Up to the present, many scientists have made great contributions to the understanding of the universe.
One of them is Albert Einstein, a German physicist, who revolutionized our concepts of space and time with his theories of relativity. In 1905 Einstein published his special theory of relativity. The foundation of the theory is the observation that light moves at a constant speed as measured by all observers, whatever its state of motion. One consequence is that measurements of distance and time are not absolute quantities but very relative to the motion of an observer. Another is that energy(E) can be converted into mass(m), and vice versa, according to the formula E=mc2 ,where c is the speed of light. Ten years later, Einstein’s general theory of relativity extended this work to include acceleration and gravity — stating that the two are equivalent. Einstein showed that gravity is actually the warping of space by matter. One consequence is that light will follow a curved path when it passes a massive body. Observations of a small apparent shift in the positions of stars near the sun during a solar eclipse in 1919 showed that the theory was correct.
General relativity was later used to interpret Edwin Hubble’s discovery of the expanding universe. Edwin Hubble discovered a universe of galaxies outside the Milky Way and showed that the entire universe is expanding. In the early 20th century, most astronomers believed that the universe did not extend beyond our Milk Way galaxy. Then in the early 1920s, Hubble pointed the largest telescope in the world (on Mt. Wilson in California) at the mysterious Andromeda nebula. He recognized individual stars within the "nebula" and concluded that it was not a cloud of interstellar gas within our Milky Way, but an entire galaxy very far away. Suddenly, the universe was far larger than most astronomers had imagined. Hubble continued to find galaxies at greater and greater distances. He developed the system for classifying galaxies by their structure, which is still used today. Hubble also discovered that the distant galaxies are moving away from each other and the greater the distance between two galaxies, the faster they are moving away. This simple proportionality between the distance and the speed of galaxies moving away from each other, set forth in 1929, is now known as Hubble’s law. It shows that the universe is expanding.
When we talk about early sky gazers, we can not forget the Danish astronomer Tycho Brahe Before the invention of the telescope, Tycho Brahe revolutionized astronomy by establishing the importance of accurate observations. He invented an improved sextant and used it to make precise observations of the positions of stars and planets.
Brahe completely recalculated Ptolemy’s astronomical tables, which contained many errors, and catalogued over one thousand stars during his lifetime He built Europe’s first observatory and taught the art of observation to a generation of astronomers. In 1572 Brahe made careful observations of a "new star" (actually a supernova) that appeared suddenly in the constellation Cassiopeia He showed that the object was not in the Earth’s atmosphere but was actually beyond the orbit of the moon. This discredited the prevailing theory that the heavens are static and unchanging.
Armed with Tycho Brahe’s very accurate observations of the planets and his own painstaking measurements, Johannes Kepler discovered what we now call Kepler’s laws of planetary motion. These three laws precisely describe the motions of the planets around the sun.
Kepler’s first law states that planets move in elliptical orbits around the sun. This overturned the ancient dogma that the planets move in perfect circles. His second law describes how a planet travels faster in its orbit when it is closer to the sun. The third law describes how the orbital periods of the planets increase with the size of their orbits. Kepler’s laws were based on the Copernican view of a heliocentric universe Newton relied on them when he developed his more general law of universal gravitation.
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