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 that the whole constellation is visible to observers south of 23°N.[b] Horologium is bordered by five different constellations: Eridanus (the Po River), Caelum (the chisel), Reticulum (the reticle), Dorado (the dolphinfish/swordfish), and Hydrus (the male water snake). The three-letter abbreviation for the constellation, as adopted by the International Astronomical Union in 1922, is 'Hor'. 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 02h 12.5m and 04h 20.2m, while the declination coordinates are between −39.63° and −67.04°.
The constellation Horologium as it can be seen by the naked eye.
Horologium has only one star brighter than apparent magnitude 4, and 41 stars brighter than or equal to magnitude 6.5.[c] Lacaille charted and designated 11 stars with the Bayer designationsAlpha through to Lambda Horologii in 1756. Francis Baily removed the designations of Epsilon and Theta Horologii as he held they were too faint to warrant naming. He was unable to find a star that corresponded to the coordinates Lacaille's Beta Horologii. Determining that the coordinates were wrong, he assigned the designation to another star. Kappa Horologii, too, was unable to be verified, yet was most likely the star HD 18292. Gould assigned designations to what became Mu and Nu Horologii as he felt they were bright enough to warrant them.
At magnitude 3.9, Alpha Horologii is the brightest star in the constellation, located 115.0 ± 0.5 light-years distant from Earth. Bode depicted it as the clock's pendulum, while Lacaille made it one of the weights. It is an orange giant 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. With 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 temperature of 5,028 K. Delta Horologii is the second brightest star in the constellation, and forms a wide pair with Alpha.
Beta Horologii is a white giant of magnitude 5.0, 314 light-years from Earth.
Lambda Horologii is an ageing yellow-white giant star of spectral type F2III that spins around at 140 km/second, and is hence mildly oblate. It is 161 ± 1 light-years from Earth.
However, Horologium does have several variable stars. R Horologii is a red giantMira variable with one of the widest ranges in brightness known, around 1000 light-years from Earth. It has a minimum magnitude of 14.3 and a maximum magnitude of 4.7; its period is approximately 13 months. T and U Horologii are also Mira variables.TW Horologii is a semiregular variable red giant star that is classified as a carbon star. It is 1370 ± 70 light-years distant from Earth.
Iota Horologii is a yellow-white dwarf star 1.23 ± 0.12 time as massive and 1.16 ± 0.04 time as wide as the Sun with a pectral type of F8V, 56.51 ± 0.05 light-years distant from Earth. Chemical profile, movement and age indicate it formed within the Hyades cluster but has drifted around 130 light-years away from other members. It was found to have a planet at least 2.5 times as massive as Jupiter orbiting it every 307 days.HD 27631 is a Sun-like star located 164.3 ± 0.3 light-years from Earth. that was found to have a planet at least 1.45 times as massive as Jupiter that takes 2208 ± 66 days (or 6 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 arounf 4.85 times as massive as Jupiter that completes its orbit every 3.6 days, and has an estimated surface temperature of 1880 ± 70 K.
Gliese 1061 is a red dwarf 12 light-years away from Earth. In August 2019, it was announced to have three planets, one of which lay in a habitable zone.
Horologium is also home to many deep-sky objects; there are several globular clusters in the constellation. NGC 1261 is a globular cluster of 8th magnitude, located 53,000 light-years from Earth. It lies 4.7 degrees north north east of Mu Horologii. The globular cluster Arp-Madore 1 is found in the constellation, the most remotely known globular cluster in the Milky Way at a distance of 398,000 light years.
NGC 1512 is a barred spiral galaxy 2.1 degrees west south west of Alpha Horologii with an apparent magnitude of 10.2. It located about 38 million light-years away from Earth.
^While parts of the constellation technically rise above the horizon to observers between the latitudes of 23°N and 50°N, stars within a few degrees of the horizon are to all intents and purposes unobservable.
^Objects of magnitude 6.5 are among the faintest visible to the unaided eye in suburban-rural transition night skies.
^ abcWagman, Morton (2003). Lost Stars: Lost, Missing and Troublesome Stars from the Catalogues of Johannes Bayer, Nicholas Louis de Lacaille, John Flamsteed, and Sundry Others. Blacksburg, Virginia: The McDonald & Woodward Publishing Company. pp. 6–7, 169–70. ISBN978-0-939923-78-6.
^Zechmeister, M.; Kürster, M.; Endl, M.; Lo Curto, G.; Hartman, H.; Nilsson, H.; Henning, T.; Hatzes, A. P.; Cochran, W. D. (2013). "The planet search programme at the ESO CES and HARPS. IV. The search for Jupiter analogues around solar-like stars". Astronomy and Astrophysics. 552. A78. arXiv:1211.7263. Bibcode:2013A&A...552A..78Z. doi:10.1051/0004-6361/201116551.
^Marmier, M.; Ségransan, D.; Udry, S.; Mayor, M.; Pepe, F.; Queloz, D.; Lovis, C.; Naef, D.; Santos, N. C.; Alonso, R.; Alves, S.; Berthet, S.; Chazelas, B.; Demory, B.-O.; Dumusque, X.; Eggenberger, A.; Figueira, P.; Gillon, M.; Hagelberg, J.; Lendl, M.; Mardling, R. A.; Mégevand, D.; Neveu, M.; Sahlmann, J.; Sosnowska, D.; Tewes, M.; Triaud, A. H. M. J. (2013). "The CORALIE survey for southern extrasolar planets XVII. New and updated long period and massive planets". Astronomy and Astrophysics. 551. A90. arXiv:1211.6444. Bibcode:2013A&A...551A..90M. doi:10.1051/0004-6361/201219639.
^Turner, O. D; Anderson, D. R; Collier Cameron, A; Delrez, L; Evans, D. F; Gillon, M; Hellier, C; Jehin, E; Lendl, M; Maxted, P. F. L; Pepe, F; Pollacco, D; Queloz, D; Ségransan, D; Smalley, B; Smith, A. M. S; Triaud, A. H. M. J; Udry, S; West, R. G (2016). "WASP-120 b, WASP-122 b, AND WASP-123 b: Three Newly Discovered Planets from the WASP-South Survey". Publications of the Astronomical Society of the Pacific. 128 (964): 064401. arXiv:1509.02210. Bibcode:2016PASP..128f4401T. doi:10.1088/1538-3873/128/964/064401.
^ abBakich, Michael E. (2010). 1001 Celestial Wonders to See Before You Die: The Best Sky Objects for Star Gazers. Springer. p. 402, 416. ISBN978-1-4419-1777-5.
^Fleenor, Matthew C.; Rose, James A.; Christiansen, Wayne A.; Hunstead, Richard W.; Johnston-Hollitt, Melanie; Drinkwater, Michael J.; Saunders, William (September 2005). "Large-Scale Velocity Structures in the Horologium-Reticulum Supercluster". The Astronomical Journal. 130 (3): 957–967. arXiv:astro-ph/0505361. Bibcode:2005AJ....130..957F. doi:10.1086/431972.