Faintest Constellations

There are a dozen constellations with no star brighter than +4.0 magnitude.  Many of them are deep in the southern sky.  They are:

ANTLIA, the Air Pump
Brightest Star: Alpha Antliae, apparent visual magnitude +4.25

ANT-lee-uh

CAELUM, the Engraving Tool
Brightest Star: Alpha Caeli, apparent visual magnitude +4.45

SEE-lum

CAMELOPARDALIS, the Giraffe
Brightest Star: Beta Camelopardalis, apparent visual magnitude +4.02

cuh-MEL-oh- PAR-duh-liss

CHAMAELEON, the Chameleon
Brightest Star: Alpha Chamaeleontis, apparent visual magnitude +4.047

cuh-MEAL-yun, or cuh-MEAL-ee-un

COMA BERENICES, Berenice’s Hair
Brightest Star: Beta Comae Berenices, apparent visual magnitude +4.25

COE-muh BER-uh-NICE-eez

CORONA AUSTRALIS, the Southern Crown
Brightest Star: Meridiana, apparent visual magnitude +4.087

cuh-ROE-nuh aw-STRAL-iss

MENSA, the Table Mountain
Brightest Star: Alpha Mensae, apparent visual magnitude +5.09

MEN-suh

MICROSCOPIUM, the Microscope
Brightest Star: Gamma Microscopii, apparent visual magnitude +4.654

my-cruh-SCOPE-ee-um

NORMA, the Carpenter’s Square
Brightest Star: Gamma2 Normae, apparent visual magnitude +4.02

NOR-muh

SCULPTOR, the Sculptor
Brightest Star: Alpha Sculptoris, apparent visual magnitude +4.27

SCULP-ter

SEXTANS, the Sextant
Brightest Star: Alpha Sextantis, apparent visual magnitude +4.49

SEX-tunz

VULPECULA, the Fox
Brightest Star: Anser, apparent visual magnitude +4.45

vul-PECK-yuh-luh

Eugène Delporte and the Constellation Jigsaw

Belgian astronomer Eugène Joseph Delporte (1882-1955) discovered 66 asteroids from 1925 to 1942, but he is best remembered for determining the official boundaries of the 88 constellations, work he completed in 1928 and published in 1930.  The constellation boundaries have remained unchanged since then.

The International Astronomical Union (IAU), founded, incidentally, in Brussels, Belgium in 1919, established the number of constellations at 88—the same number, coincidentally, as the keys on a piano—in 1922 under the guidance of American astronomer Henry Norris Russell (1877-1957).  The IAU officially adopted Delporte’s constellation boundaries in 1928.

All the constellation boundaries lie along lines of constant right ascension and declination—as they existed in the year 1875. Why 1875 and not 1900, 1925, or 1930? American astronomer Benjamin Gould (1824-1896) had already drawn up southern constellation boundaries for epoch 1875, and Delporte built upon Gould’s earlier work.

As the direction of the Earth’s polar axis slowly changes due to precession, the constellation boundaries gradually tilt so that they no longer fall upon lines of constant right ascension and declination. Eventually, the tilt of the constellation boundaries will become large enough that the boundaries will probably be redefined to line up with the equatorial coordinate grid for some future epoch. When that happens, some borderline stars will move into an adjacent constellation. Even now, every year some stars change constellations because proper motion causes them to move across a constellation boundary. For faint stars, this happens frequently, but for bright stars such a constellation switch is exceedingly rare.