Transcript Apparent motion

Topic 4
Motions of the Earth,
Moon, and Sun
Apparent Motions of
Celestial Objects
• Celestial sphere – imaginary dome above
an observers head on which all celestial
objects appear
• Celestial objects – objects outside of the
earth’s atmosphere that can be seen in the
sky
• Zenith – highest point on celestial sphere,
directly above observer’s head
• Apparent motion – the motion an object
appears to have, but which isn’t real
Daily Apparent Motion
• Daily apparent motions caused by the
rotation of the Earth
• Paths of objects form circles, or parts of
circles called arcs
→Sun, moon, stars, etc.
→Some stars make circles around Polaris in one
day, others make arcs
• Daily motions occur at a rate of 15o/hour
Star trails from New Mexico
How long did it
take to make
this photograph?
Apparent Motion of
Planets
• Daily apparent motion of the other planets is
similar to that of the stars
• Apparent motion of the planets over the long
term is different
→Planets change position relative to the stars
around them
 Planets moving in their own orbits around the
sun
→Stars will always stay in the same location
relative to other stars
 Big dipper always looks the same
Apparent Motion of the
Moon
• Moon rises in the east and sets in the west
• Location changes in relation to the
background stars
Apparent Motion of the
Sun
• Solar noon—when sun reaches highest
point in sky for day
→Changes for a given location throughout the
year
→Height depends on time of year and latitude
 Earth is tilted on its axis 23 ½ Degrees
 Different parts of the earth pointed toward
sun at different times of year
Apparent Motion of the
Sun
• Solar Noon
→In Northern Hemisphere
 Highest in summer
 Lowest in winter
 In between spring and autumn
 Never reaches zenith or directly overhead
Apparent Motion of the
Sun
• Solar Noon
→Only locations that ever see sun directly
overhead are between 23 ½ degrees North and
23 ½ degrees south latitudes
 Only locations ever pointed directly at sun
 Equator on March 21 and September 23
 23 ½o North (Tropic of Cancer) on June 21
 23 ½ o South (Tropic of Capricorn) on Dec. 21
• Apparent path of sun changes throughout
year
→In Northern Hemisphere
 Sun always passes through southern sky
 Longest path occurs on June 21(summer
solstice)
–Sun rises and sets furthest North
 Shortest path occurs on December 21 (winter
solstice)
–Sun rises and sets furthest South
 Mid-length paths occur on March 21 and
September 23 (equinoxes)
–Sun rises due east and sets due west
Models of Celestial
Motions
Geocentric Model
• Stated that celestial objects revolved around
Earth
→Earth is stationary
• Could not explain motions of planets
→Adapted to have planets move in epicycles, or
smaller circles as the revolved around Earth
Heliocentric Model
• Stated that the Earth rotated on an axis and
moved around the sun, along with the other
planets
• Also did not explain motions of the planets
until elliptical orbits were proposed
Actual Earth Motions
Rotation
• Earth rotates on an imaginary axis
→Rotates from west to east
• Rotates at an angle of 23 ½o from the plane
of its orbit
Rotation
• Evidence for rotation
→Foucault Pendulum
 If a large pendulum swings freely, its swing
will appear to change at a rate of 15o/hour
–Actually Earth is rotating underneath it
Rotation
• Foucault Pendulum
Rotation
• Evidence for rotation
→Coriolis Effect
 Objects are deflected from a straight path as
they are projected up or down the Earth
–Deflected to the right in the Northern
Hemisphere
–Deflected to the left in the Southern
Hemisphere
Revolution
• Earth revolves around the sun
→Revolves in an elliptical orbit with the sun at
one foci
→Revolves at a rate of about 1o/day
Revolution
• Evidence for revolution
→Seasons
 Earth moves so that different parts of its
surface receive longer periods of sunlight at
different times of the year
Revolution
• Evidence for revolution
