Astronomy Principles and Practice Fourth Edition.pdf

A year 7
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 1 2
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