NASA's Cosmos

1. Evolving perspectives - a historical prologue

Both ancient Earth-centered models and modern Sun-centered ones have been used to describe and predict the motions of the wandering planets, which can suddenly turn around, apparently moving in the opposite direction before continuing on their usual course.

The stars seem to be revolving around the Earth each night, but the Earth is instead spinning beneath the stars. This rotation causes the Sun to move across the sky each day.

Nicolaus Copernicus argued in 1543 that the Earth is whirling endlessly about the Sun, completing one circuit each year.

Almost four centuries ago, Johannes Kepler used accurate observations, obtained by Tycho Brahe, to infer a precise mathematical relation between the mean orbital distance and period of each planet in its revolution about the Sun.

Isaac Newton introduced the laws of universal gravitation in his “Principia", published in 1686, showing that the Sun’s gravitational force holds the solar system together.

In the early 17^th century, Galileo Galilei used his pioneering telescopic observations of the four large moons of Jupiter and the phases of Venus to support the Sun-centered model of the solar system.

Two kinds of telescopes, the refractor and the reflector, enable astronomers to detect faint objects that cannot be seen with the unaided eye, and to resolve fine details on luminous planets that otherwise remain blurred.

The known size of the solar system doubled when Uranus was discovered in 1781, and was enlarged by almost this amount once again with the discovery of Neptune in 1846.

The asteroid belt between the orbits of Mars and Jupiter contains billions of asteroids, but it is largely empty space and has a total mass that is much less than that of the Moon.

Estimates for the Earth-Sun distance were gradually refined over the centuries, setting the scale of the solar system and weighing the Sun from a distance.

The nearest star is about 270 thousand times further away from the Earth than the Sun.

The composition of the Sun is encoded in the visible spectrum of sunlight, and some of the ingredients of planetary atmospheres can be inferred from the ways that they change that light.

The lightest element, hydrogen, is the most abundant element in the Sun, and the next most abundant solar element, helium, was first discovered in the Sun.

Saturn’s rings are completely detached from the planet; they consist of innumerable tiny satellites each with an independent orbit about Saturn.

Jupiter, Saturn and Uranus have a retinue of large satellites, Neptune has only one large moon, and Mars has two small ones.

Pluto is a very small world with an oversized companion.

A host of small, unseen icy objects can be found in the disk of the solar system, beyond the orbit of Neptune. This Kuiper belt is a reservoir for short-period comets.

As many as a million million (10¹²) small, unseen icy objects orbit the Sun in the Oort cloud, a spherical shell that lies at distances that are a quarter of the way to the nearest star. A small trickle of them are deflected, by passing stars or molecular clouds, into the heart of the solar system where they are seen as long-period comets.