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Mercury is the planet closest to our sun, and as it's visible to the naked eye, it's intrigued humanity for as long as we've been here.
We see it as an evening or a morning star close to where the sun has just set or is about to rise.
And it helped Copernicus to understand that we orbit the sun, and Einstein to prove his general theory of relativity.
And for the last 50 years, we've been sending missions there to reveal something of its secrets and how those relate to the wider universe.
With the latest bepicolombo out there in space.
Now with me to discuss Mercury are Emma Bunce, professor of planetary plasma physics and director of the Institute for Space at the University of Leicester, David Rothery, professor of planetary geosciences at the Open University and Caroline Crawford, emeritus fellow of Emmanuel College, University of Cambridge, and emeritus member of the Institute of Astronomy, Cambridge.
Carolyn, before the invention of telescopes, what did we know, or think we knew.
About Mercury as one of the five planets that are visible to the inaided eye?
Mercury has been known about since ancient times.
And in fact, there are recorded observations of it from the first millennium BC on babylonian k tablets.
And it's been observed.
It changes in brightness.
It can be as bright as any of the stars in the sky, and then it'll fade to being barely discernible to the unaided eye.
But the key thing that makes observing Mercury quite difficult is that proximity to the sun.
Most of the time, it's completely lost in the sun's glare.