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CatalogueAndromeda Galaxy (M31)

Charles Messier (1730-1817) of France compiled the first modern catalogue of celestial objects. Called the Messier Catalogue, it is still used today to refer to many common astronomical objects, such as the Andromeda Galaxy, known as M31 (right) because it was the 31st object in his catalogue. The catalogue was made during the 1760's, and included 101 objects that were not comets.

Olber's Paradox

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 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 had been very successful - almost perfect - at describing planetary motion. So now Uranus' orbit couldn't be explained.

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

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