Mapping the sky

Part of the Dark Matters exhibit is a series of lithographs by Alastair Clark, partly inspired by glass photographic plates of the sky. This type of photographic plate is called a Schmidt plate, taken on a special sort of telescope with a very wide field of view. Using glass plates might sound slightly antediluvian, but up until the mid 1990s Schmidt plates were the most efficient way of capturing information about hundreds of thousands of stars and galaxies in a single exposure, and they have only recently been superseded by digital photography. Astronomers used these plates to make maps of the stars in our galaxy, the Milky Way, as well as maps of more distant galaxies.

Astronomers like to make maps – and there are some magnificent ones now. We can map distances to objects (by using their measured redshift) and also time. This allows us to picture how the Universe evolved from the initial big bang, when it went through its period of rapid expansion and when the first stars and galaxies were formed.

It sounds like a relatively straightforward activity – but as with any map, judgments need to be made about what to include. What is important and what is extraneous? And can a map be made without a supporting framework in which to view it? There is something about this activity that reminds me of Borges’ famous short story ‘On Exactitude in Science’ about the perfect (and therefore perfectly useless), map that has a one-to-one scale with reality.

Astronomers even make maps of the stuff they cannot see – the dark matter…