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Astronomy Notes
Historically Earth was thought to be center of solar
system (geocentric model)
Heliocentric model is correct (Sun centered solar
system)
On Earth celestial objects appear to move from east
to west in an arc pattern
Sun/Stars Apparent Path
#1 Looking North at Polaris
#2 Looking West
#3 Looking South
#4 Looking East
This apparent motion is due to Earth’s
counterclockwise rotation at 15 o /hour
A solar day is from one solar noon (meridian or
longitude) to the next
The earth revolves in a tilted position 360 o around
the sun (approx. 1 o /day) counterclockwise
direction in an elliptical orbit
Earth seasons are caused by the tilted revolution
around the sun - not distance!
Closer in the winter time (tilted away), further away
in the summer time (tilted towards)
Aphelion – Greatest distance away from sun in orbit
(summer in N. Hemisphere)
Sun’s diameter appears smallest in sky, slowest
orbital velocity
Perihelion – Closest distance to sun in orbit (winter
in N. Hemisphere)
Sun’s diameter appears largest in sky, fastest orbital
velocity
Dec. 21 Winter Solstice Direct Ray 23.5o S. Tropic
of Capricorn
Nights get longer, days shorter as you move higher
in latitude 90o N. Endless night!
June 21 Summer Solstice Direct Ray 23.5o N.
Tropic of Cancer
Nights get shorter, days longer as you move higher
in latitude 90oN. Endless day!
The earth rotates counterclockwise from west to
east at 15 o /hour = 1 time zone
15 o /hour x 24 hours = 24 time zones on a 360 o
sphere
Evidence of Earth’s rotation 1) Foucault’s
Pendulum (appears to turn)
2) Coriolis Effect
(right deflection in N.
Hemisphere)
MOON
The moon orbits the earth counterclockwise in an
elliptical orbit on a 5o tilt
Gravity and Inertia keeps moon in revolution, 1/2
illuminated by sunlight
Moon ¼ dia. of Earth, 1/6th gravity, 120 lbs on
earth = 20 lbs on moon
One moon revolution = one moon rotation, back
side of moon never faces Earth
27 1/3 Days to make one complete revolution
around Earth, 29 ½ days for complete phases
Apogee – Greatest distance away from Earth
(appears smaller diameter)
Perigee – Closest distance to Earth (appears larger
diameter)
Because the Earth and moon revolve as well as
rotate the Earth must rotate more than 360o to return
to the same meridian (solar noon or daily moon
position)
Lunar eclipse– occurs only at Full Moon, Earth’s
shadow cast on moon
Sun-Earth-Moon
Solar eclipse – occurs only at New Moon, moon’s
shadow cast on Earth
Sun-Moon-Earth
PLANETS
Inner solid terrestrial planets Mercury, Venus,
Earth, Mars
Mercury 1st planet, 2nd smallest, no atmosphere, no
moons
Venus has CO2 atmosphere, hottest planet due to
greenhouse effect, sister planet to Earth, clockwise
rotation (retrograde motion), no moons
Earth only planet with water/life, 1 moon
Mars – red planet, thin CO2 atmosphere, 2 moons –
Phobos & Deimos
Asteroid Belt between Mars and Jupiter
Jupiter – largest plane, ring system, fastest rotation
(shortest day), 4 major moons, has rings & the
Great Red Spot
Saturn – ring system, many moons (18)
Uranus – has rings, many moons (18), “tipped over”
about 90 degrees to orbit plane
Neptune – ring system, 8 moons Triton has a
retrograde orbit (clockwise), can orbit further out
than Pluto
Pluto – half ice/half rock, highly eccentric orbit
(very oval), over 90 degree axis tilt (points south),
has one moon Charon.
10th planet – UB 313 has moon
Meteor-Meteorite- Meteoroid
Outer Gas Planets Jupiter, Saturn, Uranus, Neptune
- all have “rings”
Jupiter largest planet, gas giant, less dense
Pluto/Charon and 10th Planet UB 313/moon are
small, solid moon-like.
Comets – “dirty snowballs”, huge elliptical orbits,
tails point away due to solar wind
Geometry of Orbits
Revolution – movement of one celestial body
around another
Orbit – path taken during revolution
All
planets
and
comets
have
elliptic
al
orbits
(oval)
The
eccentr
icity or “out-of-roundness” of an ellipse can be
calculated:
Eccentricity = Distance between major foci
Length of major axis
Gravitation is the attractive force between all
objects
The greater the mass the greater the gravity
STARS
H-R Diagram - Luminosity vs. Temperature/Color
The Sun is an average star
Star power = fusion H  He
Sunspots – dark magnetized spots (usually pairs)
travel across equator of Sun as Sun rotates
Milky Way – Spiral galaxy, Solar System/Earth
located halfway across on spiral band
The universe is expanding as evidenced by the RedShift or Doppler Effect
Galaxies are moving out and expanding, this
stretches light beams out/longer wavelength
Spectral shift of objects moving away is towards the
red wavelength (red shift)