History of Astronomy

V

Copernicus and Galileo

Astronomy took a dramatic turn in the 16th century as a result of the contributions of the Polish astronomer Nicolaus Copernicus. Educated in Italy and made a canon (member of the clergy) of the Roman Catholic Church, Copernicus spent most of his life pursuing astronomy. His greatest contribution is entitled On the Revolution of Heavenly Bodies (1543), in which he analyzed critically the Ptolemaic theory of an Earth-centered universe and showed that the planetary motions can be explained much more simply by assuming that all the planets, including Earth, orbit the Sun. His ideas were not widely accepted until more than 100 years later.

The Italian astronomer Galileo ushered in a new era of science, one in which observations and experiments play the key role in testing models and hypotheses. Most historians believe that Dutch spectacle-maker Hans Lippershey invented the first telescope in the year 1608, but Galileo built one of his own in 1609, shortly after news of this invention reached him. Others had used telescopes to observe objects on Earth, but Galileo was the first to report astronomical observations, and his observations confirmed that Copernicus was right and that Ptolemy’s model of the planetary motions was wrong. Copernicus had predicted that if Venus orbits the Sun rather than Earth, Venus should go through phases just as the Moon does. Galileo discovered the phases of Venus. He also detected four moons orbiting Jupiter, which showed that not everything orbits Earth. One argument against the idea that Earth orbits the Sun was that the Moon would be left behind. Galileo’s observations clearly disproved that argument. After all, Jupiter’s moons were able to keep up with Jupiter.

Convinced that at least some planets did not circle Earth, Galileo began to speak and write in favor of the Copernican system. His attempts to publicize the Copernican system caused him to be tried by the Inquisition for heresy, and he was condemned to house arrest. Although he was forced to repudiate his beliefs and writings, Galileo and other Renaissance scientists showed that nature can be studied and understood through experiments and observations.

VI

Kepler and Newton

From the scientific viewpoint, the Copernican theory was only a rearrangement of the planetary orbits. The ancient Greek theory that planets move in perfect circles at fixed speeds was retained in the Copernican system. Precise new observations, however, showed that this could not be the case. From 1580 to 1597 Danish astronomer Tycho Brahe observed the Sun, Moon, and planets from his island observatory near Copenhagen, Denmark, and later in Germany. Based on the data compiled by Brahe, his German assistant, Johannes Kepler, showed that the planets revolve around the Sun, not in circular orbits with uniform motion, but in elliptical orbits at varying speeds. He also discovered that their relative distances from the Sun can be calculated from the observed periods of revolution.

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The English physicist Sir Isaac Newton was the genius who developed the mathematical equations that describe the motions of the planets. He had to invent new forms of mathematics, including calculus, to help him solve this problem. What Newton showed was that the most natural state of motion is a straight line. Since planets move along curved (elliptical) paths, some force must be acting on them. Newton called this force gravity. He showed that the force of gravity between two objects must be directly proportional to their mass and inversely proportional to the square of the distance between them. Newton was able to prove mathematically that if gravity behaved in this way, then the only orbits permitted were exactly those described by Kepler. In Newton’s day, gravity had been associated with the Earth alone; if you drop something, it falls to the ground. Newton’s great insight showed that this force is universal. It acts everywhere, including on the planets.

VII

Toward Modern Astronomy

The telescopes used by Galileo were made with lenses that typically were only about 2.5 cm (1 in) in diameter. Over the next 400 years, developments in technology made it possible to build ever larger telescopes with greater light-gathering power to detect ever fainter objects. Mirrors replaced lenses as the main optical elements in telescopes. The largest single telescopes in the world today, the twin Keck telescopes at the Mauna Kea Observatory in Hawaii, are each 10 m (400 in) in diameter, and astronomers are developing plans to build telescopes that are 3 to 5 times larger still.

Discoveries with telescopes from the 1600s through the 1800s laid the basis for modern astronomy. Many new members of the solar system were identified, including the planet Uranus in 1781 by the British astronomer Sir William Herschel and the planet Neptune in 1846, which was discovered independently by the British astronomer John Couch Adams and the French astronomer Urbain Jean Joseph Leverrier. Using telescopes astronomers also discovered the first asteroids between the orbits of Mars and Jupiter. Newton’s colleague Edmond Halley used the new theory of gravity to calculate the orbits of comets. Based on his calculations, he noted that bright comets observed in 1531, 1607, and 1682 might well be the same comet, reaching the point in its orbit closest to the Sun every 76 years. He predicted that this comet would return in about 1758. Although Halley had died by 1758, when the comet did indeed appear as he had predicted it was given the name Halley’s Comet.

