Lecture 1 - The Earth as a planet

In his first Christmas Lecture, American astronomer and cosmologist Carl Sagan explores planet Earth and the place, scale and geometry of the “pale blue dot” in the Solar System. 

Sagan provides a unique insight into the history of our knowledge of the third planet from the Sun, formed 4.5 billion years ago.

Using images and models of the planets in our Solar System, Sagan reveals how the heliocentric model of our universe, in which the Earth and planets revolve around the Sun, came to replace the earlier Aristotelian idea that our planet was at the centre and everything orbited around it.   

As the complexity of observational tools has developed from simple telescopes to complex spacecraft, so too has our understanding of the world we inhabit. Looking back on the evolution in space science in the years since Sagan’s lectures we have made huge advances in our understanding of our planet’s environment, climate, weather, geology and biology – as well as our relative place in the universe.




Carl Sagan



All lectures in the series