Although there is now ample evidence that civilizations in earlier centuries and in other parts of the world had already gathered significant pieces of astronomical and mathematical knowledge, we can trace the beginnings of science as we know it to the imaginative minds of the great Greek thinkers. In order to have a definite starting place, let us try to imagine ourselves in the position of the leading scientific community of the ancient world around 400 B.C., in Athens.

Although the use of optical instruments of precision was still 2000 years away, simple observations of the night sky had by about 400 B.C. established enough data and interpretations concerning the motions of the heavens to nourish several world theories. The "fixed stars" and the Milky Way seem to move throughout the night as if they were rigidly attached to an invisible bowl which rotates around a fixed center in the sky (now called the North Celestial Pole). From observations made from different points on the earth’s surface, it could be inferred that this bowl is really more like a very large sphere surrounding the earth, and that the earth itself is also a sphere. It was possible by the third century B.C. to obtain a fairly accurate estimate of the size of the earth; but it was not until the nineteenth century A.D. that scientists could determine even approximately the distance of some of the stars from the earth.

Although we know that the earth is rotating on its axis and revolving about the Sun, astronomers sometimes talk as though the earth were fixed and that the celestial objects move about us. This concept has its roots in antiquity and, although it is not "true", it is sometimes a good way to look at the situation. Among what ancient observers called the "fixed stars" seven objects move: the Sun, the moon, and five apparently star-like objects, the planets. Other objects that moved in the sky but were transitory, such as meteors and comets, were considered by the ancient observers, Aristotle in particular, to be atmospheric phenomena and were not considered in modeling the heavens.

The Greeks were quite familiar with the fact that the hypothetical "celestial sphere" containing the stars appears to rotate uniformly from east to west, returning to its starting point every 24 hours. This is the diurnal (daily) rotation of the earth. Of course we now know that it is the earth rather than the stars that rotates on its axis every 24 hours, and that the appearance of stars arranged on a big sphere is an illusion. But that is not something you should simply accept "on authority." Instead you should consider carefully which of the following observations can be explained equally well by assuming either that the stars move or that the earth moves.

The Pole Star, or Polaris, is very close to the North Celestial Pole. However, in 400 B.C., Polaris was several degrees away from the North Celestial Pole; there was not an obvious Pole Star at that time. The Greeks knew that the North Celestial Pole does move very slowly with respect to the stars, producing the phenomenon of precession of the equinoxes; but the available observations did not extend over a long enough period of time for them to understand the precise nature of this motion. It is now known that the North Celestial Pole itself moves in a small circle, returning to its original position after about 26,000 years.

It was also well known to the Greeks that while the Sun shares the diurnal motion of the stars it does not quite keep up with them. By observing the stars just before sunrise and just after sunset, one can discover that the Sun slowly changes its position relative to the stars every day. In fact it follows a path from west to east through the stars, going through the "signs of the Zodiac," and returning to its starting point after 365 days.

More precisely, it does not simply go eastward, but it also has a north-south motion: On about March 21 (vernal equinox) the Sun is directly overhead at noon for places on the Earth’s equator, and it then moves northward every day until about June 21 (summer solstice) when it is directly overhead at noon for places 23 ½^o north of the equator (Tropic of Cancer). The Sun then moves southward, so that about September 23 (autumnal equinox) it is directly overhead at noon on the equator again, and about December 21 (winter solstice) it is directly overhead at noon for places 23 ½^o south of the equator (Tropic of Capricorn). The Sun then moves northward again and the cycle repeats itself.

The north-south motion of the Sun is of course the major factor that determines temperatures on the surface of the earth. Between March 21 and September 23 the day will be more than 12 hours long in the northern hemisphere, and the Sun will rise relatively high in the sky (depending on latitude). Between September 23 and March 21 the day will be less than 12 hours long in the northern hemisphere, and the Sun will not rise very high. On March 21 and September 23, both day and night will be 12 hours long everywhere, hence the term equinox (Latin for equal night).

The correlation between the seasons and the Sun’s motion through the stars was a scientific finding of vital importance to the ancient agricultural civilizations. By setting up a calendar of 365 days, the ancient astronomers could predict the coming of spring and thus tell the farmer when to plant his crops. Eventually it was found that such a calendar would become more and more inaccurate unless occasional extra days were added.

Finally, the ancients noticed that certain special stars did not stay in fixed positions on the celestial sphere, but wandered around in a complicated but regular manner. These stars became known as planets (from a Greek word meaning wanderer), and the study of their motions was one the chief occupations of astronomers up to the seventeenth century A.D.

If you observe the sunrise and sunset times for an entire year, you will find that after the winter solstice, about December 22, the Sun rises at about the same time for several days; and indeed instead of starting to get earlier, sunrise gets a little later after the solstice. It is not until about January 6 that the sunrise starts to get earlier again, nearly two weeks after the solstice. Meanwhile, sunset has started to get later some days before the solstice, having reached its earliest in the afternoon on about December 13, almost two weeks before. The net result is that the Sun is above the horizon for the shortest total time about half way between December 13 and January 6.

With this behavior, how can we expect the Sun to be half way between rising and setting exactly 24 hours after the previous day? The answer is that we can’t. Through the year, the Sun has sometimes already passed due south by noon, and sometimes it is not there yet, because we measure time by mechanical methods, using devices designed to indicate precise intervals of the time from one day to the next. The reason for this discrepancy between the mean solar day, as measured by Greenwich Mean Time, and the actual behavior of the Sun is because the Earth moves around the Sun in an elliptical, not a circular orbit, so that its distance from the Sun varies through the year.

The fixed point about which the sphere of stars turns is called the celestial pole. The direction on the ground toward this fixed point is called north if we are in the northern hemisphere of the Earth. In the northern sky a bright star, Polaris, is near the north celestial pole. In the southern hemisphere, there is no bright star near the south celestial pole.