19th-century astronomers went beyond cataloguing the skies to understanding their composition and predicting what could not be seen.

Where 18th century astronomy could be characterised by precise measurement of position and the classification of heavenly bodies, the 19th century saw astronomy applying developments in maths, physics, chemistry and geology to understand the make up of these bodies and the origins of the Universe. Astronomers were now interested in finding out exactly what a star, comet, or planet consisted of and how each was formed.

Spectroscopy developed in physics and chemistry was applied to find the chemical components making up the stars while theories in geology were used to understand the formation of bodies in the solar system.

Asteroids

When William Herschel discovered Uranus in 1781, this had been the first planet to be discovered since antiquity and sparked a new interest in our solar system. Astronomers began looking for a planet between Mars and Jupiter, as predicted by a mathematical relationship known as the Titius-Bode Law. Instead of a planet, they discovered a series of small, faint bodies: Ceres in 1801, Pallas in 1802, Juno in 1804 and Vesta in 1807. These objects became known as minor planets, or asteroids, and many more were discovered during the latter half of the 19th century, starting with Astraea in 1845 and Hebe in 1847.

It was partly because of these discoveries that George Airy installed his new Transit Circle at Greenwich. In 1847, while making the case for the construction of his new telescope, he wrote:

It is with the utmost difficulty that we have observed Astraea a few times, though she has been observed repeatedly on the Continent. We frequently are unable to observe, on the meridian, stars which have been compared with Comets in equatoreal observations

George Airy

The number of asteroids is now estimated to exceed half a million.

Neptune

Mathematicians John Couch Adams in England and Urbain Jean Joseph Leverrier in France both analysed variations in the orbit of Uranus and concluded that there should be another yet another planet in the solar system.

Adams contacted Astronomer Royal George Biddell Airy at the Royal Observatory to ask him to search for this new planet. Airy was unwilling to undertake the search, considering it unimportant and unrelated to the Royal Observatory's work in navigation and timekeeping. He recommended that Adams contact Challis, at Cambridge, who could search using the Northumberland telescope. Challis studied the area of the sky predicted by Adam's calculations but, unfortunately, missed the fact that one of the objects in the search field showed a planetary disc.

Meanwhile, Leverrier had contacted Johann Gottfried Galle, of the Berlin Observatory. Neptune was discovered by Galle and his student Louis d'Arrest on 23rd September 1846.

Spectroscopy

In 1859 chemist Robert Wilhelm Bunsen (1811–99) and physicist Gustav Robert Kirchhoff (1824–87) discovered that the spectrum produced by passing sunlight through a prism could be compared with spectra produced by chemicals burned in the laboratory, and that this could show which chemicals were present in the Sun.

This one discovery led to all kinds of new branches of astronomy.

Spectroscopy was used to study the outer regions of the Sun during total eclipses (the only time that the Sun's corona is visible). It was also used to study the composition of stars

Expeditions

The 19th century was also a time for mass involvement in astronomy. Expeditions to observe eclipses were popular with both professional and amateur astronomers alike. Astronomy clubs and societies were set up. The Royal Astronomical Society was formed in 1822 and the British Astronomical Association in 1890, coming out of the popularity of regional amateur societies around the country.

The 19th century also saw the start of international collaborations between observatories – for example, the Carte du Ciel project, which involved observatories around the world photographing sections of the sky in order to build a map of the heavens.

The transit of Venus

In 1874, and again in 1882, Venus passed across the face of the Sun. These events, known as Transits of Venus, occur in pairs more than a century apart – the previous transits were in 1761 and 1769. The last were quite recent and the next pair are due 10–11 December 2117, and 8 December 2125.

Careful timing measurements of the passage of Venus across the Sun, made from different locations on Earth, can be used to calculate the distance of the Earth from the Sun. In practice, these observations were difficult to make with any great precision. Captain Cook had attempted timings of the 1769 transit, from Tahiti, but was only partially successful.

Airy sent astronomers from Greenwich to five locations around the world, including Honolulu, Thebes and Cape Town, to observe the 1874 transit. Each expedition was equipped with portable telescopes and transit instruments, and took observation huts to use as temporary observatories.

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