Pierre Méchain


Pierre François André Méchain was a French astronomer and surveyor who, with Charles Messier, was a major contributor to the early study of deep-sky objects and comets.

Life

Pierre Méchain was born in Laon, the son of the ceiling designer and plasterer Pierre François Méchain and Marie–Marguerite Roze. He displayed mental gifts in mathematics and physics but had to give up his studies for lack of money. However, his talents in astronomy were noticed by Jérôme Lalande, for whom he became a friend and proof-reader of the second edition of his book "L'Astronomie". Lalande then secured a position for him as assistant hydrographer with the Naval Depot of Maps and Charts at Versailles, where he worked through the 1770s engaged in hydrographic work and coastline surveying. It was during this time—approximately 1774—that he met Charles Messier, and apparently, they became friends. In the same year, he also produced his first astronomical work, a paper on an occultation of Aldebaran by the Moon and presented it as a memoir to the Academy of Sciences.
In 1777, he married Barbe-Thérèse Marjou whom he knew from his work in Versailles. They had two sons: Jérôme, born 1780, and Augustin, born 1784, and one daughter. He was admitted to the French Académie des sciences in 1782, and was the editor of Connaissance des Temps from 1785 to 1792; this was the journal which, among other things, first published the list of Messier objects. In 1789 he was elected a Fellow of the Royal Society.
He participated in the Anglo-French Survey to measure by trigonometry the precise distance between the Paris Observatory and the Royal Greenwich Observatory. This project was initiated by Dominique, comte de Cassini, and in 1787 Méchain visited Dover and London with Cassini and Adrien-Marie Legendre to facilitate its progress. The three men also visited the astronomer William Herschel at Slough.
With his surveying skills, he worked on maps of Northern Italy and Germany after this, but his most important mapping work was geodetic: the determination of the southern part of the meridian arc of the Earth's surface between Dunkirk and Barcelona beginning in 1791. This measurement would become the basis of the metric system's unit of length, the meter. He encountered numerous difficulties on this project, largely stemming from the effects of the French Revolution. He was arrested after it was suspected his instruments were weapons, he was interned in Barcelona after war broke out between France and Spain, and his property in Paris was confiscated during The Terror. He was released from Spain to live in Italy, then returned home in 1795.
A particularly intriguing fact about this project was that Méchain was uncertain of the precision of his measurements owing to anomalous results in verifying his latitude by astronomical observation. Ultimately, the distance from the pole to the equator, which Méchain and his associate Jean Baptiste Joseph Delambre had intended to be exactly ten million meters, was determined in the late 20th century by space satellites to be 10,002,290 meters. This small error of 2,290 meters equals 1.423 statute miles; the error in such a large measurement amounts to 14½ inches per statute mile. It represents in each meter an error of approximately 0.23 millimetres – slightly more than the width of a single strand of human hair. This discrepancy is sometimes mentioned as "Méchain's error", with the suggestion that the tiny variation in the length of the meridian can be attributed to Méchain's calculations. But analysis of Méchain's figures reveals that Méchain consistently kept the discrepancy very tiny, essentially forcing his individual reported measurements to appear more precise and consistent than would be reasonably expected of a survey involving more than a hundred measurements of mostly rough country using 18th century equipment; Méchain's putative error did not affect the final value of the length of the meter nor the measurement of the meridian.
From 1799, he was the director of the Paris Observatory.
Continuing doubts about his measurements of the Dunkirk-Barcelona arc led him to return to that work. This took him back to Spain in 1804, where he caught yellow fever and died in Castellón de la Plana.

Discoveries

Méchain discovered either 25 or 26 deep-sky objects, depending on how one counts M102. However, Méchain specifically disavowed the observation from 1783 onwards as a mistaken re-observation of M101. Since that time, others have proposed that he did in fact observe another object, and suggested what they might be. See the discussion The M102 Controversy for more details.
Messier numberNGC/IC NumberCommon nameDate of DiscoveryObject typeDistance ConstellationApparent magnitude
M63NGC 5055Sunflower Galaxy14 June 1779Galaxy, spiral37,000Canes Venatici8.5
M72NGC 6981 30 August 1780Cluster, globular53Aquarius10.0
M74NGC 628 Sep 1780Galaxy, spiral35,000Pisces10.5
M75NGC 6864 27 August 1780Cluster, globular58Sagittarius9.5
M76NGC 650, NGC 651Little Dumbbell Nebula5 September 1780Nebula, planetary3.4Perseus10.1
M77NGC 1068Cetus A29 October 1780Galaxy, spiral60,000Cetus10.5
M78NGC 2068 Begin 1780Nebula, diffuse1.6Orion8.0
M79NGC 1904 26 October 1780Cluster, globular40Lepus8.5
M85NGC 4382 4 March 1781Galaxy, lenticular60,000Coma Berenices10.5
M94NGC 4736 22 March 1781Galaxy, spiral14,500Canes Venatici9.5
M95NGC 3351 20 March 1781Galaxy, barred spiral38,000Leo11.0
M96NGC 3368 20 March 1781Galaxy, spiral38,000Leo10.5
M97NGC 3587Owl Nebula16 February 1781Nebula, planetary2.6Ursa Major9.9
M98NGC 4192 15 March 1781Galaxy, spiral60,000Coma Berenices11.0
M99NGC 4254 15 March 1781Galaxy, spiral60,000Coma Berenices10.5
M100NGC 4321 15 March 1781Galaxy, spiral60,000Coma Berenices10.5
M101NGC 5457Pinwheel Galaxy27 March 1781Galaxy, spiral27,000Ursa Major7.9
M102
M103NGC 581 Mar–Apr 1781Cluster, open8Cassiopeia7.0

He independently discovered four others, originally discovered by someone else but unknown to him at the time and included in the Messier catalogue: M71, discovered by Jean-Philippe de Chéseaux in the 1740s; M80, discovered by Messier about two weeks earlier than Méchain's observation; and M81 and M82, discovered originally by Johann Bode.
Six other discoveries are "honorary Messier objects" added to the list in the 20th century:
Messier numberNGC/IC NumberCommon nameDate of DiscoveryObject typeDistance ConstellationApparent magnitude
M104NGC 4594Sombrero Galaxy11 May 1781Galaxy, spiral50,000Virgo9.5
M105NGC 3379 24 March 1781Galaxy, elliptical38,000Leo11.0
M106NGC 4258 Jul 1781Galaxy, spiral25,000Canes Venatici9.5
M107NGC 6171 Cluster, globular20Ophiuchus10.0
M108NGC 3556 19 February 1781Galaxy, barred spiral45,000Ursa Major11.0
M109NGC 3992 12 March 1781Galaxy, barred spiral55,000Ursa Major11.0

He also discovered NGC 5195, the companion galaxy that makes M51 so distinctive.
Méchain never set out to observe deep-sky objects. Like Messier, he was solely interested in cataloguing objects that might be mistaken for comets; having done so, he was the second-most successful discoverer of comets of his time, after Messier himself.
All together, he originally discovered eight comets, and co-discovered three .
His sole discoveries are:
Méchain's co-discoveries are:
Note that only the two named comets have been connected to periodic comets that have computed orbits and in neither case was he an observer when they were computed, so by that technical definition Méchain did not discover any of these nine.

Legacy

On 24 June 2002, Asteroid 21785 Méchain was named in his honour, discovered by Miloš Tichý at Kleť Observatory on 21 September 1999, and provisionally designated 1999 SS2.

Footnotes