Constellations: Horologium 'the Pendulum Clock' Content from the guide to life, the universe and everything

Constellations: Horologium 'the Pendulum Clock'

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The shield of the Science, Mathematics and Engineering faculty of the h2g2 University.Constellations: Overview | Andromeda | Antlia | Apus | Aquarius | Aquila | Ara | Aries | Auriga | Boötes | Caelum
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Hickory Dickory Dock,
The mouse ran up the clock.
The clock struck one,
The mouse ran down!
Hickory Dickory Dock...

–  English nursery rhyme

Name:Horologium (Latin: 'clock')
Short form:Hor
Area:249 sq deg
Co-ordinates1:Right Ascension 03h, Declination −60°

Horologium is a southern constellation bordered by Eridanus, Hydrus, Reticulum, Dorado and Caelum. There are no Messier objects due to its southerly position, but there is a globular star cluster; such 'objects' are the oldest members of our galaxy. Several galaxies are on parade for those interested in deep-sky viewing; then there's the billion-light-year2 distant supercluster.


In the mid-18th Century French astronomer Nicolas Louis de Lacaille (1713 - 62) went on a three-year expedition to the Cape of Good Hope. He described 14 new constellations and 42 nebulous objects among almost 10,000 southern stars from studies he made during this time. Lacaille drew up this constellation to honour the pendulum clock  Horologium Oscillatorium created by 17th-Century Dutch inventor and astronomer Christiaan Huygens. In Lacaille's first publication of Coelum Australe Stelliferum in 1756 the given name of the star grouping was l'Horloge, but the title was changed to the Latin name Horologium Oscillitorium for the next edition, which was posthumous.

It was further listed as Horologium Pendulum, but the 'Pendulum' part was later dropped for convenience. By the 19th Century there were over 100 constellations in existence, honouring some weird and wacky objects that were in vogue at the time. In 1922, the International Astronomical Union (IAU) eliminated all but the now officially-recognised 88. Horologium survived, probably with regard to Huygens and the extensive contribution he made to science. In 1930, the IAU formed official 'borders' between each constellation, entrusting the task to Belgian astronomer Eugène Delporte. The constellations were divided by straight lines and this much-easier system is what we use today.


The scientific star names are simple to understand (if you know your Greek alphabet). For example: the 'alpha' star means that it is the brightest star in that constellation. The next brightest is designated 'beta', etc. Combined with the genitive name, this is known as the 'Bayer designation'. Some stars have proper names as well, but there are no named stars in this constellation. Other stars are known by their catalogue number.

Alpha and beta Horologii are at opposing ends of the constellation. Alpha is the more northerly, lying almost on the border with Caelum. Beta is tucked away in the corner where Reticulum and Hydrus meet. There is a star, delta, in close proximity to alpha from our vantage point; in reality there is around 60 light years distance between them.

Iota Horologii has a planet which was discovered in 1999 (see extrasolar planets section below).

R Horologii is a red giant classed as a Mira variable type. Mira (Latin meaning 'the amazing one') is the common name of omicron Ceti, the first variable star to be discovered and the prototype for a certain kind of variable star. R Horologii has one of the greatest magnitude ranges known, from +4.7 to +14.3 over a period of 407 days.

GJ 1061 is a red dwarf star which is a close neighbour of ours; it resides just 12 light years away.

Star Table

StarDesignationName or
catalogue number
Stellar magDistance
(light years)
Spectral classification
and/or comments
α Horalpha HorologiiHD 26967+3.8117Orange giant
β Horbeta HorologiiHD 18866+4.9300White subgiant
δ Hordelta HorologiiHD 26612+4.9 var175White dwarf
ζ Horzeta HorologiiHD 16920+5.2154Yellow-white dwarf
η Horeta HorologiiHD 16555+5.3145White dwarf
ι Horiota HorologiiHD 17051+5.456Yellow dwarf/planetary system
μ Hormu HorologiiHD 19319+5.1130White dwarf
R HorR HorologiiHD 18242+4.7 to +14.3 var100Red giant
GJ 1061GJ 1061LHS 1565+1312Red dwarf

New General Catalogue (NGC) and Index Catalogue (IC)

The NGC was compiled by John Louis Emil Dreyer (the director of the Armagh Observatory from 1882 to 1916). Since the NGC was created, improved detection methods have uncovered other wondrous sights which are registered in the IC (Index Catalogue) and the Sh (the Sharpless Catalogue).

