## Why is the Earth round?

Jeff De La Rosa, Managing Science Editor at World Book, explains why the Earth is round and how gravity affects the shape of Earth and all matter.

Gravitation is the force of attraction that acts between all objects because of theirmass. An object's mass is its amount of matter. Because of gravitation, an object that is near Earth falls toward the surface of the planet. An object that is already on the surface experiences a downward force due to gravitation. We experience this force on our bodies as our weight. Gravitation holds together the hot gases that make up the sun, and it keeps the planets in their orbits around the sun. Another term for gravitation is the force of gravity.

People misunderstood gravitation for centuries. In the 300's B.C., the Greek philosopher and scientist Aristotle taught the incorrect idea that heavy objects fall faster than light objects. People accepted that idea until the early 1600's, when the Italian scientist Galileo corrected it. Galileo said that all objects fall with the same accelerationunless air resistance or some other force acts on them. An object's acceleration is the rate of change of itsvelocity (speed in a particular direction). Thus, a heavy object and a light object that are dropped from the same height will reach the ground at the same time.

Newtons law of gravitation

Ancient astronomers measured the movements of the moon and planets across the sky. However, no one correctly explained those motions until the late 1600's. At that time, the English scientist Isaac Newton described a connection between the movements of the celestial bodies and the gravitation that attracts objects to Earth.

In 1665, when Newton was 23 years old, a falling apple caused him to question how far the force of gravity reaches. Newton explained his discovery in 1687 in a work called Philosophiae naturalis principia mathematica (Mathematical Principles of Natural Philosophy). Using laws of planetary motion discovered by the German astronomer Johannes Kepler, Newton showed how the sunâ€™s force of gravity must decrease with the distance from the sun. He then assumed that Earth's gravitation decreases in the same way with the distance from Earth. Newton knew that Earthâ€™s gravitation holds the moon in its orbit around Earth, and he calculated the strength of Earthâ€™s gravitation at the distance of the moon. Using his assumption, he calculated what the strength of that gravitation would be at Earth's surface. The calculated result was the same as the strength of the gravitation that would accelerate an apple.

Newton's law of gravitation says that the gravitational force between two objects is directly proportional to their masses. That is, the larger either mass is, the larger is the force between the two objects. The law also says that the gravitational force between two objects is inversely (oppositely) proportional to the distance between the two objects squared (multiplied by itself). For example, if the distance between the two objects doubles, the force between them becomes one-fourth of its original strength. Newtonâ€™s law is given by the equation F = m1m2 / d 2, where F is the gravitational force between two objects, m1 and m2 are the masses of the objects, and d2 is the distance between them squared.

Until the early 1900's, scientists had observed only one movement that could not be described mathematically using Newtonâ€™s lawâ€”a tiny variation in the orbit of the planet Mercury around the sun. Mercuryâ€™s orbitâ€”like the orbits of the other planetsâ€”is an ellipse, a geometric figure with the shape of a flattened hoop. The sun is not at the exact center of the ellipse. So one point in each orbit is closer to the sun than all other points in that orbit. But the location of the closest point changes slightly each time Mercury revolves around the sun. Astronomers refer to that variation as a precession.

Scientists used Newtonâ€™s law to calculate the precession. The calculated amount differed slightly from the observed amount.

Einstein's theory of gravitation

In 1915, the German-born physicist Albert Einstein announced his theory of space, time, and gravitation, the general theory of relativity. Einstein's theory completely changed scientistsâ€™ way of thinking about gravitation. However, it expanded upon Newtonâ€™s law, rather than contradicting it.

In many cases, Einstein's theory produced results that differed only slightly from results based on Newton's law. For example, when Einstein used his theory to calculate the precession of Mercuryâ€™s orbit, the result agreed exactly with the observed motion. That agreement was the first confirmation of Einsteinâ€™s theory.

Einstein based his theory on two assumptions. The first is related to an entity known as space-time, and the second is a rule known as the principle of equivalence.

