Mensa: A star attraction
Table Mountain inspired the naming of the Mensa constellation. Photo courtesy Dee Chetty
Table Mountain, other than being just plain fabulous in its own right, is the only terrestrial feature in the world to have a constellation named after it. The southern constellation Mensa was first introduced to Western astronomy in 1754 by Frenchman Nicolas Louis de Lacaille (he originally called it Mons Mensae – Latin for “the table mountain”). Lacaille, an astronomer, catalogued an amazing 10 000 stars during his professional career and named Mensa after the increasingly world-famous landmark in the Cape of Good Hope.
The constellation’s brightest star is Alpha Mensae, but with a visibility magnitude of 5.09 it is not that bright at all. Compare this figure to the sun’s visibility magnitude of -26.74 and Venus’s visibility magnitude: −4.4.
Visibility magnitude, also called apparent magnitude, refers to the measure of brightness of a star, planet or any other heavenly body as seen from Earth. Interestingly, the brighter the object the lower its visibility magnitude. Very bright objects have negative magnitudes.
Mensa is, according to records, the faintest constellation in the sky; compare its visibility to Alpha Crucis – the brightest star in the brightest constellation (the Southern Cross) – which has a visibility magnitude of 0.77.
To its credit, Mensa does form part of the Large Magellanic Cloud (LMC), the third-closest galaxy to ours, the Milky Way. The LMC’s visibility magnitude is 0.9, so the odds of you spotting it instead of Mensa in the southern-hemisphere sky are about five times higher.
A celestial map of the Mensa constellation. Photo courtesy Torsten Bronger
Mensa is “essentially unobservable” from the northern hemisphere and is one of the southern-most constellations, second only to the south-polar constellation of Octans, which, incidentally, was also entered into Western astronomical books by the one and only Lacaille.
Lacaille was by no means rich, but his passion and knowledge drew the interest and eventual patronage of fellow French astronomer Jacques Cassini. Roland-Michel Barrin de La Galissonière, a governor of France, officially sanctioned Lacaille’s strongly motivated astronomical expedition to the Cape of Good Hope. During his mission, Lacaille not only recorded Mensa, but also made the first measurement of a South African arc of the meridian – an imaginary line, which runs true-north to south on the Earth’s surface, that connects both geographic poles.
Since it’s not incredibly bright, it’s a good thing that the constellation has such a famous namesake to help it shine. And don’t confuse the meaning of mensa in Latin with the Mexican-Spanish term, which means “foolish” or “stupid”.
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