topic 4 motions of the earth, moon, and sun. apparent motions of celestial objects
TRANSCRIPT
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
3
2
1st Quarter
1
4
3
2
New Gibbous
1
4
3
2
5
Full Moon
1
6
4
3
2
5
Old Gibbous
1
7
6
4
3
2
5
3rd Quarter
1
8
7
6
4
3
2
5
Old Crescent
1
8
7
6
4
3
2
5
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
Earth getting pulled toward moon
Moon
High Tides
Ocean water being pulled toward moon
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
Moon
Low Tides
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
Really low, low tides
NewMoo
n
Really high, high tides
Full Moo
n
Spring Tides
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
high, low tides
1st Quarter
low, high tides
3rd Quarter
Neap Tides
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
NewMoo
n
Solar Eclipse
Area of Total
Eclipse
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
Full Moo
n
Lunar Eclipse All of darkened
side of Earth can
see eclipse
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