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.

•Which of these represent single objects?

• Which term contained all of the others?

Universe

Universe Galaxy Solar system Sun Earth

The Sun, Earth

The Terrestrial Planets• Small, dense and rocky

Mercury

Venus

Earth

Mars

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

Jupiter

Uranus

Saturn

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 LawAn imaginary line connecting the Sun to any planet sweeps out equal areas of the ellipse in equal times

Kepler’s Third LawThe 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 Eccentricity P2/a3

Mercury 0.387 0.241 0.206 1.002Venus 0.723 0.615 0.007 1.001Earth 1.000 1.000 0.017 1.000Mars 1.524 1.881 0.093 1.000Jupiter 5.203 11.86 0.048 0.999Saturn 9.539 29.46 0.056 1.000Uranus 19.19 84.01 0.046 0.999Neptune 30.06 164.8 0.010 1.000Pluto 39.53 248.6 0.248 1.001

(A.U.) (Earth years)

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

Inner planet

Earth

superior conjunction

inferior conjunction

western elongation

eastern elongation

Outer Planets

Outer planet

Earth

conjunction

opposition

quadraturequadrature

Close Outer Planet

Outer planet

EarthSize of planet varies a lot as Earth moves

Far-Out Planet

Outer planet

Earth

Size of planet varies little as Earth moves

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

• 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

• 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

South-West

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 andAuriga

with Plejades and the Double

Cluster

South

• Planets– Uranus– Neptune

• Zodiac:– Capricorn

– Aquarius