The Story of Astronomy (2 page)

The Sun and the Moon are the most prominent objects in the sky; it is pure coincidence that they appear almost the same size to us. But there is also a myriad of tiny dots of light in the night sky, and these, too, must be studied to discover the secrets of the nature and origins of the universe. The deeper we penetrate into the night sky and the more we study the objects we find, the deeper becomes the mystery of creation—a mystery that we will probably never solve to our complete satisfaction.

In the ancient world, as the first stable civilizations developed, there were always some who were fascinated by the night sky. The shepherd in his lonely vigil had plenty of time to study the sky, to learn the positions of the stars, to observe the phases of the Moon and to notice that the stars appeared in the same patterns every night. While the stars maintained these patterns, the Sun and the Moon had their own independent motion across them. The whole night sky appeared to revolve throughout the
night about a point in the north; it was as though the sky were painted on the inside of a huge globe rotating about the Earth. There were also seasonal differences in the sky. The night sky in winter did not have the same appearance as the night sky in summer. Sometimes new constellations (patterns of stars) appeared near the horizon and old constellations disappeared. There was an annual cycle, and the missing constellations always returned as the seasons changed. There was also a small number of star-like objects that wandered across the night skies; they sometimes exhibited a backward (or retrograde) motion before progressing forward once more.

Every civilization wondered about the heavens. Priesthoods developed in which men were trained to make a special study of the stars. The Sun was very obviously the most influential object in the sky. It was so strong, bright and powerful that as soon as it rose in the morning all the stars faded from view. The purpose of the Sun was obvious. It gave light in the daytime and it helped the crops to grow. The purpose of the Moon was not so obvious; although it often gave a pale light by night, there were many nights when it only made a very late appearance and there were nights when it gave out no light at all. It exhibited phases, from the thin crescent of the new Moon to the bright circle of the full Moon, with every other possible phase in between. These phases were
easily predicted, and it was not difficult to deduce that they depended on the relative position of the Sun. It did not take astronomers very long to realize that the Moon was actually a sphere, and the phases came about because it was illuminated by light from the Sun. It came as a revelation when it was found that the phases of the Moon were also related to the movement of the tides. The Moon obviously held a great sway over the seas, but it was hard to explain in physical terms how a sphere traveling across the sky in a monthly cycle could affect the tides. It was logical to deduce that every object in the skies had some small influence on the Earth. The learned people of the day worked hard to discover the nature of these influences.

As we have noted, there were also other objects in the heavens that were visible to early astronomers apart from the Sun, the Moon and the stars. There were wandering “stars” that seemed to have their own motion. It was thought that these bodies, which became known as the planets, must also have some strange influence over the Earth. One of these planets was Venus, the “morning star” that sometimes heralded the dawn, but which at other times appeared in the evening when it was known as the “evening star.” It was the brightest planet in the sky. There
was the red planet that became known in the Roman world as Mars. There were the slower-moving planets that the Romans called Jupiter and Saturn. There was a small planet they called Mercury that was difficult to see, for it stayed close to the Sun. It was assumed that the planets had all been put in the sky for a reason, and that every star had a message to tell. These reasons and messages were not easy to explain, however.

In Egypt, it was discovered that the rising above the horizon of Sirius, the Dog Star, heralded the flooding of the River Nile. It was a great triumph for the astronomer priests to be able to predict when the Nile was about to flood, for it enabled the farmers to prepare themselves for the event—fertile soil brought down by the river enriched the farmland and improved the harvest. The astronomer priests also hoped to discover the purpose of other stars in the sky. The occasional meteor flashed across the heavens. It never returned, but it was not difficult to make a link with the phenomenon and some earthly event—an important death or a birth, for example. Less frequently a comet appeared; this was so rare that it was frequently deemed to be a bad omen in many cultures. Surely, reasoned the astronomer priests, all these portents in the skies were signs from the gods trying to tell them something.

