July 13: Heinrich Louis d'Arrest

Heinrich Louis d'Arrest

July 13, 1822 – June 14, 1875

Heinrich Louis d'Arrest was a Prussian astronomer, born in Berlin.

While still a student at the University of Berlin, d'Arrest was party to Johann Gottfried Galle's search for Neptune. On September 23, 1846, he suggested that a recently drawn chart of the sky, in the region of Urbain Le Verrier's predicted location, could be compared with the current sky to seek the displacement characteristic of a planet, as opposed to a stationary star. Neptune was discovered that very night.

D'Arrest's later work at the Leipzig Observatory led him, in 1851, to the discovery of the comet named for him (formally designated 6P/d'Arrest). He also studied asteroids (he discovered 76 Freia) and nebulae.

He won the Gold Medal of the Royal Astronomical Society in 1875.

The crater D'Arrest on the Moon is named after him, as well as a crater on the Martian satellite Phobos and the asteroid 9133 d'Arrest.

June 18: William Lassell

William Lassell

June 18, 1799 – October 5, 1880

William Lassell was an English astronomer who discovered moons of Neptune, Saturn and Uranus.

Born in Bolton, he made his fortune as a beer brewer, which enabled him to indulge his interest in astronomy. He built an observatory near Liverpool with a 24-inch (610 mm) reflector telescope, for which he pioneered the use of an equatorial mount for easy tracking of objects as the earth rotates. He ground and polished the mirror himself, using equipment he constructed.

In 1846 Lassell discovered Triton, the largest moon of Neptune, just 17 days after the discovery of Neptune itself by German astronomer Johann Gottfried Galle. In 1848 he independently co-discovered Hyperion, a moon of Saturn. In 1851 he discovered Ariel and Umbriel, two new moons of Uranus.

When Queen Victoria visited Liverpool in 1851, Lassell was the only local she specifically requested to meet.

In 1855, he built a 48-inch (1,200 mm) telescope, which he installed in Malta because of the better observing conditions compared to England.

He won the Gold Medal of the Royal Astronomical Society in 1849, and served as its president for two years starting in 1870. Lassell died in Maidenhead in 1880. Upon his death, he left a fortune of £80,000 (equivalent to millions of American dollars by today's standards).

The crater Lassell on the Moon, a crater on Mars and a ring of Neptune are named in his honour.

June 9: Johann Gottfried Galle

Johann Gottfried Galle

June 9, 1812 – July 10, 1910

Johann Gottfried Galle was a German astronomer at the Berlin Observatory who, with the assistance of student Heinrich Louis d'Arrest, was the first person to view the planet Neptune, and know what he was looking at (September 23, 1846). He used the calculations of Urbain Le Verrier to know where to look.

Born in Radis, Galle studied at the University of Berlin from 1830-33. He had started to work as an assistant to Johann Franz Encke in 1835 immediately following the completion of the Berlin observatory. In 1851 he moved to Breslau (Wrocław) to become professor of astronomy and the director of the local observatory.

Throughout his career he studied comets, and in 1894 (with the help of his son Andreas Galle) he published a list with 414 comets. He himself had previously discovered three comets in the short span from December 2, 1839 to March 6, 1840.

Galle's Ph.D. thesis, finished in the year of 1845, was a reduction and critical discussion of Ole Rømer's observation of meridian transits of stars and planets on the days from October 20 to October 23, 1706. Around 1845 he sent a copy of his thesis to Urbain Le Verrier, but only received an answer a year later on September 18, 1846. It reached Galle on September 23 and in it Le Verrier asked him to look at a certain region of sky to find a predicted new planet, which would explain the perturbations of the planet Uranus. The same night, after Encke gave him the permission against his own judgment, an object fitting the description was found, and it was confirmed as being a planet over the next two evenings.

Two craters, one on the Moon and the "happy face" one on Mars, the asteroid 2097 Galle, and a ring of Neptune, have been named in his honor.

March 11: Urbain Le Verrier

Urbain Jean Joseph Le Verrier

March 11, 1811 – September 23, 1877

Urbain Le Verrier was a French mathematician who specialized in celestial mechanics and is best known for his part in the discovery of Neptune.

