1
Planetary Motion from an Earthly Perspective
Stars appear “fixed” from
night-to-night providing the
familiar background of the
constellations and
asterisms.


We see the same star
patterns that were visible
to ancient civilizations.
The stars ARE moving
but they are awfully far
away.

The changes in star
patterns becomes
noticeable over tens of
thousands of years
2
Planetary Motion
Within the Solar System our perspective on the planets changes
quickly.
Even from day-to-day we can see other planets move with respect
to the backdrop of distant stars.
3
Planetary Motion
Within the Solar System our perspective on the planets changes
quickly.
Even from day-to-day we can see other planets move with respect
to the backdrop of distant stars – the word planet comes from the
Greek for “wanderer”.
4
Characteristics of the Classical Planets
Five planets – Mercury,
Venus, Mars, Jupiter,
and Saturn, are visible
to the unaided eye.


All of them are bright
compared with the
majority of the stars.
Some – Venus, Mars,
and Jupiter – can
outshine the brightest
stars in the sky.

Venus is the
brightest of all.
5
Characteristics of the Classical Planets
The planets can always
be found near the
Ecliptic in the sky.

That means that they
will always be within
20 degrees or so of
the celestial equator.


Face South and
trace the celestial
equator from due
East to due West
and there's a good
chance you will see
a planet.
Face North and you
are unlikely to see a
planet.
6
Characteristics of the Classical Planets
The planets can always be found near the Ecliptic in the sky.

Planets will often pass and even “occult” one another.

The Moon follows a simliar path.
7
Characteristics of the Classical Planets
The planets can always be found near the Ecliptic in the sky.

Planets will often pass and even “occult” one another.

The Moon follows a similar path.
8
Characteristics of the Classical Planets
The brightness of a planet will vary over time.
Mars, for example, sometimes outshines the brightest stars, at
other times it becomes so faint that it looks like a typical star on
the sky.
9
Retrograde Motion
Planets tend to drift eastward against the background of stars, but
occasionally they will stop, back up, and continue on eastward
again.

This reversal is called “retrograde motion”.
10
Inferior vs. Superior Planets
The Earth is the third planet out from the Sun in the Solar System.

Mercury and Venus follow orbits inside the Earth's

Mars, Jupiter, and Saturn orbit outside the Earth's orbit.
The behavior of Mercury and Venus is strikingly different from
Mars, Jupiter, and Saturn as a result.
11
Inferior Planets – Mercury and Venus
Since their orbits lie entirely within the Earth's orbit we always see
them close to the Sun in the sky.

They can only be “evening” or “morning” objects.
12
Inferior Planets – Mercury and Venus
Since their orbits lie entirely within
the Earth's orbit we always see
them close to the Sun in the sky.



They can only be “evening” or
“morning” objects.
Inferior planets will never be
seen in the midnight sky.
Mercury is notoriously difficult to
see since it is never very far from
the Sun at all.
13
14
Superior Planets – Mars, Jupiter, Saturn...
These planets have orbits outside
the Earth's orbit.


They can never be between the
Earth and Sun.
Unlike an inferior planet they can
be opposite the Sun in the sky.
15
Planetary Configurations
As the planets orbit the Sun, occasionally they reach interesting
geometrical alignments with the Earth and Sun.
The simplest of these are

Opposition – the planet and Sun are in opposite directions in the
sky.




Opposition is a terrific time to see a planet. It rises when the Sun sets
and is high in the sky at midnight.
Opposition is also when a planet is closest to the Earth. It's brighter
and more interesting to see in a telescope.
Only superior planets can be at opposition.
Conjunction – the planet is in line with the Sun as seen from Earth


Conjunction is the worst time to see a planet because it is lost in the
Sun's glare.
Inferior planets have both

Superior conjunction (on the far side of the Sun from Earth)
16
17
Planetary Configurations
As the planets orbit the Sun, occasionally they reach interesting
geometrical alignments with the Earth and Sun.
Greatest elongation is the time when an inferior planet is seen at
its greatest angular separation from the Sun.

Greatest elongation is the ideal time to view Mercury or Venus.