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Retrograde motion
The apparent motions of the
outer or superior planets—that is, those orbiting the Sun at greater
distances than the Earth – are in some ways similar to the motions of the
inner plants, despite certain basic differences. In these cases, Earth is
the inferior planet moving more quickly around the Sun, and it is up to
Earth to do the chasing. There is a stage where all outer planets pass, at
one time or another, through superior conjunction. Beyond this point, the
Earth begins to gains on the outer plant, which will appear to move out from
behind the Sun in a westerly direction sky. To complicate matters, the
motion of the outer planet against the background stars appears eastward.
(The impression of the westerly motion is only created by the Sun moving
eastward at quicker rate than the outer planet.)
(the picture is taken from Encarta) |
As
the Earth then moves to a part of its orbit where it begins to swing in to
pass the planet on the inside, the planet, as the Earth approaches, will for
a short time appear stationary in relation to the background stars. Just as
the Earth is passing the planet, the plant will seem to move a westward
direction against the stars. This is referred to as a ‘retrograde motion’.
At this time the planet will be seen in a part of the sky which is opposite
to the Sun, thus ‘standing at opposition’. The plant will then appear on the
meridian at midnight, or due south to an observer in the northern
hemisphere. This is the most favorable position for observation.
After opposition, the situation is virtually reversed. The planet moves
towards the Sun, which now appears to the west, and its motion once more
becomes eastward, or direct, against the background stars as Earth leaves it
behind. Finally, the planet is lost in the evening twilight prior to
superior conjunction and the whole process can begin again.
The length of time this process takes is only governed by the Earth’s
orbital or sidereal period, but also by the orbital period of the planet
concerned. In the case of Mars, for instance, it is quite a lengthy process.
The sidereal period of Mars is 686.9 days; that of the Earth, 365.2 days, or
a little more than half that of Mars. As a result, one year after a
particular opposition of Mars, the Earth will have returned to the part of
its orbit held at the time of this opposition, while Mars will have
completed about half its orbit and will appear on the opposite side superior
conjunction. A further 14 months are required to bring Mars back to
opposition for the second time. The period from one opposition to the next
is called the ‘synodic period’ for and mars the mean, or average synodic
period is 779.9 days.
For the more remote planets this synodic period becomes shorter, the most
distant planet, in fact, having the shortest. Jupiter, for instance, has a
sidereal period of slightly more than 11 years 10.5 months and will thus
complete about one-twelfth of its orbit for each complete orbit of the
Earth. Oppositions of Jupiter occur every one year and 34 days, or slightly
over 13 months. (This is interesting because since there are 13 zodiacal
constellations, and Jupiter takes around 12 years to complete 1 circuit of
the ecliptic, it seems to pass from one constellation to the next with each
successive opposition.) Saturn, taking 29 years 167 days to complete orbit,
has synodic period of one year 13 days. Uranus, with an orbit taking 84
years and one week, is brought into opposition only 4 days later each year.
Neptune’s sidereal period is about 165 years. Thus, its synodic period is
only 1 year and 2 days. Finally, Pluto’s 247-year sidereal period results in
a synodic period of one day longer than a year.
For the inner planets the synodic period is the time between each
successive inferior conjunction. Mercury with the shortest sidereal period
of all, at only 88 days, passes through inferior conjunction several times a
year and has a synodic period 115.9 days. Venus has a sidereal period of
224.7 days and a synodic period of 583.9 days, or about 20 months, which is
the second longest after mars. It is worth noting that the synodic periods
for the 3 planets Mercury, Venus and Mars are longer than their sidereal
periods. For those planets outside the orbit of mars the opposite will be
the case, however.
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The Zodiac
As mentioned earlier, the orbits of the major planets lie roughly in the
same plane. Only Mercury and Pluto Have orbits that are highly inclined
to the rest, but even so their inclination is not excessive. Thus, all
the planets seem to keep to a region of sky forming an imaginary band
that encircles the celestial sphere. This band is about 17 degrees in
width and is called the ‘Zodiac’. Running along the center of this band
is a line, again imaginary, which is called the ‘ecliptic’.
To understand
this better, consider the whole sky as an enormous sphere encompassing
the Earth. The axis of the Earth, which is a line passing through the
poles, if projected on to this sphere indicates the celestial poles. The
north celestial pole is easy to find because of the fairly bright nearby
star Polaris, or North Star. The equator of the Earth, which may be
considered as a plane through the center of the Earth, perpendicular to
the axis, becomes the celestial equator when projected on the sphere.
The ecliptic is an imaginary line on the celestial sphere traced by the
path of the Sun. since we are orbiting the Sun, the Sun will appear to
follow this line, completing one circuit each year. The Zodiac itself is
a path on the celestial sphere, which all the planets follow, with the
single exception of Pluto, whose orbit is sufficiently inclined to the
plane of the elliptic to allow it to move occasionally into regions of
the sky bordering the Zodiac.
