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