An orbit is the path a celestial object follows when moving under the control of another's gravity. This gravitational effect is evident throughout the universe: satellites orbit planets, planets orbit stars, stars orbit the cores of galaxies, and galaxies revolve in clusters.
Without gravity, celestial objects would hurtle off in all directions. Gravity pulls those objects into circular and elliptical (oval-shaped) orbits. Indeed, gravity was responsible for the clumping together of dust and gas shortly after the beginning of the universe, which led to the formation of stars and galaxies.
Since ancient times, astronomers have been attempting to understand the patterns in which planets travel throughout the solar system and the forces that propel them. One such astronomer was the German Johannes Kepler (1571–1630). In 1595, he discovered that the planets formed ellipses in space. In 1609, he published his first two laws of planetary motion. The first law states that a planet travels around the Sun on an elliptical path. The second law states that a planet moves faster on its orbit when it is closer to the Sun and slower when it is farther away.
Ten years later, Kepler added a third law of planetary motion. This law makes it possible to calculate a planet's relative distance from the Sun knowing its period of revolution. Specifically, the law states that the cube of the planet's average distance from the Sun is equal to the square of the time it takes that planet to complete its orbit.
Scientists now know that Kepler's planetary laws also describe the motion of stars, moons, and human-made satellites.
More than 60 years after Kepler published his third law, English physicist Isaac Newton (1642–1727) developed his three laws of motion and his law of universal gravitation. Newton was the first to apply the notion of gravity to orbiting bodies in space. He explained that gravity was the force that made planets remain in their orbits instead of falling away in a straight line. Planetary motion is the result of movement along a straight line combined with the gravitational pull of the Sun.
Newton discovered three laws of motion, which explained interactions between objects. The first is that a moving body tends to remain in motion and a resting body tends to remain at rest unless acted upon by an outside force. The second states that any change in the acceleration of an object is proportional to, and in the same direction as, the force acting on it. (Proportional means corresponding, or having the same ratio.) In addition, the effects of that force will be inversely proportional (opposite) to the mass of the object; that is, when affected by the same force, a heavier object will move slower than a lighter object. Newton's third law states that for every action there is an equal and opposite reaction.
Newton used these laws to develop the law of universal gravitation. This law states that the gravitational force between any two objects depends on the mass of each object and the distance between them. The greater each object's mass, the stronger the pull, but the greater the distance between them, the weaker the pull. The strength of the gravitational force, in turn, directly affects the speed and shape of an object's orbit. As strength increases, so does the orbital speed and the tightness of the orbit.
Newton also added to Kepler's elliptical orbit theory. Newton found that the orbits of objects going around the Sun could be shaped as circles, ellipses, parabolas, or hyperbolas. As a result of his work, the orbits of the planets and their satellites could be calculated very precisely. Scientists used Newton's laws to predict new astronomical events. Comets and planets were eventually predicted and discovered through Newtonian or celestial mechanics—the scientific study of the influence of gravity on the motions of celestial bodies.
In the early 1900s, German-born American physicist Albert Einstein (1879–1955) presented a revolutionary explanation for how gravity works. Whereas Newton viewed space as flat and time as constant (progressing at a constant rate—not slowing down or speeding up), Einstein described space as curved and time as relative (it can slow down or speed up).
According to Einstein, gravity is actually the curvature of space around the mass of an object. As a lighter object (like a planet) approaches a heavier object (like the Sun) in space, the lighter object follows the lines of curved space, which draws it near the heavier object. To understand this concept, imagine space as a huge stretched sheet. If you were to place a large heavy ball on the sheet, it would cause the sheet to sag. Now imagine a marble rolling toward the ball. Rather than traveling in a straight line, the marble would follow the curves in the sheet caused by the ball's depression.
Einstein's ideas did not prove Newton wrong. Einstein merely showed that Newtonian mechanics work more accurately when gravity is weak. Near stars and black holes (single points of infinite mass and gravity that are the remains of massive stars), where there are powerful gravitational fields, only Einstein's theory holds up. Still, for most practical purposes, Newton's laws continue to describe planetary motions well.