Download Observing the Planets

Welcome to
Starry Monday at Otterbein
Astronomy Lecture Series
-every first Monday of the monthOctober 2, 2006
Dr. Uwe Trittmann
Today’s Topics
• Pluto, Armageddon, etc. – Solar
System News
• The Night Sky in October
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.)
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 …
Vote: Should Pluto be a “full”
planet?
• Yes
• No
An Inventory of the
Solar System
The Terrestrial (inner) Planets
• Small, dense and rocky
• Few moons, no rings
Mercury
Mars
Venus
Earth
The Jovian (Outer) Planets
Saturn
Jupiter
Uranus
Neptune
• Large, gaseous, lots of moons, rings
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
Extreme Example: Titan
• Titan is the only moon in the solar system
known to have an atmosphere
Infrared picture shows surface details
Proteus is
one of
Neptune’s
moons
Uranus’
Moons
Earth’s moon
Asteroids, Comets and
Meteors
Debris in the Solar System
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
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-type
Ida: S-type; with moon Dactyl
How to detect Asteroids
• Fast moving
Icarus is a line
on this longtime exposed
photo
The Solar System: Top View
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
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".
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!
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.
The candidates size-wise: Eris,
Pluto, Ceres (and Earth)
Brown dwarfs: Big planets or small
Stars
Two types of planets
More candidates …
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)
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
Pluto is different!
• Pluto is only
2200 km in diameter
• Much smaller than
the Earth’s Moon
Pluto
Resolution 1“
• 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“
Pluto
• 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
Best picture of Pluto so far
Pluto’s strange Orbit
• Very far out there: 40 A.U.
• Pluto’s year = 248 Earth years
• Orbit inclined 17° w.r.t. ecliptic
• Very eccentric orbit:
• Perihelion: 30 A.U.
(inside the Neptune orbit!)
• Aphelion: 50 A.U.
Pluto’s eccentric Orbit
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!
Observing the Planets
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.
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
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
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!
• Saturn is visible later at night
Moon Phases
• Today (Waxing Gibbous, 75%)
• 10 / 6 (Full Moon)
• 10 / 13 (Last Quarter Moon)
• 10/ 22 (New Moon)
• 10 / 29 (First Quarter Moon)
Today
at
Noon
Sun at
meridian,
i.e.
exactly
south
10 PM
Typical
observing
hour,
early
October
Uranus at
meridian
Neptune
Moon
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
Perseus and
Auriga
with Plejades
and the
Double
Cluster
South
• Planets
– Uranus
– Neptune
• Zodiac:
– Capricorn
– Aquarius
Mark your Calendars!
• Next Starry Monday: November 6, 2006, 7 pm
(this is a Monday
• Observing at Prairie Oaks Metro Park:
– Wednesday, October 27, 6:45 pm
– Friday, January 26, 6:30 pm
• Web pages:
– http://www.otterbein.edu/dept/PHYS/weitkamp.asp (Obs.)
– http://www.otterbein.edu/dept/PHYS/ (Physics Dept.)
)
Mark your Calendars II
•
•
•
•
Physics Coffee is every Wednesday, 3:30 pm
Open to the public, everyone welcome!
Location: across the hall, Science 256
Free coffee, cookies, etc.
Outlook: Armageddon from Outer
Space – Apophis is coming!
• Asteroid Apophis is making a very close
encounter with Earth on Friday the 13th
April 2029 (very close, but certain miss)
• If things go really bad that day, it’ll return
exactly 7 years later and will crash into
Earth!
• Stay tuned!
Impact on Earth
• Most probably
caused the
extinction of
the dinosaurs
Impact Craters
• Barringer Crater, AZ
1.2 km diameter, 0.2
km deep; produced
by impact about
25,000 years ago
• Quebec's Manicouagan
Reservoir. Large
meteorite landed about
200 million years ago. The
lake, 70 km in diameter,
now fills the ring.
Tunguska
• ~30 m body
struck Siberia
in 1908
• Energy equal
to that of a 10
Megaton
bomb!
• Detonation
above ground;
several craters
Frequency of Impact Events