NGC Table

CatalogueNameTypeBrightness (m)Distance
(light years)
NGC 1261Caldwell 87Globular cluster+8.353,500Dun 337

NGC 1261 is a striking globular cluster which was discovered in November 1826 by Scottish astronomer James Dunlop (1793 - 1848) while he was working at Parramatta Observatory, New South Wales, Australia. NGC 1261 features in the Caldwell Catalogue as number 87. Compiled by English astronomer Sir Patrick Moore, host of the BBC's The Sky At Night, his intention was to create a list to challenge the 'backyard astronomer' – keen amateurs like himself.

There are a number of galaxies which we can see at the co-ordinates of Horologium, but they are so far away that you'd need a large telescope to view them. Those galaxies which feature in the New General Catalogue are: NGC 1249, NGC 1411, NGC 1433, NGC 1448, NGC 1493 and NGC 1512. The Horologium-Reticulum Supercluster of galaxies is talked about below.

Horologium-Reticulum Supercluster

Superclusters are chains of galaxy clusters, all gravitationally bound. Over 80% of galaxies are found in them; our galaxy, the Milky Way, is part of the 'Local' Virgo Supercluster. The largest superclusters can be spread over many millions of light years. It is impossible to give an exact distance for a supercluster, because the galaxy clusters which are a part of it can be millions of light years from each other, which gives an adequate demonstration of the power of gravity. Our 'closest' supercluster neighbours after our own Virgo Supercluster are: the Hydra Supercluster, the Centaurus Supercluster, the Perseus-Pisces Supercluster, and the Coma Supercluster (which is over 300 million light years away from us). The Horologium-Reticulum Supercluster (HRS) is even further away: the closest member galaxy has been measured at a distance of 700 million light years, and the supercluster stretches to a mind-boggling 1.2 billion light years.

Extrasolar Planets in Horologium

There are extrasolar planetary systems in the constellation Horologium. Figures given in the table below show the length of the planet's orbital period around its parent star, which we know of as a year. The mass of the extrasolar planet is compared to that of Jupiter, our Solar System's largest planet, known by astronomers as the 'Jovian scale'.

Iota Horologii b is a gas giant twice the mass of Jupiter. The parent star is similar to our own Sun and it's a comparative neighbour at just 56 light years distance. Iota Horologii b orbits its star within the habitable zone but because it is a gas giant, it's not a candidate for the search for extra-terrestrial life. However, should the planet have any rocky moons with enough gravity to sustain an atmosphere, then the odds on possibility would swing the other way. Iota Horologii b features in the light-hearted Entry on Broadcasting to our Galactic Neighbours.

During August 2019, the Red Dots project of detecting terrestrial planets around nearby red dwarf stars announced the discovery of three Earth-like planets belonging to GJ 1061, which is just 12 light years distance. Two of the planets, GJ 1061 c and GJ 1061 d, reside within the habitable zone of the star.

Extrasolar Planets Table

Star name or
catalogue number
catalogue number
Planet size
(Jovian scale)
Orbital period
(Earth days)
Year of discoveryComments
Iota HorologiiIota Horologii b23111999Gas giant; habitable zone
HD 27631HD 27631 b1.452,2082011Gas giant
WASP-120WASP-120 b53.62015Hot superjovian
GJ 1061GJ 1061 b1.3 Earth mass3.22019Terrestrial
GJ 1061GJ 1061 c1.7 Earth mass6.72019Terrestrial; habitable zone
GJ 1061GJ 1061 d1.7 Earth mass132019Terrestrial; habitable zone

Horology Down to Earth

  • There is an amazing Horloge Fleurie (flower clock) in Geneva, Switzerland.

1Current IAU guidelines use a plus sign (+) for northern constellations and a minus sign (−) for southern ones.2A light year is the distance light travels in one year, roughly 5.88 trillion miles or 9.46 trillion kms.

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