, 57 (±0.05) light years from Earth. Its chemical profile, movement and age indicate it formed within the Hyades cluster but has drifted around 130 light years away from the other members. It has a planet at least 2.5 times as massive as Jupiter orbiting it every 307 days. HD 27631 is a Sun like star located 164 (±0.3) light years from Earth which was found to have a planet at least 1.45 times as massive as Jupiter that takes 2,208 (±66) days (six years) to complete an orbit. WASP 120 is a yellow white main sequence star around 1.4 times as massive as the Sun with a spectral type of F5V that is estimated to be 2.6 (±0.5) billion years old. It has a massive planet around 4.85 times the mass of Jupiter that completes its orbit every 3.6 days, and has an estimated surface temperature of 1,880 (±70)K.

With an apparent magnitude of 13.06, Gliese 1061 is a red dwarf of spectral type M5.5V that has 12% of the mass and 15% of the diameter of the Sun, and shines with only 0.17% of its luminosity. Located 12 light years away from Earth, it is the 20th closest single star or stellar system to the Sun. In August 2019, it was announced that it had three planets, one of which lay in its habitable zone.

Horologium is home to many deep sky objects, including several globular clusters. NGC 1261 is a globular cluster of magnitude 8, located 53,000 light years from Earth. It lies 4.7 degrees north northeast of Mu Horologii. 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 (13.8 kpc) away is the dwarf lenticular galaxy NGC 1510. The two are in the process of a merger which has been going on for 400 million years. The Horologium Reticulum Supercluster is a galaxy supercluster, second in size only to the Shapley Supercluster in the local universe (anything within 200 mpc of Earth). It contains over 20 Abell galaxy clusters and covers more than 100 deg2 of the sky, centered roughly at equatorial coordinates α = , δ = .