1. 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

2. 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

3. The Terrestrial Planets Small, dense and rocky Mercury Venus Earth Mars

4. 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)

5. Observing the Planets Jupiter Uranus Saturn Neptune

6. 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

7. The Solar System: Top View

8. 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

9. 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

10. 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.

11. Apparent Planetary Motion Motion as seen from Earth, which itself is revolving around the Sun.

12. 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

13. Epicycles Ptolemy’s explanation of retrograde motion About 40(!) epicycles necessary to explain all observations  complicated theory

14. 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)

15. 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

16. Kepler’s First Law The orbits of the planets are ellipses, with the Sun at one focus

17. Ellipses a = “semimajor axis”; e = “eccentricity”

18. Kepler’s Second Law An imaginary line connecting the Sun to any planet sweeps out equal areas of the ellipse in equal times

19. 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  a 3 a P Planet Orbital Semi-Major AxisOrbital Period Eccentricity P 2 /a 3 Mercury0.387 0.241 0.2061.002 Venus0.723 0.615 0.0071.001 Earth1.000 1.000 0.0171.000 Mars1.524 1.881 0.0931.000 Jupiter5.203 11.86 0.0480.999 Saturn9.539 29.46 0.0561.000 Uranus19.19 84.01 0.0460.999 Neptune30.06 164.8 0.0101.000 Pluto39.53 248.6 0.2481.001 (A.U.)(Earth years)

20. The heliocentric explanation of retrograde planetary motion

21. 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

22. Inner Planets Inner planet Earth superior conjunction inferior conjunction western elongation eastern elongation

23. Outer Planets Outer planet Earth conjunction opposition quadrature

24. Close Outer Planet Outer planet Earth Size of planet varies a lot as Earth moves

25. Far-Out Planet Outer planet Earth Size of planet varies little as Earth moves

26. 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

27. 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

28. Phases of Venus

29. Heliocentric Geocentric

30. 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

31. 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

32. 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

33. Apparent Mars Diameter

34. The Terrestrial Planets Comparable tilt of rotation axis

35. Martian Seasons

36. Polar Ice Caps Watch them grow and shrink in the telescope

37. Mars Atlas

38. Mars observations Look for surface features Try to determine which side of Mars we see Polar caps Seasonal changes phases

39. Dust Storms

40. 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

41. Jupiter & Moons

42. 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

43. Saturn & Moons

44. 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

45. 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

46. 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

47. 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

48. Moon Phases Today (New Moon, 0%) 10 / 10 (First Quarter Moon) 10 / 17 (Full Moon) 10 / 24 (Last Quarter Moon) 11/ 1 (New Moon)

49. Today at Noon Sun at meridian, i.e. exactly south

50. 10 PM Typical observing hour, early October no Moon Mars Uranus at meridian Neptune

51. South- West High in the sky: The summer triangle

52. Due North Big Dipper points to the north pole

53. High up – the Autumn Constellations W of Cassiopeia Big Square of Pegasus Andromeda Galaxy

54. “PR” Foto Actual look

55. East High in the sky: Perseus and Auriga with Plejades and the Double Cluster

56. South Planets –Uranus –Neptune Zodiac: –Capricorn –Aquarius