Astronomy Principles and Practice Fourth Edition.pdf

A month 5
seen to rise above the eastern horizon, climb up the sky in a circle inclined at some angle to the plane
defined by the horizon and culminate, i.e. reach a maximum altitude above the line joining the north
to the south points, then descend in a mirror image of its forenoon path to set on the western horizon.
If the Moon is seen during that day, it will appear to imitate the Sun’s behaviour in rising and setting.
1.3.2 Night
As darkness falls, the first stars become visible above the eastern horizon. With the ending of twilight
the fainter stars can be seen and, as the hours pass, the stellar groups rise from the eastern horizon,
reach their maximum altitude like the Sun, then set or become dim and invisible as daylight returns.
The impression of being on a flat plane surmounted by a dark revolving bowl to which the stars are
attached is strong, especially when it is seen that there are many stars in a particular region of the sky
that revolve, never rising, never setting, about a hub or pivot. These stars are said to be circumpolar.
It is then clear that those other stars that rise and set do so simply because their circular paths about
this pole are so big that they intersect the horizon.
The Moon also revolves across this upturned bowl. Although the Moon appears to have an angular
motion across the sky similar to that of the stars, careful observation over a few hours reveals that it
moves slightly eastwards relative to the star background.
Occasionally a bright object, called a meteor, shoots across the sky in a second, looking like a
fast-moving or ‘falling star’. It may be too that faintly luminous sheets are seen, hanging down the
bowl of the heavens like great curtains. These are the aurorae W1.1 .
If our observer is watching at any time after October 4, 1957, it is quite likely that one or more
faint specks of light will be seen to cross the sky, taking a few minutes to do so, their presence giving
reminder that man-made satellites are now in orbit about the Earth. Indeed, one of the latest satellites—
the International Space Station W1.2 —is exceedingly bright—as bright as the brightest planet Venus—
and bears testament to the continual development of manned orbiting laboratories.
1.4 A month
The month is the next period of any significance to our watcher. During this time, the ideas about the
heavens and their movements change. It will be noted that after a few nights the first group of stars
seen above the eastern horizon just after sunset is markedly higher at first sight, with other groups
under it becoming the first stars to appear. Indeed, after a month, the first group is about thirty degrees
above the eastern horizon when the first stars are seen after sunset. It is then apparent that the Sun must
shift its position against the stellar background as time passes. The rate is slow (about one degree per
day—or about two apparent solar diameters) compared with its daily, or diurnal, movement about the
Earth.
The Sun is not the only object to move independently of the stellar patterns. A few nights’
observations of the Moon’s position against the stars (its sidereal position) show that it too moves but
at a much faster rate, about thirteen degrees per day, so that it is seen to make one complete revolution
of the stellar background in twenty-seven and one-third days, returning to the same constellation it
occupied at the beginning of the month. In addition, its shape changes. From a thin crescent, like a
reversed ‘C’, seen in the west just after sunset, it progresses to the phase we call first quarter about
seven days later. At this phase, the Moon’s terminator is seen to be almost a straight line. Fourteen
days after new moon, it is full and at its brightest, appearing at its highest in the sky about midnight.
Seven days later it has dwindled to third quarter and rises before the Sun, a pale thin crescent once
more, a mirror image of its phase just after new moon. Twenty-nine and one-half days after new moon,
it is new once more.
It was a fairly easy matter for the ancients to ascertain that the Moon was nearer the Earth than the
stars. Frequently the Moon was seen to blot out a star, occulting it until it reappeared at the other edge