Caelum is a small constellation located in the southern celestial hemisphere. It was defined in the 1750s by the French astronomer Nicolas-Louis de Lacaille during his southern sky survey and later adopted among the 88 modern constellations. The name Caelum is Latin for the chisel; Lacaille originally called it Caelum Scalptorium, the engraver's chisel, reflecting a pattern in his naming of new constellations after scientific and artistic tools. For more on Lacaille's mapping of the southern sky see his work.
Appearance and notable stars
Caelum is one of the faintest and smallest constellations. Its stars are relatively dim: the brightest star has an apparent magnitude near 4.45, which makes the group difficult to detect from light-polluted locations. Because of this low brightness, only observers under reasonably dark skies can discern several of its stars with the naked eye. Deep-sky observers using binoculars or small telescopes can find a handful of faint galaxies, star clusters and double stars within its boundaries.
Alpha Caeli is the constellation's primary star in brightness, but even it is modest compared with stars in many northern constellations. Caelum does not contain any very bright or well-known variable stars, nebulae or prominent Messier objects; most of its interesting targets are subtle and require optical aid to study.
History and naming
Nicolas-Louis de Lacaille charted many previously unrecorded southern constellations during an expedition to the Cape of Good Hope in the mid-18th century. He favored names that honored instruments of science, art and the Age of Enlightenment rather than the mythological figures common in northern traditions. Caelum (the chisel) fits this pattern and was sometimes shortened from his original Latin phrase so that modern star charts use the succinct name Caelum.
Observation and visibility
Caelum is best observed from the Southern Hemisphere or low southern latitudes in the sky away from bright city lights. From most of the Northern Hemisphere it lies low on the horizon or below it for much of the year, so southern observers have the clearest view. Because its stars are faint, ideal seeing requires dark, transparent skies and modest magnification to reveal its telescopic objects.
Quick facts
- Latin meaning: the chisel (originally Caelum Scalptorium)
- Introduced by: Nicolas-Louis de Lacaille (see biography)
- Visibility: southern constellation; best from southern latitudes
- Brightest star magnitude: about 4.45
- Modern status: one of the 88 IAU constellations; charted on many atlases (constellation maps)
Though small and inconspicuous, Caelum illustrates how 18th-century astronomers expanded celestial cartography by introducing new constellations based on contemporary tools and knowledge. It remains a minor but established member of the modern star atlas, of interest mainly to observers seeking faint, offbeat objects in the southern sky.