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Astronomy Unit 2
Celestial Spheres/Constellations
Apparent Motion of the Stars
Motion of the Planets
 Vocab Words for this section are:
Reflecting Telescope
Refracting Telescope
Radio Telescope
Space Telescope
 Hans Lippershey was the inventor of the
telescope in 1608.
 Galileo was the first to use a telescope to
study the sky in 1609.
 The invention and use of the telescope
began a huge number of discoveries in the
field of astronomy.
 3 types of telescopes
– Optical Telescopes study visible light.
 Reflecting telescopes use mirrors to bounce light from an
objective lens to an eyepiece lens
 Refracting telescopes use lens to bend light and magnify
images coming through an objective lens.
– Radio Telescopes study radio waves from earth.
– Space Telescopes travel out of Earth’s atmosphere
to study the other types of radiation not visible to
the naked eye, and the radiation that cannot make it
through earth’s protective atmosphere.
 Optical Telescopes study the visible light
section of the electromagnetic spectrum.
 They uses mirrors and/or lenses to
magnify the light.
 One type of
Telescope is
– Within this
there are
mirrors to
bounce the
 Another type
of Optical
telescopes is
the Refracting
– Within this
telescope there
are lenses to
bend the light
 Radio Telescopes
use a parabolic
antenna and a
receiver to study
radio waves.
– Radio Waves are
a type of
radiation that is
not normally
visible to the
naked eye.
 Space telescopes
travel out of
atmosphere to
study all different
types of radiation
that can not make
it through earth’s
– Examples are
the Hubble
Space Telescope
and the Chandra
Celestial Sphere/Constellations
 Vocab words for this section are:
Celestial Sphere
Right Ascension
Circumpolar Constellation
North Circumpolar Constellation
South Circumpolar Constellation
Celestial Sphere/Constellations
 A celestial sphere is
similar to a globe of
the sky.
– It is an imaginary
sphere where the sun,
the moon, and all the
other stars appear to
be combined.
Celestial Sphere/Constellations
 To locate on the celestial
sphere, you use
declination and right
– Declination measures
angular distance North or
South of the celestial
 Between 0 and 90 degrees
– Right Ascension measures
distances eastward, along
the celestial equator from
the vernal equinox
 Goes up to 360 degrees
Celestial Sphere/Constellations
 The 88 recognized constellations by the
International Astronomical Union are
plotted on the Celestial Sphere.
 Some constellations are close enough to a
celestial pole that they are visible in
certain locations on earth all year long.
These constellations are called
Circumpolar Constellations.
Celestial Sphere/Constellations
 Constellations close to the N. Celestial Pole are
North Circumpolar Constellations
– They are visible from the North Pole all year long!
 Constellations close to the S. Celestial Pole are
South Circumpolar Constellations.
– They are visible from the South Pole all year long!
Celestial Sphere/Constellations
 These are the
Celestial Sphere/Constellations
 These are the
Celestial Sphere/Constellations
 The ecliptic is the apparent
path the sun traces out in the
sky throughout the year.
– It is visible on the celestial
 Along the ecliptic there is a
band of 12 constellations.
They are called the zodiac
– These are where astrological
signs come from. The
constellation that rises and
sets during the month of your
birthday is supposedly your
Celestial Sphere/Constellations
 The 12 constellations of the zodiac are:
Apparent Motion of the Stars
 Vocab Words for this section are:
Circumpolar Constellation
Diurnal Circles
Apparent Motion of the Stars
 We have previously discussed circumpolar
– This concept deals with how stars appear to
move throughout the year.
Apparent Motion of the Stars
 If you trace the path a star makes each
day, it would trace out a Diurnal Circle.
– This the circular path a star appears to take
each day.
 We can’t see stars during the daytime because of
the sun, so we don’t actually see the circle.
Apparent Motion of the Stars
Diurnal Circles
Apparent Motion of the Stars
 Stars also display parallax.
– Parallax is the apparent movement of an object due a
change in the viewer’s position.
– You can try it with your finger.
 Hold your finger in front of your face. Close your left and
view your finger from your right eye. Quickly switch eyes.
Go back and forth a few times. Your finger should appear to
move even though it isn’t actually moving.
 As the earth moves around the sun, it changes
our viewing position of stars. This causes them
to display parallax!
Apparent Motion of the Stars
 We use a stars parallax (or apparent shift)
relative to “fixed” background stars, to
determine the distance to stars.
 Stars that are further away display less of a
parallax, than stars that are close to us.
Motion of the Planets
 Vocab Words for this section are:
Sidereal Motion
Kepler’s 1st Law of Planetary Motion
Kepler’s 2nd Law of Planetary Motion
Motion of the Planets
 A sidereal day actually
takes 23 hours, 56 minutes
and 4.09 seconds
– This is the amount of time it
takes the earth to get back to
the same location it was at
– We don’t use this as a day
because the earth not only
rotates, but it moves a little
bit around the sun in its
revolution during each
rotation. This is where we get
the 24 hour day from.
Motion of the Planets
 As we watch planets move across the sky,
they move relative to their background
“fixed” stars.
– Planets normally move westward across the
night sky. This is called Prograde motion.
– Sometimes planets appear to begin moving
“backward” or eastward across the night sky.
This is called Retrograde motion.
 Please not that if you look these words up, the directions will be backwards
because Astronomers pretend like you are living on the planet looking out.
For our purposes we define the word as how it looks in the night sky!
Motion of the Planets
 The planets don’t actually switch directions.
– What is happening is our orbits around the sun (the
earth’s and whichever planet we are looking at) are
catching up with each other.
– This makes the planet appear to move backwards for
a small portion of time.
 See the animation at the following website to
help you understand prograde vs. retrograde
Prograde vs. Retrograde Motion
Motion of the Planets
 Johannes Kepler developed 3 laws for how
the planets move around the sun. He did
this after making detailed studies and
observations as to how the planets are
– We are going to study the first 2!
 These laws don’t only apply to planets, but
they also apply to how moons orbit their
parent planets!
Motion of the Planets
Kepler’s First Law of Planetary Motion
Planets follow elliptical orbits, with the Sun at one
focus of the ellipse.
Motion of the Planets
 The planets move
around the sun in an
elliptical shape with
the sun at one of the
– An ellipse appears as
a squashed circle.
 It is defined as a
closed loop where the
sum of the distances
from 2 points (the
foci) to every point on
the line is constant.
Motion of the Planets
 This means there are times throughout
the year where the earth is closer and
further from the sun.
– The earth is actually closer to the sun in
December, our winter!
 It is about 147,450,000 km from the sun
– The earth is further from the sun in June,
our summer!
 It is about 152,400,000 km from the sun
Motion of the Planets
Kepler’s 2nd Law of Planetary Motion
As a planet moves in its orbit, it sweeps out an
equal amount of area in an equal amount of time.
Motion of the Planets
 Kepler’s 2nd Law implies that when a
satellite is closer to its parent object, it
actually travels faster than when it is
further away!
– When we are closer to the sun we travel
slightly faster than when we are further
– This is due to the gravitational pull the
parent object has on the satellite.
 The pull the sun has on the earth
– On average the earth travels at 29.78 km/s
Motion of the Planets
 Kepler’s Laws apply to ALL the planets
that orbit the sun.
 Kepler’s Laws also apply to how every
moon orbits its parent planet!