Early Astronomy

Early Observations: Click image For tens of thousands of years, human beings have been fascinated by the patterns of stars in the sky above Earth. Early on, they noticed that the Moon changed shape from night to night as well as its position among the stars. Early people noticed constellations of stars in the sky that looked like animals and people, and made up stories about what they thought they saw. In fact, the oldest records we have of astronomical observations are 30,000-year-old paintings found on the walls of caves. Ancient Egyptians were very interested in the night sky. In particular, they were drawn to two bright stars that always could be seen circling the North Pole. The Egyptians referred to those stars as "the indestructibles." Today we know them as Kochab, in the bowl of the Little Dipper (Ursa Minor), and Mizar, in the middle of the handle of the Big Dipper (Ursa Major). Early Greek astronomers learned from the Babylonians. The Greek philosopher Pythagoras about 550 B.C. noticed that the so-called evening star and morning star were the same body, the planet Venus. In the 4th century B.C., Aristotle of Stagira knew the Earth was round because of eclipses observed when Earth passed between the Moon and the Sun. In the 3rd century B.C., Eratosthenes was a Greek astronomer working in Egypt when he noticed the Sun directly over one city cast a shadow in another city 500 miles north. Eratosthenes understood correctly that meant Earth's surface is curved. He calculated correctly that Earth is a ball about 25,000 miles around.

Early Interpretation and Belief: Egyptians aligned their pyramids and temples toward the north because they believed their pharaohs became stars in the northern sky after they died. To assure that a king would join the circumpolar stars, the pyramids were laid out facing due north toward the "indestructible" stars. They thought that aligning the pyramids toward north gave the deceased pharaohs direct access to the northern sky. Each of the two stars was about 10 degrees from the celestial pole which lay directly between them. When one star was exactly above the other in the sky, astronomers could find a line that pointed due north. That alignment was only true for a few years around 2,500 B.C. An Egyptian astronomer might have held up a plumb line and waited for the night sky to slowly pivot around the unmarked pole as the Earth rotated. When the plumb line exactly intersected both stars -- one about 10 degrees above the invisible pole and the other 10 degrees below it -- the sight line to the horizon would aim directly north. However, Earth's axis is unstable. Our planet wobbles like a gyroscope over a period of 26,000 years. Modern astronomers now know that the celestial north pole was exactly aligned between Kochab and Mizar only in the year 2467 B.C. Before or after that date, the Egyptian astronomers would have been less accurate as they tried to mark true north. The Great Pyramid at Giza is known today as one of the Seven Wonders of the Ancient World. Nearly 4,500 years ago, in the year 2467 B.C., the "indestructible" stars lay precisely along a straight line that included the celestial pole. Research suggests that the Great Pyramid at Giza was constructed within 10 years of 2,480 B.C. The ancient Babylonians viewed the Universe as a flat disk of land surrounded by water. They were the first people to keep detailed records of the paths of planets. Like most ancient people, Babylonians believed that studying planetary movements could help them predict the future. In fact, according to a biblical story, the people of a Babylonian city tried to build a stairway to the stars. That was the Tower of Babel. The first to suggest the Heliocentric theory of gravitation was the Greek philosopher Aristarchus of Samos in the 4th century B.C. who put forth the then-radical view that Earth and other planets revolve around the Sun. Greeks thought Earth was flat. One described the world as a disk floating inside a hollow ball. Greek astronomers later came to think correctly of Earth as shaped like a ball. Others suggested correctly that moonlight was reflected sunlight. The idea of Earth as a sphere was abroad as early as the 6th century B.C. with Pythagoras of Samos.

Early Application: The first astronomers created calendars from changes they saw in the Moon. Some ancient people around 5,000 years ago set up large stones to mark the movement of the Sun and other stars. One of those old observatories is Stonehenge in what we now call England. Ancient natives of North American lined up circles of stones with the Sun and stars to chart the rising Sun and the begining of summers. In southern Mexico, the Mayans built special buildings to watch the Moon and the planet Venus. They had a calendar by 800 A.D. that was more accurate than the calendar used in Europe. Ancient Egyptians depended on the Nile River to flood their fields and make it possible to grow crops. They became the first to use a calendar with a 365-day year after their priests discovered that flooding returned about every 365 days. There are stars and constellations that always appear in Earth's northern sky. For instance, the Big Dipper is one of those constellations. Ancient sailors used stars and constellations to guide their travels. Polynesians, for example, sailed among the Pacific Ocean islands by watching stars. To explain why planets seemed to change direction, Ptolemy used old calculations by Hipparchus to understand planetary motion. Hipparchus worked about 130 B.C. and Ptolemy about A.D. 150, which suggestes it took 280 years to come up with the complicated scheme used to predict future positions of planets. For instance, al-Battani working about 900 A.D. devised new ways of calculating planetary positions. If it hadn't been for Arabs, Greek science would have been lost. Astrolabes. "Star-finders" or astrolabes were created by Arab astronomers to solve complicated astronomy problems. One side of an astrolabe contained a detailed star map.

Miscellaneous: Polaris is today's north star. It was not in the same position 4,500 years ago as it is today, and would not have helped the pyramid builders. In July 1054, a star could be seen blazing in the heavens. For three weeks it was so bright it could be seen in daylight. The only reason we know of it today is because Arab, Chinese, Japanese and Native American astronomers noted it. That supernova created what we now call the Crab Nebula. Chinese astronomers wrote about a "guest star" in the constellation Taurus that became four times brighter than Venus and was visible in daylight for 23 days. The yellow colored "guest star" was visible to the naked eye at night for 653 days. Anasazi Indian artists living in Arizona and New Mexico and the Mimbres Indians of New Mexico are thought to have recorded the supernova in their pottery. The Japanese poet Sadiae Fujiwara wrote about the star Zeta Tauri. There are no records of European or Arab observations of the 1054 supernova that have survived to modern times.

Adapted from: Ancient Astronomy Calendars, Navigation, Predictions