One camp thought that the planets orbited around the Sun, but Aristotle, whose ideas prevailed, believed that the planets and the Sun orbited Earth. For Aristotle, this meant that the Earth had to be stationary, and the planets, the Sun, and the fixed dome of stars rotated around Earth. A geocentric worldview became engrained in Christian theology, making it a doctrine of religion as much as natural philosophy. Despite that, it was a priest who brought back the idea that the Earth moves around the Sun.
In , a Polish priest named Nicolaus Copernicus proposed that the Earth was a planet like Venus or Saturn, and that all planets circled the Sun. The theory gathered few followers, and for a time, some of those who did give credence to the idea faced charges of heresy. But the evidence for a heliocentric solar system gradually mounted. When Galileo pointed his telescope into the night sky in , he saw for the first time in human history that moons orbited Jupiter.
If Aristotle were right about all things orbiting Earth, then these moons could not exist. Galileo also observed the phases of Venus, which proved that the planet orbits the Sun. At about the same time, German mathematician Johannes Kepler was publishing a series of laws that describe the orbits of the planets around the Sun.
In , Isaac Newton put the final nail in the coffin for the Aristotelian, geocentric view of the Universe. While Copernicus rightly observed that the planets revolve around the Sun, it was Kepler who correctly defined their orbits.
At the age of 27, Kepler became the assistant of a wealthy astronomer, Tycho Brahe, who asked him to define the orbit of Mars. Brahe, who had his own Earth-centered model of the Universe, withheld the bulk of his observations from Kepler at least in part because he did not want Kepler to use them to prove Copernican theory correct.
Using these observations, Kepler found that the orbits of the planets followed three laws. Eventually, however, Kepler noticed that an imaginary line drawn from a planet to the Sun swept out an equal area of space in equal times, regardless of where the planet was in its orbit.
For all these triangles to have the same area, the planet must move more quickly when it is near the Sun, but more slowly when it is farthest from the Sun. It was this law that inspired Newton, who came up with three laws of his own to explain why the planets move as they do. By unifying all motion, Newton shifted the scientific perspective to a search for large, unifying patterns in nature.
Law I. Every body perseveres in its state of rest, or of uniform motion in a right line, unless it is compelled to change that state by forces impressed theron. The law is regularly summed up in one word: inertia. Law II. The alteration of motion is ever proportional to the motive force impressed; and is made in the direction of the right line in which that force is impressed.
The strength of the force F is defined by how much it changes the motion acceleration, a of an object with some mass m. Law III. We wrap the ends of a loop of string around two tacks pushed through a sheet of paper into a drawing board, so that the string is slack. If we push a pencil against the string, making the string taut, and then slide the pencil against the string all around the tacks, the curve that results is an ellipse. At any point where the pencil may be, the sum of the distances from the pencil to the two tacks is a constant length—the length of the string.
The tacks are at the two foci of the ellipse. The widest diameter of the ellipse is called its major axis. Half this distance—that is, the distance from the center of the ellipse to one end—is the semimajor axis , which is usually used to specify the size of the ellipse.
Figure 3: Drawing an Ellipse. Each tack represents a focus of the ellipse, with one of the tacks being the Sun. Stretch the string tight using a pencil, and then move the pencil around the tacks. The length of the string remains the same, so that the sum of the distances from any point on the ellipse to the foci is always constant. The distance 2a is called the major axis of the ellipse. The shape roundness of an ellipse depends on how close together the two foci are, compared with the major axis.
The ratio of the distance between the foci to the length of the major axis is called the eccentricity of the ellipse. If the foci or tacks are moved to the same location, then the distance between the foci would be zero. This means that the eccentricity is zero and the ellipse is just a circle; thus, a circle can be called an ellipse of zero eccentricity. In a circle, the semimajor axis would be the radius. Next, we can make ellipses of various elongations or extended lengths by varying the spacing of the tacks as long as they are not farther apart than the length of the string.
The greater the eccentricity, the more elongated is the ellipse, up to a maximum eccentricity of 1. The size and shape of an ellipse are completely specified by its semimajor axis and its eccentricity. The eccentricity of the orbit of Mars is only about 0. Kepler generalized this result in his first law and said that the orbits of all the planets are ellipses. Here was a decisive moment in the history of human thought: it was not necessary to have only circles in order to have an accepTable cosmos.
The universe could be a bit more complex than the Greek philosophers had wanted it to be. He expressed the precise form of this relationship by imagining that the Sun and Mars are connected by a straight, elastic line. When Mars is closer to the Sun positions 1 and 2 in Figure 4 , the elastic line is not stretched as much, and the planet moves rapidly. Farther from the Sun, as in positions 3 and 4, the line is stretched a lot, and the planet does not move so fast.
As Mars travels in its elliptical orbit around the Sun, the elastic line sweeps out areas of the ellipse as it moves the colored regions in our figure. Kepler found that in equal intervals of time t , the areas swept out in space by this imaginary line are always equal; that is, the area of the region B from 1 to 2 is the same as that of region A from 3 to 4.
If a planet moves in a circular orbit, the elastic line is always stretched the same amount and the planet moves at a constant speed around its orbit. It was this law that inspired Newton, who came up with three laws of his own to explain why the planets move as they do. By unifying all motion, Newton shifted the scientific perspective to a search for large, unifying patterns in nature.
Law I. Every body perseveres in its state of rest, or of uniform motion in a right line, unless it is compelled to change that state by forces impressed theron.
The law is regularly summed up in one word: inertia. Law II. The alteration of motion is ever proportional to the motive force impressed; and is made in the direction of the right line in which that force is impressed. The strength of the force F is defined by how much it changes the motion acceleration, a of an object with some mass m. Law III. To every action there is always opposed an equal reaction: or the mutual actions of two bodies upon each other are always equal, and directed to contrary parts.
Within the pages of Principia, Newton also presented his law of universal gravitation as a case study of his laws of motion. All matter exerts a force, which he called gravity, that pulls all other matter towards its center. The strength of the force depends on the mass of the object: the Sun has more gravity than Earth, which in turn has more gravity than an apple.
Also, the force weakens with distance. Earth would move straight forward through the universe, but the Sun exerts a constant pull on our planet. His theories also made it possible to explain and predict the tides. The rise and fall of ocean water levels are created by the gravitational pull of the Moon as it orbits Earth.
All of us moving through the universe on the Earth are in a single frame of reference, but an astronaut in a fast-moving spaceship would be in a different reference frame.
In general, few things are moving at speeds fast enough for us to notice relativity.
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