The constellation's brighteststar—and the only one brighter than anapparent magnitude of 4—isAlpha Horologii (at 3.85), an aging orangegiant star that has swollen to around 11 times the diameter of the Sun. Thelong-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 haveexoplanets; at least one—Gliese 1061—contains an exoplanet in itshabitable zone.
The French astronomerNicolas-Louis de Lacaille first described theconstellation asl'Horloge à pendule & à secondes (Clock with pendulum and seconds hand) in 1756,[3][4] after he had observed and catalogued almost 10,000 southern stars during a two-year stay at theCape of Good Hope. He devised fourteen new constellations in previously uncharted regions of thesouthern celestial hemisphere, which were not visible from Europe. All but one honouredscientific instruments, and so symbolised theAge of Enlightenment.[a] The constellation name wasLatinised toHorologium in a catalogue and updated chart published posthumously in 1763.[5] The Latin term is ultimately derived from theAncient Greek ὡρολόγιον, for an instrument for telling the hour.[6]
Horologium constellation: showing the tangent line, or viewer's horizon, at latitude approx 23°N, which is parallel to the line of −67.04 declension, the lower declination boundary of the constellation.
Covering a total of 248.9square degrees or 0.603% of the sky, Horologium ranks 58th in area out of the 88 modern constellations.[7] Its position in the southern celestial hemisphere means the whole constellation is visible to observers south of23°N.[7][b] Horologium is bordered by five constellations:Eridanus (thePo river orNile river),Caelum (thechisel),Reticulum (thereticle),Dorado (thedolphin/swordfish), andHydrus (themalewater snake). The three letter abbreviation for the constellation, as adopted by theInternational Astronomical Union in 1922, is "Hor".[8] The official constellation boundaries are defined by a twenty-two-sided polygon (illustrated in infobox). In theequatorial coordinate system, theright ascension coordinates of these borders lie between02h 12.8m and04h 20.3m, while thedeclination coordinates are between −39.64° and −67.04°.[9]
Horologium has one star brighter thanapparent magnitude 4,[10] and 41 stars brighter than or equal to magnitude 6.5.[c][7] Lacaille charted and designated 11 stars in the constellation, giving them theBayer designationsAlpha (α Hor) throughLambda Horologii (λ Hor) in 1756. In the mid-19th century, English astronomerFrancis Baily removed the designations of two—Epsilon andTheta Horologii—as he held they were too faint to warrant naming. He was unable to find a star that corresponded to the coordinates of Lacaille'sBeta Horologii. Determining that the coordinates were wrong, he assigned the designation to another star. Kappa Horologii, too, was unable to be verified—although it most likely was the starHD 18292—and the name fell out of use. In 1879, American astronomerBenjamin Apthorp Gould assigned designations to what becameMu andNu Horologii as he felt they were bright enough to warrant them.[5]
At magnitude 4.93,Delta Horologii is the second-brightest star in the constellation,[17] and forms a wideoptical double with Alpha.[18] Delta itself is a truebinary 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.[19] The system is located 179 (±4) light-years from the earth.[12]
At magnitude 5.0, Beta Horologii is awhite giant 63 times as luminous as the Sun with an effective temperature of 8,303K.[16] It is 312 (±4) light-years from Earth,[20] and has been little-studied.[17] Lambda Horologii is an ageing yellow-white giant star of spectral type F2III that spins around at 140km/second, and is hence mildly flattened at its poles (oblate).[21] It is 161 (±1) light-years from Earth.[12]
With a magnitude of 5.24,[22] Nu Horologii is a white main sequence star of spectral type A2V located 169 (±1) light-years from Earth[23] that is around 1.9 times as massive as the Sun. Estimated to be around 540 million years old, it has adebris disk that appears to have two components: an inner disk is orbiting at a distance of96+9 −37AU, while an outer disk lies410+24 −96 AU from the star. The estimated mass of the disks is0.13%±0.07% the mass of the Earth.[22]
Horologium has severalvariable stars.R Horologii is ared giantMira variable with one of the widest ranges in brightness known of stars in the night sky visible to the unaided eye.[24] It is around 1,000 light-years from Earth.[25] It has a minimum magnitude of 14.3 and a maximum magnitude of 4.7, with a period of approximately 13 months.[26] T and U Horologii are also Mira variables.[18] TheAstronomical Society of Southern Africa reported in 2003 that observations of these two stars were needed as data on their light curves was incomplete.[27]TW Horologii is asemiregular variable red giant star that is classified as acarbon star,[26] and is 1,370 (±70) light-years from Earth.[28]
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 aspectral type of F8V,[29] 57 (±0.05) light-years from Earth.[30] Its chemical profile, movement and age indicate it formed within theHyades cluster but has drifted around 130 light-years away from the other members.[31] It has a planet at least 2.5 times as massive asJupiter orbiting it every 307 days.[32]HD 27631 is a Sun-like star located 164 (±0.3) light-years from Earth[33] 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.[34]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.[35]
With an apparent magnitude of 13.06,Gliese 1061 is ared 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 itsluminosity. 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 itshabitable zone.[36]
Composite image of NGC 1512 (left) and the dwarf galaxy NGC 1510
Horologium is home to manydeep-sky objects, including severalglobular clusters.NGC 1261 is a globular cluster of magnitude 8, located 53,000 light-years from Earth.[26] It lies 4.7 degrees north-northeast of Mu Horologii.[37] The globular clusterArp-Madore 1 is the most remote known globular cluster in theMilky Way at a distance of 123.3 kiloparsecs (402,000 light-years) from Earth.[38]
^While parts of the constellation technically rise above the horizon to observers between the latitudes of 23°N and50°N, those stars within a few degrees of the horizon are difficult to see.[7]
^Objects of magnitude 6.5 are among the faintest visible to the unaided eye from locations between suburban and rural areas innight skies.[11]
^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. 6–7,169–170.ISBN978-0-939923-78-6.
^"horologe, noun". Oxford English Dictionary. Retrieved26 December 2019. (subscription required)
^abMeshkat, Tiffany; Bailey, Vanessa P.; Su, Kate Y. L.; Kenworthy, Matthew A.; Mamajek, Eric E.; Hinz, Philip M.; Smith, Paul S. (2015). "Searching for Planets in Holey Debris Disks with the Apodizing Phase Plate".The Astrophysical Journal.800 (1): 5.arXiv:1412.5179.Bibcode:2015ApJ...800....5M.doi:10.1088/0004-637X/800/1/5.S2CID119211833. 5.
^abcRidpath, Ian; Tirion, Wil (2017).Stars and Planets Guide. London: Collins. p. 158.ISBN978-0-00-823927-5.
^Cooper, Tim (2003). "Presidential Address: Amateur Observations – Successes and Opportunities".Monthly Notes of the Astronomical Society of Southern Africa.62:234–240.Bibcode:2003MNSSA..62..234C.
^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.S2CID53694238.
^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.S2CID59467665.
^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.S2CID53647627.
^abBakich, Michael E. (2010).1001 Celestial Wonders to See Before You Die: The Best Sky Objects for Star Gazers. Springer. pp. 402, 416.ISBN978-1-4419-1777-5.