Space-time. In the complex mathematics of relativity, time and space are not absolutely separate. Instead, physicists refer to space-time, a combination of time and the three dimensions of spaceâ€”length, width, and height. Einstein assumed that matter and energy can distort (change the shape of) space-time, curving it; and that gravitation is an effect of the curvature.

The principle of equivalence states that the effects of gravity are equivalent to the effects of acceleration. To understand this principle, suppose you were in a rocket ship so far from any planet, star, or other celestial object that the ship experienced virtually no gravitation. Imagine that the ship was moving forward, but not acceleratingâ€”in other words, that the ship was traveling at a constant speed and in a constant direction. If you held out a ball and released it, the ball would not fall. Instead, it would hover beside you.

But suppose the rocket accelerated by increasing its speed. The ball would appear to fall toward the rear of the ship exactly as if gravity had acted upon it.

Predictions of general relativity

In the years since the calculation of Mercuryâ€™s precession confirmed Einsteinâ€™s theory, several observations have verified predictions made with the theory. Some examples include predictions of the bending of light rays and radio waves, the existence of gravity waves and black holes, and the expansion of the universe.

Bending of light rays. Einsteinâ€™s theory predicts that gravity will bend the path of a light ray as the ray passes near a massive body. The bending will occur because the body will curve space-time. The sun is massive enough to bend rays by an observable amount, and scientists first confirmed this prediction during a total eclipse of the sun in 1919.

Bending and slowing of radio waves. The theory also predicts that the sun will bend radio waves and slow them down. Scientists have measured the sunâ€™s bending of radio waves emitted (sent out) by quasars, extremely powerful objects at the centers of some galaxies. The measurements agree well with the prediction.

Researchers measured a delay of radio waves that pass near the sun by sending signals between Earth and the Viking space probes that reached Mars in 1976. Those measurements still represent one of the most precise confirmations of general relativity.

Gravitational waves. General relativity also indicates that massive bodies in orbit around each other will emit waves of energy known as gravitational waves. Since 1974, scientists have confirmed the existence of gravitational waves indirectly by observing an object known as a binary pulsar. A binary pulsar is a rapidly rotating neutron star that orbits a similar, but unobserved, companion star. A neutron star consists mostly of tightly packed neutrons, particles that ordinarily occur only in the nuclei of atoms.

A pulsar emits two steady beams of radio waves that flow away in opposite directions. As the star rotates on its own axis, the beams sweep around in space like searchlight beams. If one of the radio beams periodically sweeps over Earth, a radio telescope can detect the beam as a series of pulses. By closely observing changes in the pulse rate of a binary pulsar, scientists can determine the pulsarâ€™s orbital periodâ€”the time it takes the two stars to completely orbit each other.

Observations of the binary pulsar called PSR 1913 + 16 indicate that its orbital period is decreasing, and astronomers have measured the amount of the decrease. Scientists have also used equations of general relativity to calculate the amount by which the orbital period would decrease if the binary pulsar was radiating away energy as gravitational waves. The calculated amount agrees with the measured amount.

In addition, the pulsarâ€™s orbit precesses as the pulsar revolves around the companion star. General relativity predicts the precession rate, and measurements match the prediction with great precision.

Black holes. Einsteinâ€™s theory predicts the existence of objects called black holes. A black hole is a region of space whose gravitational force is so strong that not even light can escape from it. Researchers have found strong evidence that most very massive stars eventually evolve into black holes, and that most large galaxies have a gigantic black hole at their centers.

Expansion of the universe. In a paper published in 1917, Einstein applied general relativity to cosmology, the study of the universe as a whole. The theory showed that the universe must either expand or contract. In 1917, however, scientists had not yet found any evidence of expansion or contraction. To prevent his theory from disagreeing with the available evidence, Einstein added a term, the cosmological constant, to the theory. That term represented a repulsion (pushing away) of every point in space by the surrounding points, preventing contraction.

