Modern World to 1900

Messier Catalogue

Charles Messier (1730-1817) of France compiled the first modern catalogue of celestial objects. Messier created the catalogue because he was actually looking for comets - discovering comets was a way to make a name for oneself in astronomy at the time. The catalogue was a list of objects that were NOT comets, and it was compiled during 1758-1782.

The first edition of the Catalogue appeared in the Mémoires de l'Academie for 1771 (not published until 1774). It contained objects M1-M45. The first supplement included up through M68 and was published in the Connaissance des Temps for 1783 (published in 1780). M69 and M70 were added in a separate contribution to the end of the same volume. The final published version appeared again in Connaissance des Temps for 1784 (published in 1781) which contained objects through M103. Messier later added M104, and objects that were associated with some of his findings received numbers M105-M110 later on.

At the time of Messier, it was not known that the universe contained a myriad of galaxies - that would have to wait for the 1920s and Edwin Hubble. Consequently, Messier divided his objects into eight categories:

bright nebulae
faint nebulae
very faint nebulae
planetary nebulae
very large nebulae
very compressed and rich star clusters
compressed clusters of small and large stars(i.e. faint and bright)
coarsely scattered clusters of stars

It was in this that Messier coined the term "planetary nebulae" in order to refer to the circular clouds that he saw. In the primitive telescopes of the times, they actually looked like planets.

Called the Messier Catalogue after its creator, it is still used today to refer to many common astronomical objects. For example, the Andromeda Galaxy (right) is referred much more commonly to as M31 - because it was the 31^st object in his catalogue - than as it's New General Catalogue (see below) number NGC 206.

New General Catalogue

The New General Catalogue (NGC) is probably the most widely used catalogue of non-stellar astronomical objects. These include open and globular star clusters, diffuse and planetary nebulae, supernova remnants, and galaxies of all types (plus some entries that were errors and correspond to nothing of notice at all).

NGCwas originally compiled by John Louis Emil Dreyer (1852-1926) from observations by William Herschel, and it was published in 1887. Dreyer later came out with two supplements - the Index Catalogue (IC) I in 1895, and IC II in 1907. NGCincluded 7840 objects (modern versions have 7793 real objects), IC I and IC II added another 5386 objects (IC I adding 1529 and IC II adding 3857).

In 1823, Heinrich Olber put forth a seemingly simple question: Why is the night sky dark? This actually is not a very easy question to answer, and an exploration of the solution tells a lot about the nature of the universe.

First of all, it says that the universe is of finite size with a finite number of stars. If there were an infinite number of stars, then every spot in the sky would have an infinite number, so would shine as brightly as the sun. This also tells us that stars are obscured by dust. Dust obscures more distant stars, so only close ones are actually visible. Third, stars are not in a uniform distribution, but exist in clumps with voids in between.

These first three explanations are actually wrong. Any dust would heat up and so eventually glow as brightly as the stars that it hid. The second and third explanations are partially true. The universe does not contain an infinite number of stars, but it does contain enough to be more than enough to resolve the paradox. Also, stars are not in a uniform distribution, at least not on small scales, but we do not yet have an accurate enough map of extragalactic structure to know if this can still hold on a universal level.

The following are two explanations that do resolve the paradox, correctly. First off, the universe is expanding, so distant light is shifted to longer wavelengths and made harder to see, an effect known as the Doppler Shift. Second, the universe is still young at between 11.2-20 billion years. Distant light has yet to reach us from anything beyond the visible universe.

Ancient astronomers knew of six planets: Mercury, Venus, Earth, Mars, Jupiter, and Saturn. The other planets were discovered in the last few hundred years.

The story of the discovery of Uranus is actually quite uneventful. William Herschel discovered it in 1781. No one had any idea that there might actually be another planet in the solar system, and they were quite surprised once orbital calculations showed it to be a planet.

Once it had been observed for several decades, however, there were some irregularities that were seen in its orbit. By this time, Newton's Laws had revolutionized the way orbital mechanics was done, and they had been very successful - almost perfect - at describing planetary motion. So now, since Uranus' orbit could not be explained by the known laws of physics with the current configuration of the solar system, something else must be out there.

By 1846, enough observations had been made to show that Uranus' orbital perturbations could be attributed to another, unseen planet that was tugging at it, as suggested by Fredrich Wilhem Bessel. The calculations were refined by John Couch Adams and Urbain Leverrier. They told astronomer Johann Gottfried Galle where to look for this unknown planet, and within hours the planet Neptune was discovered.