Asked by Susan James
After he completed his first reflecting telescope in 1668, Isaac Newton found that he could observe the four Galilean moons of Jupiter – Europa, Ganymede, Io and Callisto – as well as the crescent phase of the second planet from the Sun, Venus.
Newton’s intention wasn’t to discover new objects with his telescope. He built his instrument with the aim of proving his theory that white light is made up of a spectrum of colours. When we observe bright white targets, some telescopes reveal colour-fringing or chromatic aberration around the object’s edges, caused by a defect in a telescope’s optical system. It’s this problem that Newton was looking for to use as proof.
Newton didn’t find the colouration because his telescope was a reflector, which employs mirrors. He soon realised that it was the refractor that suffered from chromatic aberration, helping us to grab a better understanding of telescope design.
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