But in 1929, the American astronomer Edwin Hubble discovered that distant galaxies are moving away from Earth and that, the more distant a galaxy, the more rapidly it is moving away. Hubbleâ€™s discovery indicated that the universe is expanding. In response to Hubbleâ€™s discovery and confirming observations by other astronomers, Einstein abandoned the cosmological constant, calling it his greatest blunder.

The discovery of the expansion of the universe, together with other observations, led to the development of thebig bang theory of the origin of the universe. According to that theory, the universe began with a hot, explosive eventâ€”a â€œbig bang.â€ At the beginning of the event, all the matter in the part of the universe we can see was smaller than a marble. Matter then expanded rapidly, and it is still expanding.

Dark energy. Although Einstein called the cosmological constant his greatest blunder, it may turn out to be one of his greatest achievements. Measurements reported in 1998 suggest that the universe is expanding more and more rapidly. Furthermore, the rate of expansion has been increasing as predicted by general relativity with a cosmological constant.

Until the measurements were reported, astronomers generally thought that the rate of expansion was decreasing due to the gravitational attraction of galaxies for one another. The measurements showed that exploding stars known as supernovae in distant galaxies were dimmer than expected and that the galaxies therefore were farther away then expected. But the galaxies could be so far away only if the rate of expansion had begun to increase in the past.

Astronomers have concluded that the increase in the expansion rate is due to an entity that presently opposes gravitation. That entity could be a cosmological constant or something much like it called dark energy. Scientists are still developing theories to account for the existence of dark energy, but they know how much of it probably exists. The amount of dark energy in the universe is more than twice as much as the amount of matter. In 2008, after gathering evidence for several years using the Chandra X-ray Observatory, the National Aeronautics and Space Administration announced the first direct evidence of the gravitational effects of dark energy. Scientists determined the effects by studying the movements of several galaxy clusters (dense collections of galaxies) spread across the universe.

The matter in the universe includes both visible matter and a mysterious substance known as dark matter.Scientists do not know the composition of dark matter. But measurements of the motion of stars and gas clouds in galaxies have led scientists to believe that it exists. Those measurements show that the masses of galaxies are many times larger than the masses of the visible objects in them. Astronomers estimate that the universe has about five times as much dark matter as visible matter.

Gravitation and the age of the universe. Other observations have helped show that the theory of general relativity applies to the whole universe. Cosmologists have calculated the age of the universe using equations of general relativity, the measured rate of expansion of the universe, and estimates of the amounts of dark energy and dark matter. The calculated age, about 14 billion years, agrees well with results determined by two methods that do not involve general relativity: (1) calculations based on the evolution of stars and (2) the radioactive dating of old stars.

Stellar evolution. As a star evolves, its surface temperature and its brightness change in a well-understood way. Astronomers can determine the ages of certain stars by measuring their temperature and brightness, then performing calculations based on their knowledge of stellar evolution. By means of such techniques, astronomers have found stars that may be about 13 billion years oldâ€”but no stars that are clearly older than that.

Radioactive dating of stars is based on the fact that certain chemical elements undergo radioactive decay. In radioactive decay, an isotope (form) of an element turns into an isotope of another element. Radioactive isotopes decay at known rates.

In 2007, astronomers reported on their work in applying the radioactive dating technique to an old star in our galaxy, the Milky Way. The scientists used the European Southern Observatoryâ€™s Very Large Telescope in Chile. They studied isotopes of uranium and thorium, comparing the amounts of these elements that the star must have had when it formed to the amounts it has now. They also compared the amounts of these isotopes to each other and to the amounts of other heavy elements. The astronomers then applied their knowledge of decay rates to calculate the age of the star. They determined an age of 13.2 billion years, with a 2-billion-year margin of error. Scientists think that the universe is about 13.7 billion years old

Contributor:

Joel R. Primack, Ph.D., Professor of Physics, University of California, Santa Cruz.