Download Chapter 17 - Earth`s Place in Space

Motions in the Heavens
Earth rotates on its axis and simultaneously revolves or
orbits around the Sun. Which of these two motions is
responsible for each of the following:
Earth
Sun
Earth
The sun rises in the east and sets
in the west
Is this because the
Earth rotates on its
axis, or because the
earth orbits the sun?
The sun rises in the east and sets
in the west
Is this because the
Earth rotates on its
axis, or because the
earth orbits the sun?
The earth rotates on
its axis from east to
west.
Different constellations appear in the
summer than in the winter
Is this because the
Earth rotates on its
axis, or because the
earth orbits the sun?
Different constellations appear in the summer than in the
winter because of the Earth’s orbit around the sun.
+10
+10
Sun
Earth
−10
Ophiuchus
Observed region of night
sky during one season
Earth
−10
Orion
Observed region of
night sky six months
later
Orion’s Belt
The ancient Greeks saw this…….and imagined this!
At night, stars in the Northern Hemisphere
appear to revolve around the North Star
Is this because the
Earth rotates on its
axis, or because the
earth orbits the sun?
At night, stars in the Northern Hemisphere
appear to revolve around the North Star
Is this because the
Earth rotates on its
axis, or because the
earth orbits the sun?
The stars seem to
rotate because the
Earth rotates on its
axis.
Moon revolves around Earth while
Earth rotates on its axis
Which of these two motions is responsible for the
following?
The moon rises in the east and sets in the west,
just like the sun.
Is this because the earth rotates on its axis or
because the moon orbits around the earth?
The moon rises in the east and sets in the west,
just like the sun
And like the sun, it is because the earth rotates on
its axis.
The moon rises approximately 53
minutes later each day.
Is this because the
earth rotates on its
axis or because the
moon orbits around
the earth?
The moon rises approximately 53
minutes later each day.
The moon rises once a
day because the earth
rotates on its axis.
Its 53 minutes later each
day, because the moon
orbits the earth and
therefore moved
“forward” by a small
amount.
Why the moon rises later each
night
• Full moon rises at about
sunset.
• In 24 hours, the earth
makes a complete
rotation, but the moon
has move ahead on its
orbit, so the earth must
rotate “a bit extra” for the
moon to rise above the
horizon. This extra bit is
about 53 minutes.
The moon passes through monthly phases. Is this
because the earth rotates on its axis or because
the moon orbits the earth?
The moon passes through monthly phases.
The moon passes through
monthly phases. Is this
because the earth
rotates on its axis or
because the moon
orbits the earth?
The phase of the moon
depends where it is in
its orbit around the
earth.
Why are there phases of the
moon?
• The moon only reflects
light from the Sun.
• When the moon is at
point E (farthest from
sun), entire lit face can be
seen.
• At point A, only dark side
points towards Earth.
• B-D, F-H, occur in
between full moon and
new moon.
Planets
• Move independently of
stars.
• Sometimes appear to
move “backwards”
relative to other
heavenly bodies
(“retrograde motion”).
• Larger and brighter than
most stars.
• Don’t twinkle because
they are much closer
than stars.
Aristotle and the Geocentric (Earthcentered) Universe
• All heavenly bodies orbited the Earth,
including the Sun:
– People have no sensation of motion so it must
be the sky that moved
– People do not fall off the Earth, so it must not
be moving (at least not suddenly)
– Stars position relative to each other never
changes
• Modified by Ptolemy to explain retrograde
motion of planets.
Aristotle’s Geocentric Universe
The Renaissance
and the Heliocentric
Solar System
The Beginning: Copernicus (1473-1543)
– Polish astronomer found Ptolemaic system too
complicated
– Proposed a heliocentric (sun-centered) model that was
more simple than Ptolemy’s adaptation of Aristotles’s
model
– Not censored by the Church at the time, but also not
widely circulated.
Brahe & Kepler
• Brahe (1546-1601)
mapped all known objects
in the sky, but did not
explain their motions.
• Kepler (1561-1630)
Brahe’s assistant, who
believed in the
heliocentric theory,
proposed the idea of
elliptical orbits for planets.
Galileo (1564-1642) – the Father of
Modern Science
– the Scientific Method:
the laws of nature must
be understood by
observation,
experimentation, and
analysis.
– Developed a telescope
far better than those of
previous astronomers
– Used his observations
of heavenly motions
and analysis of earthly
motion to question the
geocentric model
Galileo – Study of Physics
Law of Inertia – the
tendency for an object
to resist a change in
motion. People living
on a planet moving at
a constant speed
would realize no
motion, unless it were
suddenly accelerated.
Galileo – Study of Milky Way and Moon with
telescope contradicts Aristotle’s Ideas
• Saw Milky Way was a
collection of individual
stars, not a cloud of light.
• Realized the Moon was
not an unblemished body.
Fig. 22.13a, p.567
Final Proof for Galileo of the
Heliotropic Solar System
• Moons of Jupiter
orbited Jupiter, not
the Sun.
• Phases of Venus not
possible for in a
geocentric
configuration.
Galileo was persecuted by the
Roman Catholic Church for his
beliefs
Isaac Newton (1642-1727)
Why do planets orbit around
the Sun, instead of flying
off into space?
Newton's work combined
the contributions of
Copernicus, Kepler,
Galileo, and others to
answer this question by
explaining the unversal
laws of gravity and
motion
22.4 The motions of the
Earth and the Moon
• By 1700, the heliocentric system was well
established
– rotation – the Sun and planets spin on their
axes
– Revolution – the Earth traveling around the
sun in its orbit
– Precession – the wobble of Earth’s axis
Fig. 22-15, p.535
Fig. 22-16, p.536
The Moon
• Revolves around Earth in 29.5 days
• Rotates on its axis in 29.5 days
– New moon – when the moon is dark
– Crescent moon – four days later
– Gibbous moon – bright moon 10 days after
– Full moon – 14-15 days after the new moon
– Waning moon – as it shrinks toward the next
new moon
Orbital plane of earth and moon
• The Moon’s plane of
orbit is tilted 5.2o
relative to that of the
Earth
• Thus the Earth’s
shadow misses the
full moon.
• The new moon’s
shadow usually
misses the Earth.
Eclipses of the Sun and Moon
• If the Moon passes
through the Earth–Sun
plane when the three
bodies are aligned
properly, then an eclipse
will occur
Eclipses of the Sun and Moon
• Solar eclipse – when the
Moon passes directly
between Sun and Earth
– Umbra – the region of
shadow exhibiting total
eclipse
– Penumbra – the edge of
shadow showing partial
eclipse
• Lunar eclipse – When the
Earth lies in line between
the Moon and the Sun.
– Last longer and more
common as Earth has a
bigger shadow
Fig. 22-22, p.540
Total Solar Eclipse
Partial Solar Eclipse
Fig. 22-23, p.540