Download Terestialplanets

Today’s Topics
• The Earth and celestial bodies in space.
• Description of the principles of relative
motion and perspectives.
• The grouping of planets due to some
common characteristics
Aim: What are the characteristics of the terrestrial and
Jovian planets?
•Arrange the following words in order of size; Sun,
galaxy, universe, solar system, and Earth.
Universe
Galaxy
Solar system
Sun
•Which of these represent single objects?
The Sun, Earth
• Which term contained all of the others?
Universe
Earth
The Terrestrial Planets
• Small, dense and rocky
Mercury
Mars
Venus
Earth
The inner planets
• Mercury, Venus, Earth, and Mars
• Nearest to the Sun
• Rocky crusts, dense mantle layers, and very
dense cores.
• All Earth-like characteristics
• Also known as terrestrial planets (Earth-Like)
Observing the Planets
Saturn
Jupiter
Uranus
Neptune
The outer planets
•
•
•
•
•
Jupiter, Saturn, Uranus, Neptune, and Pluto
First four are called Jovian Planets (Jupiter-like)
Massive in nature
They are gaseous
Outer layers mostly hydrogen gas, and
compressed to a hot liquid
• Closer to the planet’s center
The Solar System: Top View
Characteristics of Planets
•all planets orbit in same direction (ccw as seen from above
the north pole)
•all orbits lie nearly in a single plane (Mercury (7deg) and
Pluto (17deg) being most notable exceptions)
•inner planets are small, dense, rocky (Terrestrial); outer
planets are large, gaseous, low density (Jovian)
•density = mass/volume
•inner planets close together, outer planets further apart
Side view: Inclination of Orbits
• Orbits (here: Mars) are very slightly tilted with
respect to the sun-earth plane
 Planets appear close to the path of the sun in the
sky, the ecliptic
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
The heliocentric explanation of
retrograde planetary motion
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
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
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 2005
•
•
•
•
Date of opposition: November 7, 2005
Constellation: Aries
Date of closest distance: October 30, 2005
Closest distance to Earth: 69.42 million km
(43 million miles, or 0.46406 AU)
• In 2003 (historically close): 55.8 million km
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
Apparent Mars Diameter
The Terrestrial Planets
Comparable tilt of rotation axis
Martian Seasons
Polar Ice Caps
• Watch them
grow and
shrink in the
telescope
Mars Atlas
Mars observations
• Look for surface features
• Try to determine which
side of Mars we see
• Polar caps
• Seasonal changes
• phases
Dust Storms
Jupiter
•
•
•
•
Color: yellowish-white
Brightness: up to –2.5m
Size: 40”
When to observe: most of the year, except
for some months around conjunction
• Difficulty: easy, moons visible in binoculars
Jupiter & Moons
Saturn
•
•
•
•
Color: yellowish
Brightness: up to –1.5m
Size: 20”
When to observe: most of the year, except
for some months around conjunction
• Difficulty: easy, rings and moons visible in
small telescopes
Saturn & Moons
Uranus
•
•
•
•
Color: greenish
Brightness: around 5.7m
Size: 4”
When to observe: most of the year, except
for some months around conjunction
• Difficulty: challenging, with binoculars
Neptune
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•
•
•
Color: greenish
Brightness: around 7.8m
Size: 2.5”
When to observe: most of the year, except
for some months around conjunction
• Difficulty: challenging, good binoculars
Pluto
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•
•
•
Color: white
Brightness: 14m
Size: star-like, no disk
When to observe: most of the year, except
for some months around conjunction
• Difficulty: very tough, outermost planet,
always very far away, very faint; big
telescope and several nights to identify
The Night Sky in October
• The sun is past autumn equinox -> longer nights!
• Autumn constellations are coming up: Cassiopeia,
Pegasus, Perseus, Andromeda, Pisces
 lots of open star clusters!
• Mars is getting close to opposition
• Saturn is visible later at night
Moon Phases
• Today (New Moon, 0%)
• 10 / 10 (First Quarter Moon)
• 10 / 17 (Full Moon)
• 10 / 24 (Last Quarter Moon)
• 11/ 1 (New Moon)
Today
at
Noon
Sun at
meridian,
i.e.
exactly
south
10 PM
Typical
observing
hour, early
October
no Moon
Mars
Uranus at
meridian
Neptune
SouthWest
High in the
sky:
The summer
triangle
Due
North
Big Dipper
points to the
north pole
High up – the
Autumn
Constellations
• W of
Cassiopeia
• Big Square
of Pegasus
• Andromeda
Galaxy
Andromeda
Galaxy
• “PR” Foto
• Actual look
East
High in the
sky:
Perseus and
Auriga
with Plejades and
the Double
Cluster
South
• Planets
– Uranus
– Neptune
• Zodiac:
– Capricorn
– Aquarius