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Investigating Astronomy
Timothy F. Slater, Roger A. Freedman
Chapter 1
Predicting the
Motions of the
Stars, Sun, and
Moon
The Scientific Method
Exploring the physical world using
observation, logic, and skepticism
• Hypothesis: A testable
idea
• Theory: A description of
nature, based on a great
deal of data.
A theory explains what
we see.
A Quick
Guide to
Objects in
the Sky
ConceptCheck:
• Which is held in higher regard by
professional astronomers, a
hypothesis or a theory?
• Explain your answer.
Constellations and Asterisms
Asterisms: Recognizable, dot-to-dot patterns
Constellations: One of 88 sections of the sky
Using Asterisms
to Navigate
the Sky
ConceptCheck:
• If Jupiter is reported to be in the
constellation of Taurus the Bull, does
Jupiter need to be within the outline of
the bull’s body? Why or why not?
All of the observed celestial motions can be described
if our planet spins once each day,
and orbits around our Sun each year.
The Sun, the Moon, and the
constellations appear to rise in
the East and set in the West,
everyday…
and, the constellations shift
over the course of the year.
(Different at/near the North and
South Poles)
Daily Motion and the Earth’s Rotation
• Half of the Earth is ALWAYS lit
by the Sun.
• The Earth spins, changing which
part is lit by the Sun.
Yearly Motion and Earth’s Orbit
• The Earth orbits the
Sun―in an almost-perfect
circle.
• The side turned toward the
Sun sees its light.
• The side turned away from
the Sun sees a changing
pattern of stars.
Imagine a giant “celestial sphere”
surrounding Earth.
• Project the equator
and poles into space.
• The point directly
above you in the sky is
the zenith.
• Objects near the
North celestial pole
seems to move in a
circle, never setting:
circumpolar.
Motions of the Celestial Sphere
• Objects near the celestial pole seem to move
in a circle, never setting: circumpolar.
• Your latitude impacts how the stars appear to
move.
ConceptCheck
• People in which of the following cities in North
America experience sunrise first: New York, San
Francisco, Chicago, or Denver?
• If the constellation of Cygnus rises along the
eastern horizon at sunset, at what time will it be
highest above the southern horizon?
• If the Earth suddenly rotated on its axis three
times faster than it does now, then how many
times would the Sun rise and set each year?
• Where would you need to be standing on Earth
for the celestial equator to pass through your
zenith?
The Earth is tilted on its side.
• The Earth’s North Pole is always tilted
toward the North Star. This will not
change in your lifetime.
• Earth does not change its tilt “toward” or
“away” from the Sun.
Remember that
Earth orbits the
Sun in an almostperfect circle.
Don’t let this
picture fool you!
The angle and hours of sunlight change
during the year.
The Northern Hemisphere’s Winter:
The North Pole cannot spin into the
sunlight.
Light at the Tropic of Cancer is “weak.”
The Northern Hemisphere’s Summer:
The North Pole cannot spin out of the
sunlight.
Light at the Tropic of Cancer is “strong.”
The angle and hours of sunlight change
during the year.
The tilt of the Earth causes sunlight to hit the Earth
more directly, and for a longer periods of time, during
Summer.
Summer Solstice
• When the sunlight
hits the ground
directly, it heats the
ground more
efficiently.
• When the sunlight
hits the ground for
longer periods of
time, it gets hotter!
(June in the
north/December in the
south
Winter Solstice
(December in the nor
June in the south)
The Sun’s Path on the Celestial Sphere
• The Sun appears to cover
one constellation after
another, along the ecliptic.
• In reality, the changes we
see in the Sun’s position
occur because WE are
moving.
• Bonus: The eight planets also appear
to travel on the ecliptic.
Equinoxes and Solstices
• When the ecliptic and
celestial equator
intersect, day and night
are each 12 hours long:
the equinox.
• When the Sun reaches
its most Northern and
Southern points in the
sky: the solstice.
The Sun appears to move on the
celestial sphere.
• The Sun appears to
rise and set at
different locations:
– Winter: toward the
South
– Summer: toward the
North
– Fall and Spring: due
East and West
ConceptCheck
• If Earth’s axis was not tilted, but rather was straight up
and down compared to the path of Earth’s orbit, would
observers at Earth’s north pole still observe periods in
which the Sun never rises and the Sun never sets?
• How long does the Sun take to move from being next
to a bright star all the way around the celestial sphere
and back to that same bright star?
• How often each year does an observer standing on
Earth’s equator experience no shadow during the
noon-time Sun?
• Approximately how many days are there between the
northern solstice and the March equinox?
The Moon is lit by sunlight.
• Just like the Earth,
half of the Moon is lit
by sunlight.
• The Moon does not
produce its own light.
This image of the Earth and Moon was taken
by the Galileo spacecraft.
Understanding the Moon’s Phases
The phase of the Moon is a result
of our point of view.
The “pictures” of the Moon show
what you would see from Earth
when the Moon is in that location.
The Moon’s Synchronous Rotation
• The Moon makes one
orbit around Earth,
and spins one time on
its axis, in the exact
same amount of time.
• We always see the
same side of the
Moon―the other side
is not the “dark side,”
but the “far side.”
Sidereal and Synodic Months
• The Moon orbits the Earth as Earth orbits the Sun, so
the Moon has to “catch up.”
• The Moon returns
to its place on the
background stars:
sidereal month.
• The Moon
completes a full
cycle of phases:
synodic month.
ConceptCheck
• If an observer on Earth sees just a tiny sliver of the
crescent moon, how much of the Moon’s total
surface is being illuminated by the Sun?
• If the Moon appears in its waxing crescent phase,
how will it appear in two weeks?
• If astronauts landed on the Moon near the center
of the visible surface at full moon, how many Earth
days would pass before the astronauts experienced
darkness on the Moon?
• If Earth was orbiting the Sun much faster than it is
now, would the length of time between full moons
increase, decrease, or stay the same?
Eclipses occur when the Sun, Moon, and
Earth are perfectly aligned.
The Moon’s
orbital plane is
just a little off
of the ecliptic.
The Sun, Moon, and Earth rarely line up.
Lunar Eclipses
• When the Moon is opposite
the Sun, it can travel through
the Earth’s shadow.
• The Earth’s
shadow is
complete in
the center
and partial
on the
edge.
Total Lunar Eclipse,
January 20, 2000
Total Solar Eclipses
• The Moon totally
covers the face of the
Sun.
• From inside the darkest
part of the Moon’s
shadow.
• Those inside of the
Moon’s partial shadow
see a partial eclipse.
Spectacular, Rare Total Eclipses
Annular Solar Eclipses
• When the Moon is
at its farthest
position, the cone
of its shadow
doesn’t reach Earth.
• The Moon appears
to be too small to
cover the Sun.
ConceptCheck
• Why don’t lunar eclipses occur each time the
Moon reaches full moon phase?
• Why does the eclipsing Moon spend more time in
the penumbral shadow than in the umbral
shadow?
• Why can total lunar eclipses be seen by people all
over the world, whereas total solar eclipses can
only be seen from a very limited geographic
location?
• If you had a chance to observe a total solar
eclipse and a total lunar eclipse, in general, how
much longer would you expect one type to last
than the other?