A constellation planted in the southern sky by Edmond Halley in 1678 as a patriotic gesture to his monarch, Charles II of England. It commemorates the oak tree in which King Charles hid after his defeat by Oliver Cromwell’s republican forces at the Battle of Worcester in 1651. Halley formed the constellation out of stars that were previously part of Argo Navis.

Halley sailed to the island of St Helena in the south Atlantic Ocean in 1676 to observe the southern stars. He presented his results to the Royal Society in London on his return in 1678 and the following year published his catalogue of southern stars, Catalogus Stellarum Australium, with an accompanying map.

Halley listed 12 stars in Robur Carolinum. The brightest of them, in the tree’s roots, was the second-magnitude star we now know as Beta Carinae. The fourth star in Halley’s list, placed among the branches, was the peculiar eruptive variable now known as Eta Carinae; Halley’s catalogue is the first record of it.

Above: Robur Carolinum shown under the name Robur Caroli II on Chart XX of the Uranographia of Johann Bode (1801). It was positioned where the hull of Argo Navis (bottom of picture) was cut off, a place occupied on other maps by either the Clashing Rocks or clouds.

Halley described his new constellation as being a ‘perpetual memory’ of the King, but it turned out to be less permanent than either of them would have hoped. The royal oak was uprooted by the French astronomer Nicolas Louis de Lacaille, who mapped the southern stars more comprehensively 75 years after Halley. Most astronomers followed suit in ignoring this genuflection to an English king, although Bode included it on his Uranographia atlas of 1801 as Robur Caroli II.

© Ian Ridpath. All rights reserved