Le Verrier studied at the Ecole Polytechnique. Following a brief period studying chemistry under Gay-Lussac, Le Verrier switched to astronomy, particularly celestial mechanics. He accepted a job at the Paris Observatory, where he spent most of his professional life, and eventually became that institution's Director.

Le Verrier's most famous achievement is his prediction of the existence of the then unknown planet Neptune, using only mathematics and astronomical observations of the known planet Uranus. Encouraged by physicist Arago, Director of the Paris Observatory, Le Verrier was intensely engaged for months in complex calculations to explain small but systematic discrepancies between Uranus's observed orbit and the one predicted from the laws of gravity of Newton. 

At the same time, but unknown to Le Verrier, similar calculations were made by John Couch Adams in England. Le Verrier announced his final predicted position for Uranus's unseen perturbing planet publicly to the French Academy on August 31, 1846, two days before Adams's final solution, which turned out to be 12° off the mark, was privately mailed to the Royal Greenwich Observatory. Le Verrier transmitted his own prediction by September 18 letter to Johann Galle of the Berlin Observatory. The letter arrived five days later, and the planet was found with the Berlin Fraunhofer refractor that same evening, September 23, 1846 by Galle and Heinrich d'Arrest within 1° of the predicted location near the boundary between Capricorn and Aquarius.

There was, and to an extent still is, controversy over the apportionment of credit for the discovery. There is no ambiguity to the discovery claims of Le Verrier, Galle, and d'Arrest. Adams's work was begun earlier than Le Verrier's but was finished later and was unrelated to the actual discovery. Not even the briefest account of Adams's predicted orbital elements was published until more than a month after Berlin's visual confirmation. But Adams himself made full public acknowledgement of Le Verrier's priority and credit (not forgetting to mention the role of Galle) when he gave his paper to the Royal Astronomical Society in November 1846:

I mention these dates merely to show that my results were arrived at independently, and previously to the publication of those of M. Le Verrier, and not with the intention of interfering with his just claims to the honours of the discovery ; for there is no doubt that his researches were first published to the world, and led to the actual discovery of the planet by Dr. Galle, so that the facts stated above cannot detract, in the slightest degree, from the credit due to M. Le Verrier.

– Adams (1846)

Galvanized by his success with Neptune, Le Verrier proceeded to interpret variations in the orbit of Mercury as being due to an unknown planet, tentatively named Vulcan. This triggered a wave of false detections, which lasted until 1915, when Einstein explained Mercury's anomalous motion with his theory of general relativity.

The last quarter century of Le Verrier's life was engaged in establishing the orbits of all eight planets, a project which he only narrowly lived to see completed and printed.

Honors

• Gold Medal of the Royal Astronomical Society - 1868 and 1876
  
• Namesake of craters on the Moon and Mars, a ring of Neptune, and the asteroid 1997 Leverrier
  
• One of the 72 names engraved on the Eiffel Tower
  

January 21: John Couch Adams

John Couch Adams 

June 5, 1819 – January 21, 1892

John Couch Adams was a British mathematician and astronomer. His most famous achievement was predicting the existence and position of Neptune, using only mathematics. The calculations were made to explain discrepancies with Uranus's orbit and the laws of Kepler and Newton. At the same time, but unknown to each other, the same calculations were made by Urbain Le Verrier. Le Verrier would assist Galle in locating the planet on September 23, 1846, which was found within 1° of its predicted location, a point in Aquarius. There was, and to some extent still is, some controversy over the apportionment of credit for the discovery.

Although he was not an active lunar observer, he was (as Wilkins and Moore point out) an outstanding mathematician and greatly improved the "lunar theory" (i.e., our understanding of the Moon's orbit).

The Lunar crater Adams is jointly named after him, Walter Sydney Adams and Charles Hitchcock Adams. Neptune's outermost known ring and the asteroid 1996 Adams are also named after him. The Adams Prize, presented by the University of Cambridge, commemorates his prediction of the position of Neptune. His personal library is now in the care of Cambridge University Library.