Comets and asteroids

As 1 Ceres reaches opposition on 28 August, we look back at its discovery

Ceres is a world of many designations; it was the first minor planet or asteroid formerly identified. This event occurred on 1 January 1801, its discoverer being an Italian monk called Giuseppe Piazzi. He was working at the Palermo Astronomical Observatory in Sicily at the time. While searching for a particular star, he observed moving Ceres and believed he had stumbled upon a comet. He followed the object for 41 days after which time he became ill, just as the Sun began to interfere with the field of view.

With just 41 days of observations, many doubted that the mathematics of the time could predict where Ceres would be after re-emerging from the Sun’s glare. Mathematician Carl Friedrich Gauss saved the day, discovering a method for computing its orbit. He sent his predictions to astronomers Franz Xaver von Zach and Heinrich WM Olbers, who found the object on 31 December 1801.

Although Piazzi believed he had observed a new planet, other similar objects were soon discovered and it was realised that this was a new class of object. William Herschel coined the term asteroid, meaning ‘star-like’, to describe them.

In 2006 the International Astronomical Union, faced with the dilemma of several large Pluto-sized bodies discovered in the Solar System, created the new classification of ‘dwarf planet’.

This is the decision which demoted Pluto from planetary status while elevating 939km-diameter Ceres to a dwarf planet.

Ceres reaches opposition on 28 August. At the time it will lie within Aquarius and shine at mag. +7.7. It starts its August passage at mag. +8.1 close to mag. +3.7, 88 Aquarii. Consequently, it will be visible through binoculars all month. During favourable oppositions Ceres can reach mag. +6.6.