This
article derives from a series of three talks I gave, which charted the cataloguing
of deep sky objects over one hundred and twenty years, and through the work
of Charles Messier, William Herschel and John Dreyer and their associates.
The story moves from France, through Britain and on to Ireland, through telescopes
ranging from enormously long, very small aperture refactors to what was at
the time the world's largest reflector. Few of the characters are quite what
they seem at first glance, but they left their work as a legacy to us and
to future generations of astronomers.
We begin with a man who wasn't even interested in deep sky objects....
CHARLES MESSIER (1730-1817)
Imagine, if you can, the year 1730. Telescope astronomy, which we take for granted today, was relatively in its infancy. Galileo had made his first observations with a refractor in 1609, discovering the moons of Jupiter and the concept of the solar system that we have today. By the middle of that century the likes of Galileo, Huygens and Cassini were studying Saturn's rings. Newton published his theories on gravity in 1687 and came up with the idea of the reflecting telescope shortly after that. The telescopes of that time had optics far inferior to the ones we use today. For a world that had only just come to terms with gravity, the idea of galaxies outside our own, or indeed that our own Milky Way was a giant, star filled galactic structure, was just inconceivable. Deep sky objects were 'nebulae' - clouds. No-one could see most of them in any great detail and no-one was interested in them. What were they interested in, then? Moving bodies in the sky. Things they could use Newton's gravitational theories to predict - transits and occultations, when objects pass in front of or behind other objects, eclipses of the Sun and Moon. And comets. Halley used Newton's theories to predict the return of a comet in 1758. He didn't get it quite accurate, it was a little late, but it was still named in his honour.
Into
this period of mathematical astronomy comes Charles Messier. Typical of the
time, he was the tenth of twelve children, six of which died young. His father
died when young Charles was just eleven. At the age of 21, in 1751, Charles
went to Paris to make his living. He had already witnessed some notable astronomical
events - the six tailed comet of 1744 and the annular solar eclipse of 1748
- and this may have influenced his choice of employment. He was hired as a
draughtsman and clerk by Joseph Delisle, who was then astronomer to the Navy.
His first task was to make a copy of a map of the great wall of China - no
photocopiers then - but he was also taught how to use astronomical instruments,
and his first documented observation was a transit of Mercury in 1753. He
spent eighteen months searching for the return of Comet Halley, and actually
did discover it, but was never credited with that discovery.
Go back again, if you will, to 1758. Halley wasn't the only one to try to predict the return of this particular comet. Nor was Messier the only one to discover it on its return. News travelled very slowly - no internet then, no global communications network. The spread of news was someone getting on a horse to hand deliver a letter to the next town, or someone walking to the next village and telling a friend, who then told his friend.... Delisle had actually made an error in his calculation of the comet's position and refused to believe that Messier had discovered the right one. He would not allow Messier to announce his discovery, but in any case it had already been discovered a month earlier by an astronomer from Saxony. It took three months for that news to reach Paris.
Messier worked at the Marine Observatory, which was housed on a tower on the Hotel de Cluny. Sadly although the tower exists today the observatory has been removed. A year after his discovery of Halley, he found the first of his own comets, named Comet 1759 II Messier. For a period of about fifteen years Messier made almost all the comet discoveries. He claimed to have discovered twenty one, of which about thirteen were independant discoveries. Messier is sometimes a bit derided today because he didn't actually do all he claimed to have done, but this is a bit harsh and a sad reflection of the world we live in today that we feel it necessary to criticize Messier for what he didn't do rather that give him credit for what he did do. As well as comets he studied occultations, transits and eclipses, and in that time before computers it was Messier or his assistants who did all the complex calculations by hand. This was science still in its infancy, when new discoveries might not be known about for months, when verifying a discovery could take even longer. We should look at Messier's work in context, and not use our modern world to judge him.
He became a member of the Royal Society and its equivalent in Germany and Russia, though in his native France he wasn't immediately so honoured, probably because of embarrassing Delisle over Halley. Delisle retired in 1765, but Messier wasn't named his sucessor until 1771, a year after he was belatedly admitted to the Royal Academy of Science.
