The planets – The Earth as a planet (1977)

Carl Sagan

Carl Sagan explores our place in the solar system.

Watch time: 59:48
A still of Carl Sagan during his CHRISTMAS LECTURE
Credit: Royal Institution

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, Carl 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 Carl 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.


What exists beyond Earth? Over six Lectures presented in 1977, American astronomer and cosmologist Carl Sagan explores the vast expanse of space that surrounds the third planet from the Sun.

Life on Earth

Where at first we could only discern the size of our planet and some knowledge of its atmosphere and configuration, the evolution of planetary exploration has revealed not only intricate details of Earth’s climate and geology, but a multitude of stars and planets besides our own.

Beginning with a closer look at the world we inhabit, Carl explores of the diversity of life on our own planet and the building blocks behind it, before questioning whether the same organic chemistry is occurring on planets in the outer solar system.

The Red Planet

In Lecture three onwards, Carl takes a closer look at our neighbouring planet, Mars. From early interpretations of terrestrial life on its surface to the surprising discoveries made by NASA’s Viking Program, the Red Planet has become the focus of efforts to discern whether intelligent life exists elsewhere in the universe.

When Carl delivered his Lectures in the late 1970s, NASA had only just begun its Voyager program to the furthest planets in our solar system and no extra-solar planets were known to exist. Now, over three decades later, astronomers are looking at planets that lie beyond our solar system to ask the very same question we pondered over Mars: is there life out there?