The development of astronomy and astrology were not confined to Egypt and the Mediterranean world. The Chinese, the Aztecs and the Incas all studied the stars and tried to predict eclipses and other events in the heavens. Some of the stars appeared to form distinct constellations, and the vivid imagination of the ancient astronomers saw all kinds of patterns formed by them—mainly creatures and ancient gods. There were dogs and bears, there were archers, and there were dragons, serpents and scorpions. Twelve of these constellations formed a wide path through which the Sun, Moon and planets appeared to move during the year. These were the constellations of the zodiac, and they held special significance. The stars were not visible in the daytime, but it was easy to calculate which sign of the zodiac “contained” the Sun at any given time. It was also possible to calculate in which part of the zodiac the Moon and each of the planets lay. Thus came the beginnings of astrology and the belief that the positions of the heavenly bodies in the sky affected our life on Earth. Chinese records relating to the study of the stars go back to the third millennium
BC
, and many of the dates in their chronology can be identified from their records of eclipses. Thus the Shang Dynasty, dating from about 1760
BC
to 1122
BC
, is well recorded astronomically and it contains much data from these early centuries.

It was only a question of time before the first theories were put forward about the origins of the world. The earliest ancient civilizations thought the Earth to be flat. They thought it was natural for everything to be drawn to the ground. There was no need to explain gravity; it was a familiar phenomenon. If voyagers traveled far enough in any direction they would find that the Earth had a limit, a place in the ocean where any ship would fall off the edge of the world or perhaps a place beyond the mountains where there was a sudden end to the land. To explain the night sky, a great sphere of clear crystal was envisaged that carried all the stars and the planets on their diurnal journey. The Sun and the Moon had their own spheres and their own independent motions. The extent of the Earth was not known, but it was obvious to all that it was the center of the universe; it seemed larger than the Moon or the Sun and far larger than any other celestial object.

All the early civilizations recognized the need for a calendar. The Aztecs had two calendars, one with a solar year of 365 days and one with a sacred year of 260 days. The latter consisted of 13 sequences of 20 days, each of the sequences being ascribed to one of 20 deities.
Arithmetic shows that, after 52 years of the 365-day calendar, the 260-day calendar had revolved through 73 cycles and the two calendars had come into phase with each other again. The Aztecs studied the Moon's motion, and they were able to predict the occurrence of an eclipse. The Mayans had the same calendar as the Aztecs with the same solar and secular year. Both civilizations built temples and pyramids to the gods of the Sun and the Moon. The Incas of Peru had a similar religion to the Mayans. The importance of the Sun and Moon was recognized. Temples to the Sun were decorated with gold, and temples to the Moon were decorated with lavish silver furnishings.

Every ancient civilization built temples to their gods. In most cases it was the duty of the priests to look after the calendar, and they earned their keep by predicting the seasons and telling farmers when to plant seed and reap the harvest. The priests needed to observe the objects in the heavens and to record unusual happenings in the sky. They all recognized the importance of the Sun and the Moon. They knew that the stars appeared in the same positions and in the same constellations every night. Every early civilization had also noticed the “wandering stars.” The Sun, the Moon and the wandering planets roamed across the zodiac from constellation to constellation. They had obvious functions, but what was the meaning of the
multitude of other objects in the night sky? What was the meaning of the signs that the heavens were trying to convey to the Earth?

Ancient civilizations began to create calendars as a way of reminding them about important annual events, such as the times to plant and then to store crops. As cultures developed, the calendars became more sophisticated. Later, observations from eclipses led philosophers and early astronomers to develop ways of measuring the distance from the Earth to the Sun, and even the size of the Earth itself. However, for many millennia the belief that the Earth was at the center of the universe held back the advancement of our understanding of the true nature of the solar system.

In the fertile plain between the great rivers of the Tigris and the Euphrates there developed the ancient civilizations of Sumeria and Babylon. The Sumerian civilization, which was even older than the first Egyptian dynasty, dates back to about 8000
BC
. The Sumerians
developed a form of writing executed with a stylus on a clay tablet. The writing became known as “cuneiform” text, with the symbols written with the stylus representing either nouns or verbs.

