Astronomy Principles and Practice Fourth Edition.pdf

6 Naked eye observations
Figure 1.1. The change in length of a shadow according to the time of day and the time of year.
of the Moon’s disc. And occasionally the Moon was eclipsed, the Earth progressively blocking off the
sunlight until the satellite’s brightness had diminished to a dull, coppery hue. An even more alarming,
but rarer, occurrence took place at times during daylight: the Moon revealed its unseen presence near
the Sun by eclipsing the solar disc, turning day into night, causing birds to seek their nests and creating
superstitious fear in the mind of primitive man.
The observer who studies the night sky for a month or so also discovers something new about the
one or two star-like objects noted that do not twinkle. Careful marking of their positions with respect
to neighbouring stars shows that they too are moving against the stellar background. There does not
seem to be much system, however, about these movements. In the course of a month, one may move
in the direction the Moon travels in, while a second object, in another part of the sky, may move in the
opposite direction. Indeed, towards the end of this month’s observing sessions, either object may cease
to move, seem almost to change its mind and begin to retrace its steps on the celestial sphere. These
wanderers, or planets (‘planet’ is a Greek word meaning ‘wanderer’), are obviously of a different
nature from that of the fixed, twinkling stars.
1.5 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