So much for the comets, what of the deep sky objects that appear in Messier's now famous catalogue. He discovered what we now call M1, the Crab Nebula in Taurus, while he was following a comet in 1758. He didn't know what it was, but he plotted it on the comet's chart, presumably so he wouldn't mistake it for another comet. He carried on plotting deep sky objects in a hit and miss way until 1764, stumbling across them in the couse of his work. Most of these early ones were already known about but ignored. Messier was the first to pay them much attention, and in 1764 he started compiling the catalogue in earnest. Five years later he published the first edition - forty five objects, or 'nebulae', ending with M45, the Pleiades.
In 1770, at the age of forty, Messier married. We know already that he came from a family which by our modern standards seems huge, even considering the early deaths of half the children, and that by the time he reached adulthood his father was already dead. Madame Messier, or Marie-Francoise, gave her husband a son two years after their marriage, but less than two weeks later both mother and son were dead. As shown by Messier's own family, this sort of tragedy was not at all uncommon in the eighteenth century.
In
1774 Messier was introduced to Pierre Mechain. Mechain seems to have been
a bit of a rival comet hunter, but he collaborated with Messier on the nebula
catalogue. His first discovery was one of the galaxies in Canes Venatici,
M63. In 1780 the catalogue, now with sixty eight objects, was incorporated
into what appears to have been the equivalent of our Astronomical Almanac.
The following year's revision totalled one hundred objects, a quarter of them
Mechain's.
Shortly afterwards Messier was severely injured in a fall, and at this point Mechain took over the catalogue, which stopped at one hundred and nine. This was the same year that William Herschel discovered Uranus. Herschel will be the next link in the chain for deep sky objects, but he appears here because his planet was originally assumed to be either a star or, more likely, a comet. Messier was sent word of it, studied its motion and thought it unlikely to be a comet. It was proved mathematically to be a planet, by another Frenchman.
There were oddities in Messier's lists, not surprising really when you consider that this was a secondary interest. He certainly observed NGC205, the second companion galaxy of M31 in Andromeda, in 1773, but never listed it. Some now refer to it as M110, but Messier and Mechain didn't.
M101 and M102 are one and the same. They were Mechain's, and he recognized his mistake, but never corrected it. The identity of M91 is a puzzle. From Messier's own notes it was very faint, but there is actually nothing in the position he gave. Opinions vary. M91 is listed now as NGC4548, but could it be a duplicate and incorrectly positioned observation of M58, or a comet, or NGC4571, which is in about the right position but is far too faint to have been observed by Messier? There was similar confusion over the identities of M47 and M48. Then there's M40, not a 'nebula' at all but merely a pair of faint stars. In Messier's defence. it wasn't his discovery. It was one reported to him and he knew there was nothing there, or nothing that he could see. He always was a comet hunter first and foremost. The nebulae were always a sideline.
Messier co-discovered his last comet in 1801, at the age of 71. Through the French Revolution he'd struggled to continue his work, though he received no salary and the Navy weren't even paying the rent on the observatory. He died in 1817, having suffered a stroke two years earlier. Mechain died of yellow fever in 1804, in Spain where his work as director of the Paris Observatory had taken him.
Although Messier used a large number of different telescopes, most of them were small. The largest magnification he is recorded as using was 138x on a twenty five foot focal length refractor. None of the telescopes he used was any better than a modern four or six inch reflector. This goes a long way towards explaining why Messier's list is so important to us today. And the fact that he was working from Paris means that anything that he could see, we can see from Britain, though some of them are a bit low in the sky.
Every newcomer to astronomy starts with Messier's catalogue, the 'M' objects. They're all listed in the NGC, but they're always refered to by Messier's number. Anyone with a relatively small telescope can go out and know they're there to find, and one hundred and nine are enough without being too many. Some people even take on the challenge of finding them all in one night.
What would Messier have thought if he looked in on someone's 'Messier Marathon'? Would he ever have thought that today we would attach so much importance to objects that he merely sometimes mistook for comets, indeed that most observers attach more importance to the 'nebulae' than his beloved comets?