Planetary Motions
• The sky seems to revolve around us because
of Earth’s rotation
• Additionally, planets move with respect to
the fixed stars, that’s why they are called
planets (greek: wanderers)
• Due to the planet’s movement in their orbit,
and Earth’s orbital motion, this additional
motion – the apparent motion of the planet
as seen from Earth - looks complicated.
Apparent Planetary Motion
• Motion as seen
from Earth,
which itself is
revolving
around the Sun.
Explanation 1: Ptolemy (~140 AD)
• Planets move on circles
sitting on circles around
Earth  geocentric model
• dominates scientific
thought during the Middle
Ages
• Longest lasting (wrong)
theory ever: 1000yrs
Epicycles
• Ptolemy’s
explanation of
retrograde motion
• About 40(!) epicycles
necessary to explain
all observations
complicated theory
Explanation 2: Copernicus (1473–1543)
• All planets – including Earth –
move around the Sun
• Planets still on circles
needs 48 epicycles to explain
different speeds of planets
• Not more accurate than Ptolemy
Major Work : De
Revolutionibus
Orbium Celestium
(published posthumously)
Correct Explanation: Kepler, Newton
• All planets move around the sun according
to Newton’s theory of gravity
• Kepler’s laws tell us how the orbits look
like, and where a planet is in its orbit
Kepler’s First Law
The orbits of the planets are ellipses, with
the Sun at one focus
Ellipses
a = “semimajor axis”; e = “eccentricity”
Kepler’s Second Law
An imaginary line connecting the Sun to any planet sweeps
out equal areas of the ellipse in equal times
Kepler’s Third Law
The square of a planet’s orbital period is proportional to the cube of its
orbital semi-major axis:
P 2  a3
a
P
Planet Orbital Semi-Major Axis Orbital Period
Mercury
0.387
0.241
Venus
0.723
0.615
Earth
1.000
1.000
Mars
1.524
1.881
Jupiter
5.203
11.86
Saturn
9.539
29.46
Uranus
19.19
84.01
Neptune
30.06
164.8
Pluto
39.53
248.6
(A.U.)
(Earth years)
Eccentricity
0.206
0.007
0.017
0.093
0.048
0.056
0.046
0.010
0.248
P2/a3
1.002
1.001
1.000
1.000
0.999
1.000
0.999
1.000
1.001
Inner and Outer
Planets
• Inner Planets: closer to sun than Earth
– Mercury & Venus
– Always close to sun in the sky
• Outer Planets: further from sun than Earth
– Mars, Jupiter, Saturn, Uranus, Neptune, Pluto
– Best viewing when opposite of sun in the sky
Superior Planets** (Ex: Mars)
1) Superior Conjunction – Mars
is on the other side of the Sun
(like Superior Conjunction for
Venus). “Full” and small
(Ex: Mars – Sun – Earth )
2) Opposition – Mars is on the
opposite side of Earth from the
Sun. “Full” and very big.
Superior planets are closer and
bigger at opposition than at
any other time!
(Ex: Mars – Earth – Sun)
**Definition: A superior planet
is one that is farther from the
Sun than Earth.
Inferior Planets* (Ex: Venus)
1) Superior Conjunction –
Venus is on the other side of
the sun. “Full” = entirely lit
up still
and very small
Testgen questions
(Ex: Venus – Sun – Earth )
do not copy to 1)other
Inferior Conjunction – Venus
applications.
is between the Earth and
Sun; “New” = entirely dark
and large
(Ex: Sun – Venus – Earth)
**Definition: An inferior planet
is one that is closer to the
sun than Earth.
1) Inferior Planets (Ex: Venus)
Since Venus and Mercury are lit
up, they must be in Superior
Conjunction (on the far side of the
sun from Earth)
2) Superior Planets (Ex: Mars) Since
they are appearing near sunrise,
Mars and Jupiter are in Superior
Conjunction as well.
The last opposition
was in 2012. Earth
and Mars were just 63
million miles apart.
The next opposition
will be in 2018. Earth
and Mars will be 36
million miles apart.
Mars’ orbital plane is slightly tilted
relative to Earth’s orbital plane.
When the two orbital planes overlap at the
same time as opposition occurs, the
planets can pass even closer to each other.
Inner Planets
superior conjunction
Inner planet
eastern
elongation
western
elongation
inferior conjunction
Earth
Outer Planets
quadrature
conjunction
Earth
quadrature
Outer planet
opposition
Close Outer Planet
Size of planet
varies a lot as
Earth moves
Earth
Outer planet
Far-Out Planet
Earth
Size of planet
varies little as
Earth moves
Outer planet
Tutorial
• Make sure to discuss the diagrams and
questions with your partner before you write
your answer. The learning is in the discussion.
Mercury
•
•
•
•
Color: yellow-golden
Brightness: up to –1m
Size: 10”
When to observe: several times a year for
short periods
• Difficulty: pretty tough, innermost planet,
always very close to the sun
Venus
•
•
•
•
Color: white
Brightness: up to –4.5m
Size: up to 40”
When to observe: all year, except for
period around superior conjunction; either
west of the sun (morning star), or east of the
sun (evening star)
• Difficulty: very easy
Phases of Venus
Phases of
Venus
Heliocentric
Geocentric
Mars
•
•
•
•
•
Color: orange
Brightness: up to –2.2 m
Size: up to 25”
When to observe: about every 2 years
Difficulty: very easy around opposition
Mars Opposition 2016
• Date of opposition: May 22
• Constellation: Scorpius
• Date of closest distance: Mars Close Approach is
May 30, 2016.
• Closest distance to Earth: Mars will be at a
distance of 46.8 million miles (75.3 million
kilometers).
• Miss seeing Mars Close Approach in 2016? The
next Mars Close Approach is July 31, 2018.
Mars
• Fairly bright, generally
not too hard to see
• Smaller than Earth
• Density similar to that
of the moon
• Surface temperature
150–250 K
• Day ~ 24.6 hours
• Year ~ 2 Earth years
Martian Seasons