In the night sky there is always something awesome to look at, like brightly shining stars, huge planets, meteors burning up and leaving glowing trails behind, and comets appearing and then suddenly disappearing. No wonder the night sky has fascinated astronomers like Galileo Galilei, Johannes Kepler, and Isaac Newton.

About 3,000 years ago, Britain's earliest humans built Stonehenge, a collection of huge stone slabs standing upright in a circle. It was used to track the movement of the Sun. Only seven stones still stand today. Thousands of miles away in Central America, between about 200 and 900 AD, Mayan Indians built many tall pyramids. The pyramids were used to keep track of the Earth and the Sun. Even though these two monuments are very different from each other, the people who built them had one common purpose in mind. They used these structures to keep track of the Sun, planets, and stars. Besides these two monuments, there have been many other similar ruins discovered around the world with the same purpose.

For the builders of Stonehenge, the Mayan pyramids, and other ancient "observatories," keeping track of the heavens was a very important job. It was especially important for farmers. They needed a way to follow the movements of the Sun so they would know when to plant and harvest their crops. So for many farmers, the observatories helped them keep track of time.

Eventually many cultures used what they knew about the movements of the Sun, Moon, and stars to develop better calendars. In most of the calendars the phases of the Moon were months, and when the Sun set it was a day. When there was a full Moon it would be the end of one new month and the beginning of a new month.

It took a lot of careful research to build monuments and to develop calendars for tracking the heavens’ movements. Even though the people in ancient times accomplished all this, there were a lot of them who didn’t understand the way the universe worked. To many of those people, the Sun, planets, and stars were gods and goddesses who controlled all human lives from their homes, above the Earth. Solar eclipses and comets were viewed with suspicion and fear.

Ancient astronomers came up with some important achievements. For example, to help predict solar eclipses, Babylonian mathematicians came up with a complex system similar to modern-day algebra. This ancient mathematical system is responsible for some things we still use today like the 360-degree circle, the 60-minute hour, and the 60-second minute.

The Babylonians also began predicting events in peoples' lives based on the movements of the planets. Today we call this astrology. The Babylonians invented the 12 birth signs that we still use today like Cancer, Scorpio, and Virgo. Around the same time the Greeks named most of the stars and constellations like Hercules, Perseus, and Cassiopea. The Greeks also named the planets after their gods like Mercury, Venus, Mars, and Jupiter.

The Greeks came up with many ideas based on the Babylonians' discoveries. The most important question had to do with the Earth's shape. Many scientists had thought the Earth was flat, but Aristotle, a Greek scientist who lived in the 4th century BC, thought it was round. He came up with this idea by watching an eclipse of the Moon and noticing that the Earth's shadow on the Moon's surface was curved. He thought that if the world was flat, then it must be rectangular. If Earth was shaped like a rectangle, then its shadow would also be a rectangle. So, it would be impossible for a rectangular Earth to make a curved shadow. Since Earth made a curved shadow, Aristotle thought Earth must be round.

The Greeks also tried to solve the mystery of Earth's position in the universe. Most people thought Earth did not move and was the center of the universe. They also thought the Sun, Moon, and stars orbited around the Earth. Around the 3rd century BC, Aristrachus, a Greek scientist, believed that the Earth and everything else circled the Sun. This was a very different idea compared to earlier beliefs. Aristrachus also suggested that the Earth spins on an axis, which causes day and night.

Not many people believed Aristrachus, and his theory was rejected. Another Greek astronomer named Claudius Ptolemy said that the Earth did not rotate and that Aristrachus was incorrect. For the next 1,700 years people supported the idea that the Earth did not move. It wasn't until the 1500's when a Polish astronomer named Nicolaus Copernicus began supporting Aristrachus' idea that the Sun was the center of the universe and all the planets revolved around it. Copernicus published his report in 1545, but the Roman Catholic Church did not accept it. However, other astronomers believed it and began working on trying to prove it. One of these astronomers was Johannes Kepler of Germany. Kepler used the findings of other astronomers before him and discovered three mathematical laws that explain how planets move in relation to the Sun. These laws have become so important in astronomy that Kepler is called the father of modern astronomy.

Around 1600, another astronomer named Galileo Galilei of Italy built his homemade telescope. He used the telescope for watching the sky and confirmed the idea that the Sun was the center of the universe. Galileo watched Venus over a period of time and discovered it goes through a complete set of phases just like Earth's Moon. Galileo thought that if Earth was the center of the universe, as most people still believed, he wouldn't be able to see all the phases he had observed in Venus. The Church still did not accept Galileo's findings.

Isaac Newton was a famous mathematician from England in the late 1600's. At this stage people were gradually coming to accept some of the ideas Galileo and others before him had spoken about. Isaac Newton's telescope was one of his big contributions to astronomy. The new telescope design helped answer many questions about the universe and improved people's views of the Sun, stars, and Moon. Isaac Newton came up with some important ideas about moving objects and forces that act on objects. He helped explain the force that we now call gravity. Finally scientists could understand how everything in the universe was held together. Isaac Newton also discovered how the Moon affects the tides in the ocean.

Early astronomers discovered and invented many things with very little equipment. Many times, if they disagreed with the Church, they were forced to change their ideas. Their discoveries have helped us put people in space and land men on the Moon. Who knows what new discoveries people will make in the future?

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Unless otherwise noted, all images courtesy of NASA. Permission for use at http://www.nasa.gov/gallery/photo/guideline.html.

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