If you study the sky on a regular basis, you can see that many changes occur in a cyclical manner. The rising and setting of the Sun., the phases of the Moon, and the movement of constellations in the night sky can all be used to keep track of the passage of time.
Stonehenge, the Mayan calendar and the North Star are all based on the celestial objects that form the basis of astronomy. Long before there was any understanding of what stars and planets were or why meteorites occurred, people used the objects in space to know when to plant crops, when to migrate to warmer climes, and to navigate; they also used these as part of their religions, and they contributed to their beliefs about creation, seasons, and day and night.
First Signs of Man's Understanding of Astronomy
In deciding whether or not an ancient structure is based on astronomy, scientists look at the orientation of a structure to see if it is aligned with any special celestial events. Examples of these are the rising or setting of the Sun or the Moon, or a particular star on a special day such as a solstice. The Newgrange tomb, built in Ireland around 3200 B.C. is the earliest known structure that is aligned in this special way. During the winter solstice, light shines through a passageway to a central tomb chamber.
Stonehenge and over 900 other rings of stone were built between 3000 and 2000 B.C. They are imprecise, but Stonehenge does show signs of being influenced by astronomy. The main axis is aligned with the Sun's rising on midsummer morning of the longest day of the year. However, because this alignment is imprecise, it may be that the architects were only using this for religious purposes.
It was around 2000 B.C. that the first solar-lunar calendars appeared. Based on the cycles of the sun and the moon, these calendars represent man's earliest written efforts to mark time in relationship to celestial objects. These were developed in Egypt and Mesopotamia.
The European Astronomers
In 246 B. C. Eratosthenes, a Greek scientist, made the first good measurement of the size of the earth. Using the ingenious method of measuring the length of shadows in different locations during the summer solstice, he was able to make a reasonable calculation of the circumference of the Earth.
Around 140 A.D., Ptolemy, a Greek astronomer, developed the geocentric theory of the universe. Basing his work on that of Aristotle and Hipparchus, he believed that the Earth was the center of the universe, and the other celestial bodies orbited it.
Between 1120 A.D., several observatories were built. These include ones in Egypt, Iran and Central Asia.
In 1543, Nicholas Copernicus, a Polish astronomer, developed the heliocentric model. This maintained that all of the planets, including Earth, orbited around the Sun. He was able to provide support for his theory by showing how it could accurately determine the positions of the planets.
In 1608, Hans Lippershey invented the telescope. In 1609, Galileo was the first person to use a telescope to study the night sky.
Also in 1609, Kepler announced two laws of planetary motion. Based on these, he was able to calculate the speed and distances of planets. In 1619, he announced his third law that states that if you know how long it takes for a planet to go around the sun you can calculate that planet's distance from the sun.
The Telescope Transforms Astronomy
Following the invention of the telescope, major discoveries were made in quick succession. Among these were Saturn's rings, the markings on Mars, and the Martian polar ice caps
In 1668, Isaac Newton created the first reflecting telescope. In 1675, Ole Rome measured the speed of light.
The Modern Age of Astronomy
The year 1687 marked the beginning of Modern Astronomy with Newton's publication of his theory of universal gravitation. With the creation of better and more powerful telescopes, new discoveries were made with frequency. These include the discovery of Uranus, galaxies, nebula and star clusters, asteroids and sunspot cycles.
In 1905, Albert Einstein published a paper on the special Theory of Relativity, followed in 1916 by his general Theory of Relativity. The Theory of Relativity makes predictions about the link between gravity, space and time.
In 1926 Robert Goddard discovered that liquid fuel is highly efficient and believed that with this tool travel into space and to the Moon was possible. In 1937, Grote Reber built the first radio telescope.
Nineteen fifty-seven marked the beginning of man's adventures beyond the earth's atmosphere. Russia was first with the launching of the Sputnik, the first man-made object to orbit around the Earth. The U.S. followed in 1958 with the launching of the Explorer 1. In 1961, Yuri Gagarin became the first man in space. Nineteen sixty-two found American John Glean orbiting the earth as well. In 1969, Apollo 11 landed on the moon, and Armstrong and Aldrin took man's first steps on the moon.
Beyond the Moon
The Russians were the first to be able to land a vehicle without its crashing on the surface of Venus in 1970. The U.S. launched the first satellite destined for Jupiter. The U.S. Mariner 10 probe sends the first pictures of Mercury. The Viking probes landed on Mars. Between 1980 and 1986, the Voyager spacecrafts sent back photos of Saturn and Uranus.
In 1981 the first space shuttle was launched. In 1986 the space shuttle Challenger exploded shortly after take off. In 1990 the Hubble Space Telescope, which has sent us images of the universe we never had even dreamed of, was placed into orbit.
Since this time, having unmanned spacecraft orbiting, landing, and transmitting images from other planets in our solar system has become almost commonplace. With the ability to land a rover on Mars, and have it send images of multiple locations on the planet, we are that much closer to being able to determine whether or not there has ever been life on the red planet. And, perhaps, this is only the beginning of the incredible discoveries to be made in our universe.