Telescopic studies of double stars, also known as binary star systems, provided evidence that gravity applies outside the solar system. The two members of a double star system follow elliptical orbits around their common center of gravity, just as the planets orbit the Sun. This proof that the law of gravity is truly universal meant that the same physical processes that we can study here on Earth can be applied to studies of distant objects, including stars.

The distances to stars were first measured in 1838. In this year, three astronomers reported distances for three different stars—61 Cygni, Alpha Centauri, and Vega. The distances were calculated from measurements of the very slight shift in position of these nearby stars relative to much more distant background stars when viewed from opposite sides of Earth’s orbit. This is the calculation that the Greeks tried to perform in order to test whether the Earth orbits the Sun. The Greeks failed because the shift in position, which is called parallax, is only about 1.5 seconds of arc for even the nearest bright star. This degree of separation is about equal to the apparent size of a quarter when viewed from a distance of 2.3 km (1.4 mi). It was much too small to be measured with the techniques available to the Greeks.

The nearest of the first three stars measured, Alpha Centauri, is at a distance of about 42 trillion km (26 trillion mi). Obviously astronomers needed a new unit to measure such large distances, and one that eventually became widely used is the light-year. One light-year is equal to the distance that light travels in one year at the speed of light, which is about 300,000 km/sec (186,000 mi/sec). So one light-year equals 9.5 trillion km (5.9 trillion mi). The distance to Alpha Centauri from Earth is about 4.4 light-years.

In the mid-1800s astronomers also obtained information about what stars are made of. They used a technique called spectroscopy. When the light from a star is spread out into its rainbow of colors and passed through an instrument known as a spectroscope, some of the colors are found to be missing. These missing colors are referred to as dark lines. Laboratory experiments showed that the pattern of dark lines can be used to identify what hot gases—hydrogen, helium, even iron—are present in the star. Each element produces its own unique pattern.

In 1864 British astronomer Sir William Huggins was the first to show that the pattern of dark lines in the spectrum of a star matched the patterns produced by elements known here on Earth. Huggins’s discovery was another important example showing that the physical processes that we study here on Earth can be used to study the whole universe. Spectroscopy also provides information about the temperatures of stars, their masses, and their motions in space.

VIII

The Foundations of Modern Astronomy

A

Einstein and Relativity

As the 20th century began, the German-born physicist Albert Einstein advanced his general theory of relativity, which fundamentally changed our understanding of gravity. Einstein described gravitation as the curvature of space and time. His theory explained certain things that Newton’s theory of gravity could not. For example, certain peculiarities in Mercury’s orbit of the Sun could not be adequately described by Newton’s theory. In 1919 a team of astronomers led by British astronomer Sir Arthur Stanley Eddington used the occasion of a solar eclipse to measure the deflection of starlight as it passed by the Sun and arrived at numbers that agreed with Einstein’s predictions.

B

Edwin Hubble and the Scale of the Universe

The 1920s proved to be a breakthrough decade for astronomers who were attempting to learn more about the size, or scale, of the universe. In 1920 two American astronomers—Heber D. Curtis of the Lick Observatory and Harlow Shapley of the Mount Wilson Observatory—debated whether so-called spiral nebulae were part of the Milky Way Galaxy or were themselves distant galaxies. Curtis argued that they were “inconceivably distant galaxies of stars,” while Shapley placed them near the Sun.

In 1923 American astronomer Edwin Hubble, using the largest telescope in existence at the time—the 2.5-m (100-in) Hooker telescope at the Mount Wilson Observatory—discovered two Cepheid variable stars in a spiral nebula known as Andromeda. The intrinsic or true brightness of these stars was already known as a result of earlier work by American astronomer Henrietta Leavitt. The distance to Andromeda could then be calculated by a comparison of the apparent brightness of the Cepheids with their intrinsic brightness. Over the next six years, Hubble found a total of 40 Cepheids in Andromeda, and in 1929 he published a paper in which he calculated that the Andromeda nebula was about 900,000 light-years from Earth (current estimates of this distance are about 2.2 million light-years). Hubble’s observations therefore proved that Andromeda was a vast distance from the Milky Way Galaxy, which had a diameter of 100,000 light-years, and so must be a separate galaxy.

In 1929 Hubble published another and even more astounding discovery. His studies of distant galaxies revealed that the universe was not static, as had been previously believed, but was expanding in size. In 1927, the Belgian scientist Georges Lemaître had proposed a new model for the universe based on Einstein’s theory of general relativity. In this model, Lemaître assumed that the universe is expanding, a result that is consistent with the equations of general relativity. Hubble’s measurements of the red-shifts of distant galaxies, however, were the first to demonstrate that Lemaître’s assumption was indeed correct. This finding paved the way for the big bang theory of the origin of the universe.

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