Passion of the people – 1.1.11

Imaginary world – the 19^th century

Where 18^th-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. At the same time, theories in geology were used to understand the formation of bodies in the 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.

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.

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 British Royal Astronomical Society was formed in 1822 and the British Astronomical Association in 1890, coming from 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.

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. Astronomers from Greenwich were sent out 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.

Tookit

Passion of the people

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Venus Transit: A Planet’s Day in the Sun