Because the
Earth’s axis is tilted to the plane of the elliptic by 23 degrees and 27
minutes the apparent paths of the Sun and planets on the celestial
sphere take a rather unusual course, relative to the celestial equator.
Perhaps the best way to describe the situation is to consider an
observer standing on the Earth’s equator. To him the celestial equator
would appear as an imaginary line from the eastern horizon to the
western horizon and passing directly overhead. The ecliptic, however is
inclined to the celestial equator by 23 degrees and 27 minutes, an
amount corresponding to the Earth’s axial tilt. At certain points,
therefore, the ecliptic will appear to the south of the celestial
equator and at the other points to the north, its maximum separation
being, of course, 23 degrees and 27 minutes.
In two positions, 180 degrees apart, the ecliptic intersects the
celestial equator. These are called the ‘vernal’, or ‘spring’, ‘equinox’
and the ‘autumn equinox’. The first of these points is at the present
found in the constellation of Pisces, the second in Virgo. The point
where the ecliptic has its maximum displacement north is called the
‘summer solstice’; that where it has its maximum displacement south is
the ‘winter solstice’. These are situated in the constellation of Gemini
and Sagittarius respectively. Altogether there are thirteen
constellations strung along the ecliptic, not twelve as is often
believed. They are Gemini, Cancer, Leo, Virgo, Libra, Scorpius,
Ophiuchus, Sagittarius, Capricornus, Aquarius, Pisces and Taurus. Their
signs are, with the exception of Ophiuchus, the well-known signs of the
Zodiac. |
the brightest planet
By far the brightest of all planets is Venus.
There can be a few people who have not remarked on the appearance of this
brilliant star in the morning or the evening sky, a star which is seen long
before any other in the evening and long after all the other stars have been
lost in the brightening dawn sky. Even at its dimmest the magnitude of this
planet is –3.4, but at greatest brilliance it can attain a magnitude of as
much as –4.5. Venus is indeed the brightest object in the sky, apart from
the Sun, the Moon and the occasional comet.
Venus is so bright
that when conditions are right it can cast fairly distinct shadows. During
late 1978 the author recalls seeing shadows cast without the slightest
difficulty. Many UFO sightings can doubtless be explained with Venus. Casual
and unsuspecting observers have frequently been amazed at its aspect, and
most of the flying saucer sightings of late 1978 were very certainly due to
the appearance of Venus. It is possible for keen sighted people, provided
they know exactly where to look, to see the planets as a tiny star-like
point even in broad daylight.
Of the
remaining fainter planets, only Uranus is bright enough to be seen with the
naked eye. It usually shines at around magnitude +5.5 and the slightest
opital aid renders it an easy object. Neptune requires some sort of optical
aid to be seen. Its magnitude of +7.7 puts it well below naked-eye
visibility. Binoculars will show it as faint star, but to see it well a
small telescope at least is necessary. Lastly, Pluto is so faint, at
magnitude +14, that a moderate-sized telescope is needed to show it at all,
and even than it is not an easy object. A telescope with a lens of 8 inches,
or 200 mm aperture should just about pick it up, but a 12-inch or 300 mm
aperture telescope would, at the least, be needed to show it well. Pluto is
thousands of times fainter than the faintest naked-eye stars.
reflecting power
There are many thingsthat affect the magnitude of a given planet, it actual
reflacting power, or albedo, for one thing. This, of course depends on the
material of the visible surface. Venus has a dense cloud cover wich reflects
light very efficiently. Mercury is a poor reflector sinse its surface
material is very dark. There is also the consideration of the percentage of
the sunlit hemisphere directed towards the Earth. With the inner planets,
this percentage varies from 0%
to 100%, which raises a
rather important point.
The planets are not
self-luminous bodies. They shine only by reflecting the light they receive
from the Sun. Consequently, only half of a planet is illuminated and
reflecting light. This is obviously the hemisphere directed towards the Sun,
so the planet has a day, and night, side. Because of its inner planets
exhibit ‘phases’, like those of the Moon. When either planet is on the far
side of the Sun, the whole of the sunlit hemisphere is directed towards us
and the planet will appear full. As the planet moves out to greatest
elongation the phase will change from full to half-intermediate phases
between full and half being referred to as ‘gibbous’. At greatest elongation
the angle that the planet forms between the Earth and Sun is 90°
and we see the half sunlit hemisphere and half the unilluminated
hemisphere. Then, as the planet moves towards inferior conjunction the phase
will decrease, passing through the changing crescent phases, until at
inferior conjunction the whole of the night side is directed towards us. The
planet is then ‘new’ and, in theory, invisible to us.
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