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.[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 5.0, Beta Horologii is a white giant that is radiating 63 times the Sun's luminosity from its photosphere at an effective temperature of 8,303K. It is 312 (±4) light-years from Earth. It has been little-studied. 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). It is 161 (±1) light-years from Earth.
With a magnitude of 5.24, Nu Horologii is a white main sequence star of spectral type A2V located 169 (±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+9 −37 AU, while an outer disk lies 410+24 −96 AU from the star. The estimated mass of the disks is (1.3±0.7)×10−3 times the mass of the Earth.
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, 56.51 (±0.05) light-years distant from Earth. Its 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 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 as massive as 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 star to the Sun. In August 2019, it was announced to have three planets, one of which lay in its circumstellar habitable zone.
Composite image of NGC 1512 (left) and the dwarf galaxy NGC 1510
Horologium is also home to many deep-sky objects; there are several globular clusters in the constellation. 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. 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 123.3 kiloparsecs (402,000 light-years) from Earth.
^ 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. ISBN978-0939923786.
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^Anglada-Escudé, G.; Reiners, A.; Pallé, E.; Ribas, I.; Berdiñas, Z. M.; López, C. Rodríguez; Morales, N.; López-González, M. J.; Hambsch, F.-J. (2019-08-13). "Red Dots: A temperate 1.5 Earth-mass planet in a compact multi-terrestrial planet system around GJ1061". arXiv:1908.04717v1 [astro-ph.EP].
^ abBakich, Michael E. (2010). 1001 Celestial Wonders to See Before You Die: The Best Sky Objects for Star Gazers. Springer. pp. 402, 416. ISBN978-1441917775.
^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): 957–67. arXiv:astro-ph/0505361. Bibcode:2005AJ....130..957F. doi:10.1086/431972.