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Unformatted text preview: Lecture 3 Discovering the Universe for
Yourself What are constellations? A constellation is a region of the sky The celestial sphere is
divided into 88
constellations The ecliptic is the
Sunʼ s path through the
celestial sphere The stars appear to lie on a celestial sphere
even though they are really at different distances 1 The Milky Way in the
night sky is actually
our view into the disk
of the Milky Way
galaxy The constellations along the ecliptic make up the zodiac The north celestial
pole is above the
North Pole How do we locate objects in
the sky? The celestial equator
is above Earthʼs
equator The south celestial
pole is above the
South Pole Measuring Angles in the Sky An objectʼs position in the local sky is given by
its altitude above the horizon (in degrees) and
its direction along the horizon 2 Time Exposure Photograph: Review: Coordinates on the
Earth • Latitude: position north (+) or south ( of
• Longitude: position east or west of prime
meridian (runs through Greenwich, England) In the northern hemisphere, which way do the stars move
in this picture? A. Counterclockwise or B. Clockwise Why can’t everyone see the
• Your latitude (in
degrees) = the altitude
of your celestial pole
(north or south).
• In the northern
within an angle equal to
your latitude of:
• the North Celestial
pole never set.
• the South Celestial
pole are never seen
by you. The Daily Motion
daily circles --- CCW looking north, CW looking south Celestial
Equator Your horizon Summary Slide:
Depending on your latitude:
• Some stars never set (circumpolar stars).
• Some stars never rise.
• All other stars have daily, circular paths that rise in the
east and set in the west. Annual Motion
• As the Earth orbits the Sun, the Sun appears to move
eastward with respect to the stars.
• The Sun circles the celestial sphere once every year. • Whether those other stars are up during the night or
not depends on where they are relative to the Sun on
the ecliptic - a position that changes through the year… 3 What causes the seasons on
Earth? What causes the seasons? A. The earth’s tilt, which changes the direction of
the Sun’s light as the Earth orbits the Sun.
When sunlight is the most direct, it is summer.
B. The distance from the Earth to the Sun. When
the Earth is closer, it is summer.
C. The earth’s tilt, which causes some parts of
the Earth’ s surface to be closer to the Sun
than the rest of the surface. Seasons occur because the directness of sunlight
changes over the year The directness of sunlight changes because of the tilt
of Earthʼs axis… …NOT the distance of the surface to the Sun.
June: December: Sunlight most direct in
north (summer) Sunlight least direct in
north (winter) Sunlight least direct in
south (winter) Sunlight most direct in
south (summer) Solstice: Equinox: Time when difference
between directness of
sunlight between north
and south is greatest Time when directness of
sunlight in north and
south is equal How do we mark the
progression of the seasons? 4 Solstices & Equinoxes
• Winter solstice: near Dec 21
Spring equinox: near Mar 21
Summer solstice: near June 21
Fall equinox: near Sept 21
The times and dates shift a little because
calendar is not precisely synchronized with
Earth’s orbit (365.25 days). Seasonal changes are more extreme at high latitudes
The Sun is circumpolar above the Arctic Circle at the time
of the summer solstice More Questions about Seasons
Why does the hottest part of the year come after the
– Temperatures continue to increase while the
sunlight remains more intense. (Think of heating
a pot of water.) Does the orientation of
Earth’s axis change with time? Why are seasons more moderate in the southern
– There’s more water and less land. Precession causes Earthʼs rotation axis to trace a
circle in space over the course of about 26,000 years Because of precession, Earthʼs position in orbit on a
particular date slowly shifts 5 ...
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- Fall '08