Date: March 17, 2010

Title: Why Do the Planets Stay in Orbit?


Podcaster: Stuart Clark


Description: Why do the planets stay in orbit? And why doesn’t the Moon fall out of the sky.

Bio: Dr Stuart Clark is an award-winning astronomy author and journalist. His books include The Sun Kings, and the highly illustrated Deep Space, and Galaxy. His next book is Big Questions: Universe, from which this podcast is adapted. Stuart is a Fellow of the Royal Astronomical Society, a Visiting Fellow of the University of Hertfordshire, UK, and senior editor for space science at the European Space Agency. He is also a frequent contributor to newspapers, magazines, radio and television programmes. His website is and his Twitter account is @DrStuClark.

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Hello I’m Dr Stuart Clark, astronomy author and journalist. Today I’d like to explore the question: Why do the planets stay in orbit?

Well, if the question is a simple one, the answer is anything but. Yet it led to the defining moment of science: the moment when natural philosophy was placed on its course to become modern science, with mathematics as its language.

The reason why the planets stay in orbit is contained within Isaac Newton’s theory of gravity; the central concept of which is ‘universal gravitation’, which states that everything with mass generates gravity: the Earth, the Moon, the Sun, all the planets and the moons, all the stars – everything. The amount of gravity generated is proportional to an object’s mass, and affects anything else nearby with mass; in other words, everything pulls everything else.

Newton’s Universal Gravitation was partly based upon the pioneering work of Johannes Kepler, who had mathematically described planetary motion several decades earlier.

Kepler’s first of three laws states that each planet moves in an elliptical orbit, with the Sun at one focus. The second law is that when the planet is far away from the Sun, it does not move as fast as when it is close to the Sun. This implied that whatever force was moving the planet weakened with distance. The third law follows on from the second, expressing as an equation the link between the size of the planet’s orbit and the time it takes to complete the orbit. But Kepler could not explain why the planets moved in this way.

Newton could. He visualized a cannon atop a tall tower, pointing horizontally and firing its projectile. Ignoring air resistance, the cannonball would zoom off parallel to the ground and the gravity of the Earth will immediately begin to pull it downwards, eventually dragging it to the floor. The greater the explosive charge, the faster the projectile will be ejected and the further it will travel before gravity pulls it down. Newton imagined sufficient explosive to eject the cannonball so fast that, by the time it started to fall, the curvature of the Earth resulted in the ground beneath dropping away and so the cannonball finds itself always at the same altitude above the ground. Without air resistance, the projectile would still be travelling at the same speed as when it left the cannon, and the whole situation starts again. Every time the cannonball drops a little, so the curvature of the Earth compensates, allowing the projectile to continue around the Earth forever; in effect, placing it in orbit.

This gives us the solution to the question of what stops the Moon crashing into the Earth. The Moon is falling towards us, but also travelling along so fast that it ‘overshoots’ the Earth and continues in a circular path.

Newtonian gravity changed the way astronomers thought about the night sky. No longer content to chart the positions of the stars as an aid to navigation, they could understand the motion of the celestial objects and predict future movements. The dates of future eclipses, the return of comets, the alignments of the planets – all were prescribed by Newton’s theory.

And it applied to a breathtaking range of other phenomena, too. It gave a way of estimating the mass of the planets and the Sun, and a means of explaining why the Earth and other planets bulged at their equators. It provided a method of calculating the movement of falling objects on Earth, and, importantly in the 17th century, of predicting the trajectory of projectiles fired from cannons.

The tides, so important to a sea-faring nation, could finally be explained as due to the gravitational attractions of the Moon and the Sun on the oceans. And Newton’s work showed that the same happens in reverse. Earth’s gravity deforms the Moon and because Earth more massive, the lunar tide is correspondingly larger, amounting to an elongation of the Moon by many metres. These changes to the spherical shapes of Earth and the Moon constantly sap their rotational energy. In the case of the Earth, the length of the day slowly but perceptibly increases and an extra second must be added occasionally to the midnight chimes at New Year. The slowing of the Moon’s rotation is more profound; over the billions of years since its formation, it has slowed so much that it now rotates just once every orbit, constantly presenting the same face to Earth.

Because of its huge size, Jupiter creates enormous tidal forces on its collection of moons. It transforms the innermost moon, Io into the most volcanically active place in the Solar System. Further out is the moon Europa, where the tidal force is less extreme and may provide the energy to keep a global ocean of liquid water under Europa’s icy crust. There may even be more water on Europa than there is on Earth.

Newton’s gravitational theory has allowed astronomers to find more than 400 planets orbiting other stars; they have not seen a single one of these planets, but their presence is certain because the stars are ‘wobbling’. Just as the star pulls the planet into an orbit, so the planet pulls back on the star and makes it wobble. The surprise is that instead of following slow pirouettes, like the one Jupiter induces in the Sun, most of the wobbling stars move quickly, indicating large planets in close orbits. As technology improves astronomers expect to find planetary systems more like ours.

Yet for all his success, nowhere did Newton explain the nature of gravity; he simply described it mathematically. Subsequent natural philosophers and scientists grappled with the fundamental origin of gravity, though none came close to a breakthrough. The world had to wait until the second decade of the 20th century to receive a mind-bending answer from Albert Einstein with his Theory of General Relativity.

End of podcast:

365 Days of Astronomy
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