How does gravity keep planets in orbit?

Isaac Newton explained in 1687 that gravity acts as an attractive force between masses, and a planet’s sideways motion keeps it from falling straight into the Sun. In a roughly circular orbit, the inward pull of gravity is matched by the planet’s tendency to move forward at high speed. That is why Earth can orbit the Sun at about 29.8 kilometers per second and still keep nearly the same distance over time. Orbits are not perfect circles; most are slightly elliptical, as Johannes Kepler described in the early 1600s. The same principle works for moons, satellites, and even galaxies, though the details depend on mass and distance.