Rotation & Revolution

Earth’s Rotation and Revolution
Earth’s Rotation – Causes daily
• The spinning of earth on its AXIS – the
imaginary line through the planet from the
North Pole to the South Pole – is its
• The axis of the earth is tilted 23.5° from a
line perpendicular to the plane of its orbit
of the sun.
Earth’s Rotation
• As the earth revolves around the sun,
Earth’s axis remains tilted at 23.5°
• The north end of the axis points toward the
north star (Polaris)
• Earth rotates 360° from west to east
(counter clockwise looking down from the
north pole) in 24 hours, at an angular rate
of l5° per hour (360°/24 hours = 15°/hour)
Tilt of the axis always stays the
1. Evidence of Earth’s Rotation
• 1851, Foucault’s pendulum will appear to
change in a predictable way.
• Why is this evidence? There is no force acting
on the pendulum to make its plane of oscillation
rotate or go around its own axis
• It would continue to swing in the original path if
Earth did not rotate.
No force that makes pendulum rotate – must
be earth that is rotating!!!
2. Evidence for Rotation
• Coriolis Effect – the tendency of objects moving over the
earth (air, water, planes, projectiles) to be deflected
(curve away) from a straight line path. The French
scientist, Gaspard Coriolis, first explained the deflection
of objects moving over the surface due to Earth’s
• The deflection is to the right in the Northern Hemisphere
and to the left in the Southern Hemisphere.
• This deflection occurs because Earth’s surface is
rotating with respect to the objects.
What are the APPARENT
MOTIONS of Celestial Objects
• An apparent motion is a motion an object
appears to make
• An apparent motion can be real or an
• For example, the stars appear to move
across the sky from east to west
• However, the apparent motion is caused
by Earth’s rotation
Stars in constellations are often very far
from one another but we see them in
the same direction in the sky.
Big Dipper
Pointer Stars point to Polaris
Some Definitions
• Celestial Sphere: A huge imaginary sphere around
the Earth to which the stars appear to be
• Constellation: An arbitrary grouping of stars in the
Sky that are in the same general direction. The
modern ones are fixed by international
agreement to map the entire Sky.
• Celestial Equator: Projection of the Earth’s Equator
onto the Celestial Sphere.
Imagine all the stars moved
down onto a big glass sphere
around the Earth.
sphere is
Earth Rotates
•Looking upward we see the stars
fixed on the celestial sphere
•Earth’s eastward rotation makes
the stars between the equator
and north celestial pole appear to
move westward
•Rise in the east and set in the
• Timed photography showing
stars rising in the east
Stars appear to be moving from
east to west. Stars rising in the
Stars appear to be moving from
east to west.
Rise in the east and set in the west.
Northern part of the sky around
Moving west
to east
Earth’s counter clockwise rotation
makes the stars appear to rotate
counter clockwise around the north
celestial pole (Polaris).
The complete circular path can be
seen for stars in the northern part
of the sky around Polaris
Circumpolar constellations never set.
Circumpolar constellations change with
latitude… sky changes with latitude
Daily Motions
• Some of the stars near Polaris appear to
move in a complete circle in 24 hours.
• The paths of all celestial objects moving in
the sky are circular or parts of a circle
called an arc.
• All motion occurs at a constant rate of
approximately l5° per hour or 360° in 24
Earth’s Revolution
• Earth revolves around the sun in a slightly
eccentric elliptical orbit or path once a year
• It takes Earth 365.25 days to revolve 360°
in its orbit around the sun. It moves
approximately 1° per day. (360°/365.26
days = 1° per day)
Evidence of Earth’s Revolution
around the Sun
• Seasonal Constellations: Because of the
Sun’s annual motion, some constellations
are visible at night only during certain
• Constellations are visible when the dark
side of Earth (away from sun) faces
toward the constellation.
The 13 constellations the sun
appears to travel through during
the year
Different constellations at different
times of the year
Evidence of Revolution
Parallax Shift
• Because of Earth’s revolution about the
sun, near stars seem to shift their position
against the farther stars (closer star –
greater the shift)
• If you observe the same star while earth is
at 2 different points during its orbit around
the sun, the star’s position relative to the
more distant background stars will appear
to change.
Parallax Shift
Apparent Motion of Sun due to
Rotation and Revolution
• Rotation: Sun – apparent daily path from
sunrise to sunset has the shape of an arc
moving l5° per hour.
• Revolution: However, the sun’s arc
changes both its position and its length
with the seasons
Earth’s Revolution around the Sun – the tilt of
Earth’s axis does not change producing a cyclical
change of seasons
Because of the tilt of the Earth’s axis, the sun’s
rays are only perpendicular (directly overhead) at
noon, between 23.5° N and 23.5° S during the
June 21 23.5° N
March/Sept. 21
Dec. 21 23.5° S
90° Perpendicular Rays of the Sun
• The noon perpendicular rays of the sun
travel southward from the Tropic of Cancer
(23.5° N) on June 21 passing the Equator
on September 21 and going to the Tropic
of Capricorn (23.5° S) where it turns
around and starts going back up passing
the Equator on March 21and arriving at
the Tropic of Cancer again on June 21.
Zenith Position of the Sun
• Because of the earth’s spherical shape, on
any particular date, there is just 1 place
where insolation (incoming solar radiation)
is at an angle of 90°. All other places are
less than 90°.
• Does the 90° ever reach the United
• An observer in New York State will never
see the Sun directly overhead.
Summer maximum altitude of noon sun is 73°,
Winter is 26° Spring/Autumn?
Changing Altitude of the Noon Sun
The apparent rising and setting of the sun is due to
Changes in the altitude of the path and horizon position
of sunrise and sunset are due to revolution
North of
North of
Due east
Due West
South of
South of
• Rotation: Earth rotates l5° per hour in 24
• Revolution: Earth takes 365.25 days to
revolve around the sun moving
approximately 1° per day.
Eccentricity = 0
Eccentricity = 0.5
Eccentricity = 1
Kepler’s Three Empirical
Laws of Planetary Motion
1. The orbital paths of the planets are
elliptical, with the Sun at one focus
Kepler’s Three Empirical
Laws of Planetary Motion
2. An imaginary line connecting the Sun to
any planet sweeps out equal areas in
equal time..
Area 1
Area 2
This is the best website for 1st and 2nd laws of Kepler
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