The Sun is the center around which the Earth and the other planets of our solar system revolve. It's a rather ordinary star of average size. Even so, it is more than 1,392,000 km (865,000 mi) across — more than 100 times the diameter of Earth. Its mass equals that of 333,420 Earths! It resembles a huge furnace, fired by nuclear energy at its core. The tremendous energy produced is transferred outward, away from the core, in a process known as convection. Patches of gas heat and rise toward the surface. The tops of these heated patches of gas, which are called granules, can be seen in photographs of the Sun's surface — the surface has a mottled appearance created by a pattern of bright granules separated by darker spaces called intergranular lanes. As the energy reaches the Sun's surface, circulating currents of solar gas carry it away. The energy radiates in all directions and at practically all wavelengths of the electromagnetic spectrum, from long radio waves to short ultraviolet and X rays. Because the Earth is so small and so distant from the Sun, it receives only about one-half of one-billionth of the total solar-energy output. But this energy makes all life possible. It provides us with food and oxygen by way of green plants. Directly or indirectly, it provides us with energy to light and heat our homes and power our machines.
For centuries, people dreamed of visiting the Moon. These dreams became a reality in the second half of the 20th century. First, the United States and the Soviet Union sent unmanned spacecraft to the Moon, to photograph its surface and help determine the best sites for landings. Meanwhile, manned spacecraft were being launched into orbits around the Earth, to give people a chance to test equipment and to study the effects of space travel on the human body. Then, building on these successes, the United States developed the Apollo program. Its goal was to fly astronauts around the Moon and land them there. The first Apollo spacecraft...
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