Unit 3: Motion in the Universe

Topic 1:Apparent Motions of Celestial Objects

Is the Earth Moving?Evidence that Earth stands still Evidence that Earth is moving

Apparent Motion• Apparent motion is the way an object APPEARS

to move.– Can be real, or a trick of perspective

Most celestial objects appear to move across our sky at a rate of

15o per hour

Apparent Motion of the Stars• Some stars rise in the east, and set in the west

• Polaris remains directly above the North Pole and does not rise or set.– Some stars circle Polaris– Some stars move across the

southern sky in a shallow arc.

Apparent Motion of the Stars

Apparent Motions of the Planets• Planets generally follow the same east-to-west motion as

the stars, but change position over longer periods of time.

• Retrograde Motion

• Complicated motions are due to the Earth and other planets revolving around the Sun in different orbits at different speeds.

Apparent Motions of Earth’s Moon

• The Moon has daily East to West motion, rising 50 min. later each day and shifting eastward

• One revolution around Earth = 27.3 days.

Apparent Motions of the Sun• The Sun has a daily east to west motion– has changes in position and length of arc with the

SEASONS. – is only directly overhead between 23.5o N and 23.5o S.

latitudes

Apparent Motions of the Sun

Polaris 42oN

WEST

Geocentric Theory• “Earth-centered” Theory

• Earth does not rotate or revolve

• Largely accepted until the 16th Century

Heliocentric Theory• “Sun-centered” Theory

• Earth rotates on an imaginary axis and revolves around the Sun.

• Accounts for all observations of apparent motion simply and without exceptions.