Horologium
What is Horologium?
Constellations· Southern summer · a long strip south of Eridanus. Horologium is Lacaille's tribute to the pendulum clock. It is a long chain of faint stars, and in its direction lie the strongly varying star R Horologii and a distant supercluster of galaxies.
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Stars of the figure
| Star | Magnitude |
|---|---|
| Alpha Horologii | 3.9 |
The 1 brightest of the 2 stars used in the figure (1 lines). Lower magnitude means brighter.
Constellation basics
| Season | Nov–Jan evenings; only its northern end rises, low, from New York; not visible from London |
|---|---|
| Brightest star | α Horologii, mag 3.9 |
| How to find | east of Achernar, beside the southern end of Eridanus |
| Area rank | 58th of 88 |
Figures are representative approximations. Source: compiled from NASA and IAU public data.
The giant supercluster beyond Horologium
Starting about 700 million light-years away in the direction of Horologium lies a vast gathering of galaxies. It is the Horologium–Reticulum Supercluster, made up of about 35 galaxy clusters. A galaxy cluster is a group of hundreds to thousands of galaxies, and a supercluster is an even larger structure made of many such clusters. This one is estimated to hold about 30,000 large galaxies.
Along its longest side the supercluster stretches about 550 million light-years. Light would take more than 500 million years to cross it. On the sky it covers an area about 12 degrees on each side across Horologium and Reticulum, roughly the width of 24 full moons in a row. Its total mass is calculated to be similar to that of Laniakea, the huge structure that contains our own galaxy.
Galaxies are not spread evenly through the universe. Threads packed with galaxies alternate with huge empty regions, forming a giant web. A supercluster is one of the knots in this web where galaxies crowd together. Beyond a quiet constellation with no star brighter than 4th magnitude lies a structure of this enormous size.
The constellation in depth
Horologium is one of the scientific-instrument constellations Lacaille added to the southern sky in the 1750s. It honours the pendulum clock, invented by the Dutch scientist Christiaan Huygens in the 17th century. By using a weight that swings at a steady rhythm, the pendulum clock kept time far more accurately than earlier clocks. Old charts show a tall clock with a weight and a dial, a shape that suits the long line of stars.
Even its brightest star, Alpha Horologii, is only a magnitude 3.9 orange giant, so the whole constellation is faint. One star worth noting is the variable R Horologii. About every 400 days it changes from magnitude 4.7 to 14. At its brightest it is thousands of times brighter than at its faintest, a large change even for its type of variable star.
Horologium also has a Sun-like star, Iota Horologii. It is a yellow star about 56 light-years away, and in 1999 a planet about twice Jupiter's mass was confirmed orbiting it roughly every 300 days. The star's motion and makeup resemble those of the Hyades cluster in Taurus, so some astronomers suggest it was born there and later drifted away.
Horologium lies deep in the southern sky, so from mid-northern latitudes such as New York only its northern end, around Alpha, rises low above the southern horizon. From the southern hemisphere, look on November to January evenings for a faint strip running east of the bright star Achernar. In the Sky view, set Cape Town on a December evening and look beside the southern end of Eridanus.
Frequently asked
- When can I see Horologium?
- Horologium is best seen in Nov–Jan evenings; only its northern end rises, low, from New York; not visible from London.
- How do I find Horologium?
- To find Horologium, look for east of Achernar, beside the southern end of Eridanus.
- What is the brightest star in Horologium?
- The brightest star in Horologium is Alpha Horologii, at magnitude 3.9.
Related
Figures are representative approximations. Simulated positions use J2000 mean elements.