A year

A year's patient observing, by day and night, provides the watcher with new concepts. For example, the Sun's daily behaviour, moving easterly bit by bit, is linked to the seasonal changes.

Each day, for most observers, the Sun rises, increases altitude until it culminates on the meridian at apparent noon, then falls down the sky until it sets on the western horizon. We have seen that this progress can be studied by noting the changes in direction and length of the shadow cast by a vertical rod stuck in the ground (see figure 1.1).

As the days pass, the minimum daily length of shadow (at apparent noon) is seen to change, becoming longest during winter and shortest during summer. This behaviour is also linked with changes in the rising and setting directions of the Sun. Six months after the Sun has risen between north and east and setting between north and west, it is rising between south and east and setting between south and west. Another six months has to pass before the solar cycle is completed, with the Sun once more rising between north and east and setting between north and west.

All this could be explained by supposing that the Sun not only revolved with the stars on the celestial sphere about the Earth in one day (its diurnal movement) but that it also moved much more slowly along the path among the stars on the celestial sphere, making one revolution in one year, returning to its original position with respect to the stars in that period of time. We have already seen that the observer who notes over a month what group of stars is first visible above the eastern horizon after sunset will have already come to the conclusion that the Sun moves relative to the stars. Now it is seen that there is a regular secular progression right round the stellar background and that when the Sun has returned to its original stellar position, the seasonal cycle is also completed.

The Sun's stellar route was called the ecliptic by the ancients. The groups of stars intersected by this path were called the houses of the Zodiac. The ecliptic is found to be a great circle inclined at about 23 5 degrees to the equator, the great circle on the sky corresponding to the projection of the Earth's equator, intersecting it at two points, the vernal and autumnal equinoxes, 180 degrees apart.

It was quite natural, then, for the ancients to worship the Sun. Not only did it provide light and warmth by day against the evils of the night but, in addition, its yearly progression was intimately linked to the seasons and so also to seed time and harvest. It was, therefore, necessary to keep track of progress to use it as a clock and a calendar. To this end, the science of sundial-making began, ramifying from simple obelisks that throw shadows on a fan of lines radiating from their bases, to extremely ingenious and complicated erections in stone and metal. Up to the 19th century, these constructions rivalled most pocket-watches in accuracy as timekeepers.

For calendrical purposes, lines of standing stones could be set up, pointing to the midsummer, midwinter and equinoctial rising and setting points of the Sun. In the British Isles, there still remain hundreds of such solar observatories, witnesses to our forefathers' preoccupation with the Sun-god.

The observer who watches the night sky throughout a year counts about thirteen revolutions of the stellar background by the Moon in that time. Over that period of time, it is not apparent that any simple relationship exists between the sidereal period of revolution of the Moon, the period of its phases and the year (the time it takes the Sun to perform one complete circuit of the ecliptic). That knowledge comes after much more extended observation, certainly measured in decades.

It would be noticed, however, that the Moon's sidereal path is very little inclined to the ecliptic (about five degrees) and if records were kept of the points of the ecliptic crossed by the Moon, it might be realized that these points were slipping westwards at a rate of about twenty degrees per year (see figure 1.2).

More information, too, would be acquired about the star-like objects that do not twinkle and which have been found in the course of a month to have a slow movement with respect to the stellar background. These planets, like the Moon, would never be seen more than a few degrees from the plane of the ecliptic, yet month after month they would journey through constellation after constellation. In the case of one or two, their paths would include narrow loops, though only one loop would be observed for each of these planets in the course of the year.

The year's observations would not add much to the observer's knowledge of the stars, except to confirm that their positions and brightnesses relative to each other did not alter and that each star, unlike the Sun, had its own fixed rising and setting direction, unless it was circumpolar. It is possible, however, that in a year, the extra-careful watcher might have cause to wonder if the conclusions about stars were without exception for, by regular comparison of the brightness of one star with respect to that of neighbouring ones, it might be discovered that a few stars were variable in brightness. This was certainly known to the Arabian astronomers of the Middle Ages. The appearance of a nova might even be observed, i.e. a star appearing in a position where one had not been previously noted. This occurrence might well lead to doubt about the knowledge of the now familiar constellations—in any event it could bring about the decision to make a star map for future use if the phenomenon happened again. It is also possible that in the course of a year the observer might see a comet, a star-like object

Figure 1.2. The Moon's sidereal path crosses the ecliptic twice each month at an angle of about 5°. For successive lunations the crossing points move westward, covering about 20° over a year. The Constellation of Leo is shown to give an indication of the scale of the movement.

with a long luminous tail. The development of the tail and the movement of the comet head could be detected from night to night.

Our observer by now must have come to tentative conclusions concerning the heavenly phenomena studied and noted. The interpretations, and the use made of the world-picture, will be constrained by the culture of the time. A man of Neolithic times and a Greek of Athens' golden era would develop entirely different cosmologies from identical observations. And a hunter or farmer has different needs, astronomically speaking, from a sailor.

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