A history of telescopes

How did telescopes come into focus
08 September 2020

Interview with 

Richard Dunn, Science Museum London


Old style telescope


Adam Murphy’s been taking a look at how telescopes first came into being, by speaking to the Keeper of Technologies and Engineering and the Science Museum in London, Richard Dunn...

Adam - People, including me, have been fascinated by space since time immemorial. But looking up with our puny human eyes only gets you so far. To really see out, you need something more. You need a telescope. In some ways, the telescope seems like a simple idea. It's a few bits of glass, maybe a mirror, all put in a tube, but to find out how they came about, and what their legacy is, I spoke to Richard Dunn, Keeper of Technologies and Engineering at the Science Museum in London.

Richard - People were, certainly by the 16th century, thinking about how you might use lenses to see much further away. And there are lots of texts speculating, sometimes in the kind of magical way, about wondrous devices for doing this. The crucial development seems to be at the beginning of the 17th century, where someone works out that if you use diaphragms to cut out the outer parts of lenses, you'll manage to produce an image with much better resolution. At the end of September, 1608, we have a letter saying that there's a chap with a new device for seeing things far off. And this is in the Dutch Republic. Initially announced as a device for seeing your enemies from far away. So very much about use on land and at sea, not as something for looking out into the universe. And really from there, the telescope spreads incredibly quickly. Within six months, you've got cheap telescopes on sale in Paris, places like that. And these new low powered two lens telescopes are spreading throughout Europe.

Adam - So they were popular and a huge crowd of people could have them. But of that crowd, there is one name in particular that comes into focus.

Richard - Galileo Galilei is the person very much associated with the telescope, and its first uses in astronomy, even though he's not the first person to do recorded astronomical observations. He comes on the scene in the middle of 1609, hears about the telescope and begins making his own. And it's in the autumn, kind of October, November, that we know he's using an instrument that can magnify 20 times to begin doing astronomical observations, particularly observations of the Moon, but also other parts of the night sky. And this leads to his revolutionary work, the Sidereus Nuncius, the starry messenger, that's published March the following year.

And the reason he's so associated with the telescope and its use in astronomy, is because he gets this amazing book out. And he's the first person to print a report of telescopic observations, and makes a set of extraordinary claims. And he says that the Moon is not a smooth sphere, but it has mountains and valleys. He says that the Milky Way is not this kind of, solid band, but it's made of individual stars. It says that there are stars in the heavens that the eye alone can't see, you can only see them with a telescope. And finally he says that Jupiter has four of its own moons circling around it. And some of these are claims that really undermine the current view of how the universe was constituted at that time.

Adam - Those discoveries put a stamp on history. The four moons he saw around Jupiter are still known as the Galilean moons, but how did it revolutionise our understanding of our place in things?

Richard - The observations that Galileo announces, they don't prove that the Earth moves around the Sun, but they do undermine the traditional view of an Earth centered system with concentric spheres, perfect concentric spheres, moving around the Earth. The idea that the Moon is not a perfect sphere, undermines that. The idea that Jupiter has its own moons moving around it as the centre undermines the idea that all motion is around the Earth as the centre. But that didn't necessarily show that the Earth was moving around the Sun. And even when, later than the publication of the Sidereus Nuncius, Galileo announces that he's observed phases of Venus, even that doesn't absolutely prove that the Sun-centered system is correct.

So it's not actually until the 18th century that we get the first observational proof that the Earth is in motion around the Sun. And this is from the work of James Bradley, who after a long series of observations in East London in the 1720s, actually discovers what we now call the aberration of starlight. And this is an apparent change in the position of observed stars caused by the Earth's motion. And so it's only in 1729 when he announces it, that we actually have any direct observational proof. So it's quite interesting that people by that time, had long accepted that we live in a heliocentric system, but there was not actually absolutely conclusive proof that that was the case.


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