Synchronous rotation of the Moon --- is the name for the fact that the Moon
rotates on its axis in the same time it takes to revolve around the Earth,
namely 1 sidereal month. Note that this is the only case of such a special
association between two periods that we have seen so far. Note also that it
isn't true for the Earth --- the Earth day is not equal to a month!
I hinted that the Earth is a bigger body than the Moon and so has had more
effect on the Moon than the Moon has had on the Earth. The Moon does
have an effect on the Earth --- the tides, both in the oceans and in the body
of the Earth. It is because of these tides that the Earth-Moon system is
slowly changing. The Earth is slowing down (the day is getting longer) and
the Moon is spiralling away from the Earth. Some calculations discover that
this will go on until the Earth is also in synchronous rotation. More later
when we study tides. Another observation that suggests the connection between
the Earth and the Moon is eclipses.
Eclipses --- occur because the Moon's orbit round the Earth is only about 5
degrees from the ecliptic and the Moon has nearly the same angular size as the
Sun in our sky --- about 0.5 degrees. (Note the obvious fact that this means
that the Moon must be closer to us than the Sun is, and therefore that it must
be larger than the Sun because they have the same angula
size. More on angular measurement, distance and size soon.) So at New Moon a
Solar eclipse is possible (Moon between Sun and Earth) and at Full Moon a
Lunar eclipse can happen (Earth shadow on the Moon). Full, Annular and
Partial solar eclipses occur depending on whether the Earth is in the Umbra
(complete shadow zone of the Moon) or the Penumbra (partial shadow zone).
During Lunar eclipse the Moon appears red from sunlight bent or diffracted
through the Earth's atmosphere. Note that the Moon must be new for a solar
eclipse to occur, but that such an eclipse doesn't happen every new moon
because the Moon's orbit is slightly tilted to the ecliptic and the Moon has
to be exactly on the ecliptic so that the Earth, Moon and Sun line up. The
Moon crosses the ecliptic twice a year, and so eclipses can happen in
these approximately twice-yearly "eclipse seasons". The fact that they don't
repeat exactly every six months is because the Moon's orbit isn't fixed in
space, it is wobbling ("precessing") (see below).
Saros Cycle --- is the period of precession of the Moon's
orbit (18yr 11.3d), the time between recurrences of patterns of eclipses.
Motions of the Planets --- Mercury, Venus, Mars, Jupiter and Saturn are all
visible in the night sky with the naked eye. All the planets (except Pluto)
stay near the ecliptic, implying that they are all on or near the one plane in
the sky, that of the Earth's orbit round the Sun. They travel round the Sun
at rates different from each other and from the Earth, and so `wander' over
the sky (`planets' are `wanderers'). Mercury and Venus are always seen close
the Sun, just after sunset or just before sunrise, but the other planets can
be in the sky at other times (i.e. far from the Sun). Mercury and Venus are
called Inferior planets, the others Superior. Mercury's maximum elongation
(angle from the Sun) is 28 degrees, Venus' is 47 degrees. We also defined the
other configurations of the planets (or indeed anything on the sky) with
relation to the Earth and Sun: opposition when something is the opposite
direction in the sky from the Sun, Quadrature when it is at right angles,
inferior conjunction when it is in front of the Sun, and superior conjunction
when it is behind. The inferior planets can never be in opposition or
quadrature; the superior planets can be in any configuration. Planets drift
eastward in the sky, in general ("prograde motion") but occasionally go into
"retrograde" (westerly) motion, executing loops in the sky.