Motions of Earth, Moon, and Sun

Apparent Motions of Celestial Objects

• An apparent motion is a motion that an object appears to make.

• Apparent motions can be real or illusions.• When you see a person spinning, that is a real

motion.• When that person “thinks” the room is

spinning, that is an illusion.

Daily Motion and Stars

• Most celestial objects appear to move across the sky.

• Most of these celestial objects are stars.• The paths of these celestial objects are circles. • This motion occurs at a rate of 15o per hour.

Apparent Motion of Planets

• Planets exhibit daily motion similar to stars. • Over longer periods of time (weeks, months)

the planets appear to change positions with respect to the background field of stars around them.– This is because that planet is also revolving around

the sun at a different rate than us.

Apparent Motion of the Moon

• Moon follows the same pattern as stars.• It appears to rise about 50 minutes later each

day and shifts eastward each day compared to the background field of stars.

Apparent Sun Motions in Northern Mid-Latitudes

• The sun rises in the east and sets in the west.

• The sun’s apparent path across the sky has the shape of an arc.

• In fall and spring, the sun rises due east and sets due west.

• In the winter the sun rises south of east and sets south of west.

• In the summer the sun rises north of east and sets north of west.

Changes in Altitude of the Sun at Noon

• The sun always reaches the highest position in the sky at local solar noon

• Only between 23.5 N and 23.5 S can the sun be directly over head.

Geocentric Models

• The Earth-centered model is called the geocentric model.

• The geocentric mode explained most of the motions of the stars, sun, and moon but could not explain the swinging of a pendulum.

Heliocentric Models

• Sun-centered models. • Heliocentric models stated that Earth rotated

on its axis and revolved around the sun.• This idea explains the movement of stars and

the sun because of the Earth’s rotation.• The apparent motions of the sun is explained

by the tilt of the Earth and the revolution around the sun.

Heliocentric Models

• Most early heliocentric models did not explain the motion of planets because they used circular orbits, not elliptical.

Actual Earth Motions

• The Earth moves with the milky way galaxy, as the galaxy expands.

• The Earth moves around the center of the milky way galaxy. It 225 millions years for one ‘galaxy year’

• The Earth also rotates and revolves around the sun.

Rotation

• The spinning of the Earth on its axis is rotation.

• The tilt of Earth is 23.5o.• The north end of the axis points toward the

north star/Polaris. • The Earth rotates 360o in 24 hours, at a rate of

15o per hour.

Evidence of Earth’s Rotation

• Foucault’s pendulum appears to move, but it is actually the room spinning and the pendulum is moving in the same direction constantly.

Evidence of Earth’s Rotation

• The Coriolis Effect– All particles of matter moving at Earth’s surface is

deflected from a straight line path. – This deflection occurs because the Earth is

rotating.

Evidence Earth Rotates around the Sun

• Seasons are evidence that the earth revolves around the sun.

• The apparent diameter of the sun changes in a cyclic fashion. – Sun appears largest January 3rd, when we are

closest to the sun.– Sun appears smallest on July 4th.

Actual Motions of Earth’s Moon

• The revolution of the moon around Earth takes 27.3 days. The moon also rotates which takes 27.3 days.

• As the moon rotates once, it revolves one.– A year and a day for the moon takes the same

amount of time

Moon Phases

• The varying amount of light seen reflected off the moon onto Earth is known as the moon’s phases.

Tides

• The gravitation between the Earth, moon, and sun results in a cyclic rise and fall of the ocean waters.

• Tides are primarily caused by the moon, because the moon is 400 times closer than the sun.

Tides

• A high tide occurs when the Earth is in line with the moon.

• A low tide occurs when the Earth is at a 90o angle to the moon.

• Because the Earth rotates, we have high tide and low tide every 12 hours.

Eclipses

• The Earth and Moon cast shadows out into space.

• When a celestial object comes into the shadow of another celestial object, it is called an eclipse.

Lunar Eclipse

• When the moon is in the shadow of the Earth during a full moon.

• If part of the moon is in the shadow of the Earth, it is called a partial lunar eclipse.

Solar Eclipse

• The moon blocks out the sun. • Solar eclipses occur at any one location about

every 200 years.• Solar eclipses last about 7.5 minutes.• Next total solar eclipse visible in the USA Aug

21 2017.