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FACING THE OPPOSITION (Part Two)
By Dave Furry, SCAS Secretary
In order to understand the apparent motion of the planets it is important to first understand
several key concepts. In part one of this series of two articles I will discuss opposition,
conjunction, elongation, transit, and quadrature; in part two (next month) I will finish with
sidereal orbit, synodic orbit and retrograde motion. I have relied on several references for this
information – if you would like to have a list for further investigations on your own please don’t
hesitate to contact me at [email protected].
Not long after telescopes were first used by Galileo for astronomical purposes, observers
began to note that two of the planets (Venus and Mercury) were behaving quite differently from
the other planets. (Eventually this led in part to the proof that Earth and the other planets
orbited the Sun rather than the other way around – but that’s another story!) What was so
curious was that both Venus and Mercury exhibited phases just like the Moon as well as
significant variations in their angular size (see Figure 1).
The only way this could
happen is if the two
planets were located in
orbits between the Sun
and Earth; for this reason,
Venus and Mercury are
called inferior planets.
When an inferior planet is
directly between Earth and
the Sun it is in a position
call inferior conjunction
and its dark side is seen
from Earth (Figure 2). The
planet is then at its
maximum angular size but
we can’t see it from Earth
because it is lost in the
Sun’s glare. As the planet
continues in its orbit (a
counterclockwise direction
looking down on the
planet’s north pole) we are able to see a thin crescent and that its angular size has decreased
somewhat. The crescent continues to widen and the planet’s angular size continues to
decrease as the planet continues around the Sun.
During this time the planet rises before the Sun does, and appears as a “morning star” as seen
from Earth. The planet is at its greatest angular distance from the Sun as a morning star when
it reaches greatest western elongation and it appears from Earth in a “half” phase. As it
continues along its way the planet’s angular size and angular distance from the Sun decrease
and it shows a gibbous phase until it reaches superior conjunction at which it is in its “full”
phase; once again we can’t see it because it is now lost in the Sun’s glare.
Figure 2: Orbit of an inferior
planet relative to the Earth.
After its “full” phase the planet will appear as an “evening star” after the Sun has set. At
greatest eastern elongation the planet will show us its other half when it is at its maximum
eastern angular distance from the Sun.
If the orbital plane of the inferior planet was in the same plane as Earth’s, it could be seen
apparently crossing the surface of the Sun (called a transit) at every inferior conjunction.
However, these planes are not exactly aligned and such transits are rare occurrences.
The orbits of Mars, Jupiter, Saturn, Uranus and Neptune are outside of Earth’s orbit, and for
this reason are referred to as superior planets. As shown in Figure 3, a superior planet will be
closest to Earth at opposition, when Earth is located exactly between the planet and the Sun.
At opposition a superior planet will appear in its “full” phase, have its greatest angular size, and
will cross Earth’s meridian at midnight (the meridian is the imaginary line in the sky from due
north, directly overhead and due south). Obviously opposition is the best position for observing
a superior planet from Earth.
At the other extreme, when a superior planet is at conjunction (Figure 3) it is as far way from
Earth as possible and is lost in the Sun’s glare. At quadrature the superior planet appears
slightly gibbous, somewhat like the Moon does a couple of days after full.
You may have already noticed that a superior planet can never appear as a crescent nor in a
“half” or “new” phase. Also, a superior planet can never transit the Sun as seen from Earth.
Figure 3: Orbit of a superior
planet relative to the Earth.