The legend is that Newton discovered Gravity when he saw a falling apple while thinking about the forces of nature. Whatever really happened, Newton realized that some force must be acting on falling objects like apples because otherwise they would not start moving from rest.

Gravity is most accurately described by the general theory of relativity (proposed by Albert Einstein in 1915), which describes gravity not as a force, but as a consequence of masses moving along geodesic lines in a curved spacetime caused by the uneven distribution of mass.

Above the Earth’s atmosphere, outer space dims even further, fading to an inky pitch-black. And yet even there, space isn’t absolutely black. The universe has a suffused feeble glimmer from innumerable distant stars and galaxies. This artist’s illustration shows NASA’s New Horizons spacecraft in the outer solar system.

Despite what you might assume from this image, most of the Universe is empty, intergalactic space. But the reason the Universe is this large today is because it’s expanded and cooled to reach this point. Even today, the Universe continues to expand at a tremendous rate: approximately 70 km/s/Mpc.

Far and away the brightest object in the sky, the sun is easy to find, but it’s so bright that one can’t look directly at it without vision damage. Unlike most satellites, which orbit the Earth, SOHO orbits around the sun itself, like a planet or comet.

Venus is very similar to Earth in size and mass – and so is sometimes referred to as Earth’s sister planet – but Venus has a quite different climate. Venus’ thick clouds and closeness to the Sun (only Mercury is closer) make it the hottest planet – much hotter than the Earth.

Solis is Latin for sun. Sol is the Roman equivalent of the Greek sun god Helios.

Sun

Helios

Sol

Selene