Starry Monday at Otterbein

1. Welcome to Starry Monday at Otterbein Astronomy Lecture Series -every first Monday of the month- October 2, 2006 Dr. Uwe Trittmann

2. Today’s Topics • Pluto, Armageddon, etc. – Solar System News • The Night Sky in October

3. Feedback! • Please write down suggestions/your interests on the note pads provided • If you would like to hear from us, please leave your email / address • To learn more about astronomy and physics at Otterbein, please visit • http://www.otterbein.edu/dept/PHYS/weitkamp.asp (Obs.) • http://www.otterbein.edu/dept/PHYS/ (Physics Dept.)

4. Pluto’s Demotion • Since August 2006, Pluto is not a planet anymore, but a “dwarf planet” • The International Astronomical Union (IAU) voted on a new definition of the term “planet” at its triennial meeting in Prague • There is hope: lots of astronomers are not happy with the new definition, and could vote to change it in 3 years …

5. Vote: Should Pluto be a “full” planet? • Yes • No

6. An Inventory of the Solar System

7. The Terrestrial (inner) Planets • Small, dense and rocky • Few moons, no rings Mercury Mars Venus Earth

8. Saturn Jupiter Neptune Uranus The Jovian (Outer) Planets • Large, gaseous, lots of moons, rings

9. Moons (Satellites) • Satellites or Moons orbit around planets, which orbit around the Sun Satellites thus orbit the sun! • Satellites can be big or small, rocky or icy, dark or light, spherical or irregularly shaped

10. Extreme Example: Titan • Titan is the only moon in the solar system known to have an atmosphere Infrared picture shows surface details

11. Uranus’ Moons Proteus is one of Neptune’s moons Earth’smoon

12. Asteroids, Comets and Meteors Debris in the Solar System

13. Asteroid Discovery • First (and largest) Asteroid Ceres discovered New Year’s 1801 by G. Piazzi, fitting exactly into Bode’s law: a=2.8 A.U. • Today more than 100,000 asteroids known • Largest diameter 960 km, smallest: few km • Most of them are named • about 20 of them are visible with binoculars

14. Asteroid Types • C-type (dark, carbonaceous), 75% of all Asteroids • S-Type (brighter, silicate) 15% • Other (e.g. M-type with nickel/iron) 10% Gaspra: S-typeIda: S-type; with moon Dactyl

15. How to detect Asteroids • Fast moving Icarus is a line on this long-time exposed photo

16. The Solar System: Top View

17. 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

18. Why is Pluto not a planet anymore? – The Definitions (1) A "planet"1 is a celestial body that: • (a) is in orbit around the Sun, • (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and • (c) has cleared the neighbourhood around its orbit. (2) A "dwarf planet" is a celestial body that: • (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape2, • (c) has not cleared the neighbourhood around its orbit, and • (d) is not a satellite. (3) All other objects, except satellites, orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".

19. Cleaning up the Neighborhood • Small objects are forced out of the inner Solar System by gravitational pull of bigger planets • Small planetesimals collide and form planets -- or are thrown out!

20. Why should we change the definitions? • There were no clear definitions • New data / new discoveries • Many new trans-Neptunian objects found • Many new exo-solar planets discovered It’s the way of science: we build our view of the world based on observations; if it is more convenient we modify our theories, terms, etc. to better represent the world around us.

22. The candidates size-wise: Eris, Pluto, Ceres (and Earth)

23. Brown dwarfs: Big planets or small Stars

24. Two types of planets

25. More candidates …

26. Is the demotion outrageous? – Not unprecendented! • History of our view of the solar system • Pre-1450 AD: Earth in center (not a planet!), 7 planets (including Sun & Moon!) • Copernicus: Sun and 6 planets • W. Herschel (1781): Sun and 6 planets (add Uranus) • Piazzi (1801): Sun and 7 planets (add Ceres) • A few years later: Sun and 11 planets (add 3 more asteroids: Pallas, Juno, Vesta) • 1846: a dozen planets (add Neptune)

27. Demotion of the “Minor Planets” • In the mid-1800s the asteroids were considered minor planets (too many of them by then)  8 Planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune

28. Pluto is different! • Pluto is only 2200 km in diameter • Much smaller than the Earth’s Moon

29. Resolution 1“ Pluto • Sought by Percival Lowell, discovered by Clyde Tombaugh in 1930 • Existence discovered based on perturbations of Uranus’s and Neptune's orbits • The calculations were wrong, but it was there anyway! • Satellite Charon discovered in 1978 • Mutual eclipses of Pluto and Charon reveal sizes and masses Space Telescope: distance 0.9“

30. Pluto Best picture of Pluto so far • Less dense than our Moon • About 1/5 as big as the Earth • About right for a Jovian moon • Maybe a Kuiper belt object • Icy material like Triton, one of Neptune’s moons

31. Pluto’s strange Orbit • Very eccentric orbit: • Perihelion: 30 A.U. (inside the Neptune orbit!) • Aphelion: 50 A.U. • Very far out there: 40 A.U. • Pluto’s year = 248 Earth years • Orbit inclined 17° w.r.t. ecliptic

32. Pluto’s eccentric Orbit

33. Is the demotion the final word? • Problems with the new definition abound • Example: if trans-Neptunian objects exist, then Neptune has not cleared his neighborhood  Neptune is not a planet?! • Many astronomers are not happy with the new definitions; less than 500 voted!  Stay tuned!

34. Observing the Planets

35. 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.

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

37. The heliocentric explanation of retrograde planetary motion

38. 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

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

40. conjunction Outer Planets quadrature Earth quadrature Outer planet opposition

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

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

43. 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

44. 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

45. 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

46. 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

47. 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

48. Jupiter & Moons

49. 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

50. Saturn & Moons