→Constellations
 Groups of stars that form patterns in the sky
–Help people landmark, or tell locations, in
the sky
 Different constellations can be seen from a
given location on the Earth at different times
of the year
Revolution
• Evidence for revolution
→Apparent diameter of the sun
 Changes throughout the year due to the
elliptical orbit of the Earth
–Sun appears larger when Earth is at its
perihelion
–Sun appears smaller when Earth is at its
aphelion
Earth Motions and Time
• Local time – time based on the rotation of
the Earth
→Same for all locations on a meridian
→Shown by the position of the sun in the sky
(solar time)
• Solar day – solar noon one day to solar
noon the next day
→Length varies throughout the year due to the
changing speed of the Earth in its orbit
throughout the year
• Mean solar day – average length of a solar
day
→Used to tell time
→Broken into 24 hours
Time zones
• Earth broken into 24 time zones
→Each is 15o wide
• All locations in a time zone keep the same
time
→Time corresponds to the meridian in the middle
of the time zone
Actual Motions of the
Earth’s Moon
• Moon revolves around the Earth
→Takes 27.3 days
• Plane of moon’s orbit is tilted 5o compared
to the plane of Earth’s orbit around the sun
Moon Phases
• Half of the moon always lit by the sun
• As the moon revolves around the Earth, the
amount of the lit portion that can be
observed on Earth varies
→Called phases
1
New Moon
1
2
New Crescent
1
2
3
1st Quarter
1
2
4
3
New Gibbous
1
5
2
4
3
Full Moon
6
1
5
2
4
3
Old Gibbous
7
6
1
5
2
4
3
3rd Quarter
7
8
6
1
5
2
4
3
Old Crescent
7
8
6
1
5
2
4
3
Moon Phases
• Full moon  Full moon takes 29 ½ days
→Earth moves in its orbit as the moon travels
around the Earth
→When the moon makes one full revolution
around the Earth it is not seen on Earth as
being in the same phase due to the Earth’s
change in position
 Moon must go further to be seen in the same
phase again
Full Moon
27 1/3
days
New Gibbous
29 1/2 days
Full Moon
Tides
• Caused by the gravitational attraction
between the Earth, moon, and sun
→Mostly the Earth and moon
Tides
• High Tide
→Occurs on side of Earth closest to the moon
 Moon pulling on the water
→Also occurs on the opposite side of the Earth
 Earth is being pulled away from this side –
leaves water
Ocean water
being pulled
toward moon
Moon
Earth getting
pulled toward
moon
High Tides
Tides
• High Tide
→Different locations on Earth experience high
tide as the Earth rotate through the areas of
higher water
→High tides at a given location occur a little more
than 12 hours apart
Tides
• Low Tide
→Occurs on sides to right angles of high tides
 Water pulled away from these locations
Low Tides
Moon
Tides
• Affect of the Sun on Tides
→Sun doesn’t cause the tides, but can make
them more extreme or more moderate
→Spring tides - Highest high tides and lowest
low tides
 Occur when the sun, Earth and moon are all
in a line
–New moon or full moon phases
Spring Tides
Really low,
low tides
New
Moon
Really high,
high tides
Full
Moon
Tides
• Affect of the Sun
→Neap Tides - Lower high tides and higher low
tides
 Occur when the sun and moon are at right
angles to each other
–1st and 3rd quarters
Neap Tides
3rd
Quarter
high, low
tides
low, high
tides
1st
Quarter
Eclipses
• Eclipses occur when one celestial object
ends up in the shadow of another
• Solar Eclipse – when the sun is blocked by
the moon, causing a shadow to be seen on
the Earth
→New moon phase
→Seen on a very small portion of the Earth’s
surface
Solar Eclipse
Area of Total
Eclipse
New
Moon
Area of Partial
Eclipse
Eclipses
• Lunar Eclipse – when the moon passes into
the shadow of the Earth
→Full moon phase
→Seen by all people on the darkened side of the
Earth
Lunar Eclipse
All of
darkened
side of Earth
can see
eclipse
Full
Moon
Eclipses
• Eclipses do not occur during every phase
cycle because the orbit of the moon is tilted
5o with the plane of the Earth’s orbit