In addition to this early form of writing, the Sumerians were also the first civilization to develop a working calendar. Like all who followed them they wanted to bring together the cycles of both the Sun and the Moon into their calendar. The Moon took 30 days, measured to the nearest day, to go through all its phases, and thus the Sumerian year consisted of 12 months of 30 days, giving a total of 360 days. The fourth month was written in cuneiform as the character for “seed”; the 11th month, the harvest month, was written as the character for “grain”; and it was followed by the character for “house” or “barn.” The Sumerian calendar thus predicted and noted the season to sow the seed, the season to harvest the crops, and when to store them in the barn for the winter.

For a while the calendar served them well, but after a few years they found that the harvest did not ripen by the 11th month because their year was too short by just over five days. So the Sumerians simply added an extra month to their calendar every six or seven years. They knew that their year was too short, but a year of 360
days had one great advantage. It was a number that divided exactly by 60, and because of this the complete circle of the heavens was seen as divided into 360 equal parts that we now call degrees. The base of 60 was used for both angular measure and time intervals—thus 60 seconds became one minute and 60 minutes made an hour. Eventually the angular measure of the degree was also divided into the 60 parts that we know as minutes, and the minutes themselves were divided into 60 seconds of arc. The confusion between minutes and seconds of arc, and minutes and seconds of time, still remains, however.

In the dynasty of the emperor Hammurabi (1792–1750
BC
) the capital of the Sumerian kingdom moved to Babylon and the area became known as Babylonia. Much of the ancient calendar was retained, and every lunar month began on the first appearance of the crescent Moon. The astronomers went on to divide the stars into a zodiac of six houses of unequal portions.

The Babylonians were not only great astronomers, they were also very able mathematicians. We see many examples of geometry and trigonometry appearing in their work, as well as advances in astronomy. Their calculation of pi, the ratio of the circumference of a circle to its diameter, was given as three and one eighth; it was in error
by a few percent, but it shows that they knew the importance of this ratio. One of the surviving cuneiform scripts shows that the Babylonians knew how to solve the quadratic equation. They were the first civilization to introduce a seven-day week and they named the seven days after the Sun and the Moon followed by the five known planets, to give the sequence Sun, Moon, Mercury, Venus, Mars, Jupiter and Saturn. (This naming is still very evident in languages such as French and Spanish, but in English the Norse gods have supplanted all the original names except for Sunday, Monday and Saturday.)

The Earth was often imagined by many ancient peoples as a hollow hemispherical shell floating on the world-waters—the idea of the Earth as boat-shaped is one of the oldest images of the Earth. In the Babylonian conception of the universe the Earth occupied the central place and was the accepted center of their planetary system.

According to the Babylonian system the sky forms a hollow vault above which reside the gods. In the east (left) is a door out of which the Sun rises each day, and a similar door is in the west (right) through which it returns. In the sky there are fixed stars, meteors and moving planets. The Earth is round in the form of a large hollow mountain, which rests on water. In the east is the bright mountain of the rising Sun and in the west the dark mountain of the sunset. The sea extends over
the sky and in the southwest lies the island of the blessed. Under the ground is the region of the dead, consisting of seven concentric areas.

At about the time the Babylonians were devising their calendars, the ancient Egyptians also made valuable contributions to the calendar. For the Egyptians, the annual flooding of the Nile was their lifeblood, and to be able to predict the flooding was of great significance to them. Their calendar evolved with only three seasons, which they called the flooding, the subsistence of the river and the harvesting. Each of these three seasons was divided into four lunar months, making a total of 12 months in all. The Egyptians instigated a calendar based on their 12 lunar months. This calendar remained in use for approximately 3000 years, but like the Babylonians the Egyptians needed to insert extra months every six or seven years to maintain its accuracy. In later years they simply added five or six days at the end of the year. This was the system they used in 440
BC
when the Greek traveler and historian Herodotus (484–420
BC
) gave a description of the Egyptian calendar: