Elements of astronomy

The solar system is part of a galaxy called the Milky Way. The system is made up of the sun, the nine planets and their satellites, asteroids, comets, and dust. There are believed to be many other systems in the universe that also contain planets.

The sun is a star. The planets revolve around it. The satellites, or moons, revolve around the planets. Asteroids are small, rocky bodies that orbit the sun. They are about 620 miles (1,000 kilometers) or less in diameter. They are found mostly between the orbits of Mars and Jupiter. Comets are heavenly bodies mainly made up of frozen water, frozen gases, and some gritty material similar to stone. Comets also orbit the sun. The dust in the solar system is made of fine particles or grains.

Apart from visible matter, the universe may contain large quantities of matter that do not emit any light. This matter cannot be seen by standard astronomical techniques and is, therefore, known as hidden mass, or dark matter.

Study

Astronomers study the universe in different ways. Some observe distant objects with the help of instruments. Others work in laboratories where they analyze actual samples of meteorites, rock samples from the moon, and interplanetary dust particles collected by spacecraft. Some scientists interpret the data collected through observation, construct models of celestial objects and their properties, and guide further observations.

Some people study objects in space as a hobby. They are known as amateur astronomers. There are far more amateur astronomers than there are professional astronomers. It is often the amateurs who discover a new comet or an exploding star.

Telescopes

Astronomers use a wide range of instruments to observe outer space. The earliest of these is the telescope that magnifies, or enlarges, distant objects. Galileo Galilei, an Italian astronomer, has been credited with developing the telescope in 1609. Since then much more powerful telescopes have been built.

The size of a modern telescope varies. It can range from those used in giant observatories and Earth-orbiting spacecraft to those small enough to be held in one's hand. An observatory is a place for the viewing of natural phenomena. The Hubble Space Telescope is a very sophisticated optical observatory that was placed into orbit around the Earth. It magnifies and photographs distant objects.

Photography has been used in astronomy for more than 100 years. It helps reveal faint, hard-to-detect objects, as well as record all observations. Besides the Hubble Space Telescope, many other manned and unmanned spacecraft have been launched into outer space. These have provided astronomers with invaluable data about the sun, moon, and planets. Some of the most famous of these are the Orbiting Solar Observatory, the Lunar Orbiter, and Voyagers 1 and 2.

History

Thousands of years ago, before there were any calendars or clocks, people kept track of time by watching the sun and the stars. It was important for them to know the time in order to sow their crops and reap their harvests. These people were the first astronomers.

Many early civilizations eventually developed reliable calendars. The Babylonians are thought to have recognized a number of prominent constellations as early as 3000 BC. It is believed that they developed a calendar based on the regular occurrence of certain astronomical events. Astronomer-priests of ancient Egypt used their calendars to predict the time of the annual flooding of the Nile River. The Maya people of the Yucatán peninsula in Mexico developed a complicated calendar to keep track of past and future days and predict astronomical events.

In China, a calendar had been developed by the 14th century BC. A Chinese astronomer, Shih Shen, drew up what may be the earliest star catalog, listing about 800 stars. Chinese records even mention comets and meteors.

The astronomers of ancient Greece, including Aristotle, thought that the Earth was a sphere. Hipparchus, who lived around 140 BC, classified stars according to their visible brightness, estimated the size and distance of the moon, found a way to predict eclipses, and calculated the length of the year to within 6¹/₂ minutes.

The most influential ancient astronomer was Ptolemy of Alexandria in Egypt. He lived in lived in the 2nd century AD. He predicted the motions of the planets. In his view, Earth occupied the center of the universe. His theory about the motions of the celestial bodies was held until the end of the Middle Ages.

In medieval times Western astronomy did not progress much. During those centuries Hindu and Arabian astronomers kept the science alive. The records of the Arabian astronomers and their translations of Greek astronomical information later helped in the development of Western astronomy.

In 1543 Copernicus published his theory that Earth and the other planets revolved around the sun. His suggestion went against all the beliefs of the time. Galileo supported Copernicus' theory with his observations.

The great Danish astronomer Tycho Brahe first rejected Copernicus' theory. Yet later his data on the positions of planets were used to support the theory. After Tycho Brahe died, his assistant, Johannes Kepler, analyzed the astronomer's data and developed the laws of planetary motion. In 1687 Isaac Newton's laws of gravitation and motion reinforced Kepler's laws.