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(originally published to Helium writing site, now gone)

Since the Stone Age, people have observed the path of the sun across the sky. They noticed that the sun rises and sets in a different place each day and climbs higher in the sky in the warmer months. When agriculture started, the sun and the seasons became even more important. People were soon making detailed observations about the sun. They used them to determine when to plant and harvest crops, and devised calendars based on the solar cycle. Most early civilizations regarded the sun as a god or goddess.

The earliest site used to observe the sun and its movements was probably Stonehenge in southern England. Construction activity at the site dates back to about 8000 BCE. Archaeologists found evidence of four or five large postholes that held pine posts. No doubt there was originally a greater number. The posts were used to observe the movement of the sun and to establish the time of the northern summer solstice.

From about 2600 BCE, bluestone replaced the timber at Stonehenge. Up to 80 of these stones were placed in holes dug at the middle of the site. Each stone weighs about four tons and is more than six feet high. A sandstone known as the “heelstone” is 16 feet high. In 1771, Dr John Smith noted that from the center of the site, the sun rose over this stone at the summer solstice or longest day. Smaller stones are aligned with the sun at other times of the year, such as at the equinox. In 1950, Gerald Hawkins and Fred Hoyle proposed that Stonehenge was also used to forecast solar and lunar eclipses.

A solar observatory has been found in Germany dating back to 5000 BCE. Archaeologists believe the site was made up of four concentric circles plus a ditch, mound and two wooden posts. Three sets of gates faced north, south-east and south-west. From the center of the circles, the sun could be seen rising and setting through the southern gates at the winter solstice. The site has been called the German Stonehenge. Later, these structures are thought to have been quite common, with about 200 having been found throughout Europe.

A tomb at Newgrange, Ireland, has a 60 foot long passage leading to a chamber whose roof has 97 stones around it in a circle. The site has been dated to about 3200 BCE. The passage and chamber are lit up at sunrise at the winter solstice for 17 minutes, capturing a significant astronomical event. Similar sites have been found elsewhere in Ireland. The timing of astronomical events was important for agriculture, religion and calendar development.

Solar astronomy in Egypt goes back to about 5000 BCE. A site with an alignment of stones in a circle dates to this period. At Egypt’s Abu Simbel temple, built in the 13th century BCE, the dawn sun illuminates the statue of Pharaoh Ramses II at the equinoxes. The Sphinx, dating to the third millennium BCE, faces east and gets the first of the morning sun at the autumnal equinox. In the Nabta Playa area of southern Egypt, sets of enormous stones from around 3000 BCE are aligned with the solstice.

The Egyptians were the first to develop a solar calendar. The point on the horizon where the sun rises at the time of the northern winter solstice in late December was regarded as the birthplace of their sun god Ra. Around 4500 BCE, they counted the time elapsed between his visits to his birthplace as 365 days. So they could keep track of Ra’s birthday, the people of Lower (northern) Egypt introduced a lunisolar calendar of this length. It had 12 moons or months of 29 or 30 days each and an additional or intercalary month every two or three years as the first month. This meant the celebration of the birth of Ra could always be in the last month.

Ancient Babylonians built observatories called ziggurats, such as the Tower of Babel. Astronomer-priests observed the sun as well as the moon and the planets. They recorded their findings on clay tablets and could predict the movements of these bodies. For example, they worked out that solar eclipses went in cycles. They also knew that 19 solar years almost equal 235 lunar months. By adding seven 30 day intercalary or extra months over a 19 year cycle, a calendar is accurate to one day in 219 years (later known as the Metonic cycle after the Greek astronomer Meton).

Solar astronomy was also important in the ancient Far East. The first record of a solar eclipse was in China in 2136 BCE. Their sunspot records go back to 28 BCE and perhaps earlier. Records of Indian astronomy go back to around 1500 BCE, including descriptions of the sun’s movement, solstices, equinoxes and calendars. In Japan, two stones found near the Takamatsu Zuka Kofun tomb, which dates to the seventh century, may have been used to align with solstices.

The American Indians no doubt made observations of the sun and other celestial bodies thousands of years ago. Agriculture started in North America around 5000 BCE and there would have been a need to know about the sun’s movement and the seasons. A petroglyph or rock painting at Chaco Canyon shows a circle with streamers coming out of it. These could be the sun’s corona during a total solar eclipse, possibly that of 11 July 1097. A nearby site, known as the “Sun Dagger”, allows the sun to enter a cave through an alignment of three rocks. The light, shaped like a dagger, was used to mark solstices and equinoxes.

Ancient solar astronomy began with the need to track the seasons for agricultural purposes. Many early civilizations built elaborate structures to determine solstices and equinoxes. Advances in solar astronomy resulted in the development of calendars, important for agricultural, civic and religious activities.

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