September 23: Johann Encke

Johann Franz Encke

September 23, 1791 – August 26, 1865

Johann Franz Encke was a German astronomer, born in Hamburg. He is sometimes confused with Karl Ludwig Hencke, another German astronomer.

Encke studied mathematics and astronomy from 1811 at the University of Göttingen under Carl Friedrich Gauss; but he enlisted in the Hanseatic Legion for the campaign of 1813–1814, and became lieutenant of artillery in the Prussian army in 1815. Having returned to Göttingen in 1816, he was at once appointed by Bernhardt von Lindenau as his assistant in the observatory of Seeberg near Gotha.

There he completed his investigation of the comet of 1680, for which the Cotta prize was awarded to him in 1817; he correctly assigned a period of 71 years to the comet of 1812. That comet is now called 12P/Pons-Brooks.

Following a suggestion by Jean-Louis Pons, who suspected one of the three comets discovered in 1818 to be the same one already discovered by him in 1805, Encke began to calculate the orbital elements of this comet. At this time, all the known comets only had an orbital period of seventy years and more, where the aphelion is far beyond the orbit of Uranus. The most famous comet of this family was Comet Halley with its period of seventy-six years. Therefore the orbit of the comet discovered by Pons was a sensation, because his orbit was found to have a period of 3.3 years, therefore the aphelion had to be within the orbit of Jupiter. Encke predicted its return for 1822, but this return was only observable from the southern hemisphere and was seen by Carl Ludwig Christian Rümker from Australia. The comet was also identified with the one seen by Pierre Méchain in 1786 and by Caroline Herschel in 1795.

Encke sent his calculations as a note to Gauss, Olbers, and Bessel. His former mathematics professor published this note and Encke became famous as the discoverer of the short periodic comets. The first object of this family, the Encke comet, was named after him and so it is one of the few comets not named after the discoverer, but after the one who calculated the orbit. Later this comet was identified as the origin of the Taurids meteor showers.

The importance of the predicted return based on the calculation by Encke was rewarded by the Royal Astronomical Society in London by presenting their Gold Medal to him in 1824.

In 1837, Encke described a broad variation in the brightness of the A Ring of Saturn. The Encke Division was later named in honor of his observations of Saturn's rings.

Twice, in 1824 and 1830, he received the Gold Medal of the Royal Astronomical Society.

The crater Encke on the Moon, the asteroid 9134 Encke and the Encke gap of Saturn's rings are named in his honor.