In this article, we’ll be sharing with you everything you need to know about the constellation of Horologium, including how to find it, deep space objects contained within it and how to use it to find other night sky objects more easily.

The Constellation of Horologium 

Horologium is one of the 14 southern hemisphere constellations that French astronomer Nicolas-Louis de Lacaille (1713-1762) created. Lacaille traveled to the Cape of Good Hope and studied the southern night sky from 1751 to 1754.

This constellation was originally named ‘Horologium Oscillatorium’ (Latin for ‘Pendulum Clock’) to honor Dutch mathematician, astronomer and physicist Christiaan Huygens (1629-1695), inventor of the pendulum clock.

Horologium represents the pendulum clock Lacialle used to time his observations. The constellation’s name was shortened to ‘Horologium’ meaning ‘the clock’. There are no myths associated with this modern constellation. 

To help you spot Horologium, here’s what SkySafari 6 shows.

Horologium as shown by SkySafari
Horologium as shown by SkySafari. Click for full-screen.

Horologium has an area of 249 square degrees making it the 58th largest of the 88 recognized constellations. Horologium is often depicted as a pendulum clock.

The boundaries and neighboring constellations for Horologium
The boundaries and neighboring constellations for Horologium. Click for full-screen.

In the next section discover how to find Horologium.

How To Find Horologium In The Night Sky

Horologium is part of the Lacaille family of constellations. It’s located in the southern hemisphere of the sky and is visible to observers at latitudes between +30° and -90°. It is one of the most southerly constellations visible to Northern Hemisphere observers (south of 30°N). From mid-US states, for example, only the most northerly part of the constellation is visible, as shown in the star chart below.

Northern and Southern Hemisphere observers can best see it around December.

The constellation of Horologium is bordered by the constellations Caelum, Dorado, Eridanus, Hydrus and Reticulum. The area of the sky where Horologium is located is far from the Milky Way and contains few stars. 

Horologium at 9:00 p.m. on the 26th of December
Horologium at 9:00 p.m. on the 26th of December. Click for full-screen.

To find Horologium, do a naked-eye search for Eridanus the River. Trace Eridanus from Achernar (Alpha Eridani) to Acamar (Theta Eridani) (~21° straight line) then look for Horologium in a darker patch of sky ~8° southeast of this section of Eridanus.

Alternatively, join the stars Betelgeuse and Alnitak in Orion with an imaginary line and continue it on for ~50°. You can estimate these degree distances using your hand at arm’s length.

To find Horologium’s exact position for your location on any night, use software such as Stellarium (free) or SkySafari.

Horologium’s Brightest Stars 

Horologium is a small, dim constellation and only has five stars shining at magnitude five or brighter. The brightest star is only just above magnitude four. All of Horologium’s brightest stars are shown in the chart below and described in more detail underneath that.

The brightest stars of Horologium
The brightest stars of Horologium. Click for full-screen.

Alpha Horologii – This magnitude 3.85 orange giant star is the brightest in Horologium. Alpha Horologii is 117 light-years away. Its mass is 2.0 Solar masses, its radius is 11 Solar radii and it’s 20% cooler than the Sun.

Alpha Horologii represents either the pendulum or the weights in different depictions.

Beta Horologii – This magnitude 4.98 white giant is the third brightest star in Horologium. Beta Horologii is 295 light-years away, has a mass of 2.8 Solar masses, a diameter that’s 3.1 Solar diameters and is 1.8 times hotter than the Sun.

Zeta Horologii – This magnitude 5.21 yellow-white subgiant is 160 light-years away. Its mass is 1.4 Solar masses, its diameter is 3.1 Solar diameters and it’s 1.2 times hotter than the Sun.

Eta Horologii – is a double star 149 light-years away from us. The magnitude 5.30 yellow-white main-sequence primary and the secondary component are 4.8 arcseconds apart.

Its mass is 1.4 Solar masses, its diameter is 2.0 Solar diameters and it’s 1.4 times hotter than the Sun. This may be a multiple system. 

Mu Horologii – This magnitude 5.11 yellow-white subgiant is 142 light-years away. Its mass is 1.6 Solar masses, its diameter is 2.4 Solar diameters and it’s 1.3 times hotter than the Sun.

Nu Horologii – This magnitude 5.26 white main-sequence star is 165 light-years away. Its mass is 2.4 Solar masses, its diameter is 1.8 Solar diameters and it’s 1.6 times hotter than the Sun.

R Horologii – This magnitude 7.21, orange giant, variable is a pulsating Mira-type variable ranging in magnitude from 4.69 to 14.30 with a period of 407.60 days. At its brightest, it is the second brightest in the constellation. At its dimmest, this star, which is 685 light-years away, is barely visible even with a backyard telescope.

Star Hopping From Horologium 

Horologium is too dim to be a good starting point for beginner star hopping.

Objects To See Within Horologium 

Horologium contains no Messier objects, but there are a few other deep sky objects suitable for small telescope users. 

NGC 1261 – This globular cluster has a magnitude of 8.28 and an apparent size of 6.8 arcminutes. It’s 53,500 light-years away and is at right ascension 03h 12m 55s and declination -55° 07’ 43”.

NGC 1433 – This spiral galaxy has a magnitude of 9.96 and an apparent size of 6.2 x 3.0 arcminutes. It’s 27 million light-years away and is at right ascension 03h 42m 45s and declination -47° 08’ 51”.

NGC 1448 (NGC 1457) – This spiral galaxy has a magnitude of 10.56 and an apparent size of 8.0 x 1.5 arcminutes. It’s 62 million light-years away and is at right ascension 03h 45m 18s and declination -44° 34’ 16

Summary

Horologium is a medium-sized, dim constellation containing only a few bright stars but it does make a home for some interesting deep sky objects for small telescope users, especially if you’re based south of 30° north.

Southern latitudes and dark skies are needed to spot this timekeeper constellation near Eridanus. 

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