Chapter 6: Earth’s Motions in Space Ms. Johnson Foundations

Objectives  Rotation is the spinning of the Earth around its axis.  Earth’s rotation causes day and night. It takes 24 Earth hours (1 day) to complete one rotation.  Earth’s axis is tilted 23.5°. The northern end of Earth’s axis (the North Pole) points towards the North star.  Revolution is the motion of Earth traveling around the sun. It takes Earth days (1 year) to complete one revolution around the sun.  Earth’s revolution combined with its tilt causes the sun’s most direct rays to shift farthest north on June 21 and farthest south on December 21. Thus, the seasons on Earth are caused by Earth’s revolution and its changing tilt relative to the sun’s intense rays.

Vocabulary  Rotation  Axis of rotation  Revolution  Orbit  Constellations

Earth: Member of the Solar System  Our solar system includes the sun and 9 planets.  Earth is the third planet.  Each of the planets revolves around the sun in an elliptical shape.  Each planet also revolves on its axis.

Earth’s Rotation  The motions of the sun, moon, and stars are caused by Earth’s own motion.  Earth spins like a top.  Rotation: the spinning motion of the Earth  East to West direction or counterclockwise direction  Axis of rotation: the imaginary rod through the earth from the North pole to the South pole

Earth’s Rotation  The rotation of the Earth has several results: 1. Causes the daily change from day to night.  At any given time half the world is daylight (facing the sun) and half is in dark (facing away from the sun).  The only exception is the poles- Arctic and Antarctic circles experience several weeks of light or dark.

Earth’s Rotation 2. The speed of Earth’s rotation causes the length of a day to be 24 hours. 3. The daily motions of the sun, moon, planets, and stars across the sky.  They appear to rise in the East and set in the West because the Earth rotates from east to west.

Daily Time  The time of day at a given location on Earth is based on Earth’s rotation.  The Earth rotates 360° around, and there are 24 hours in a day. 360°/24= 15  So, for each hour the Earth rotates 15°

Earth’s Revolution  Revolution: the motion of Earth as it moves around the sun.  Orbit: the path Earth travels around then sun.  Earth’s orbit is elliptical (oval-shaped)

Earth’s Revolution  Earth’s revolution has two major effects: 1. The time earth takes to revolve once around the sun is the length of a year (364 ¼ days).  For convenience its 365 days long, and then every fourth year (Leap Year) an extra day is added. 2. Earth’s revolution combined with the axis tilt causes the changing seasons on the planet.  No matter where Earth is in its orbit, the axis is always pointed towards the North Star  While other stars move across the night sky, the North Star always remains motionless because Earth’s axis points to it.

Yearly Seasonal Time  Earth revolves 360° during a 12-month (1 year) period of time.  When divided (360/12) the Earth revolves about 30° each month.

Seasonal Changes  Earth’s orbit is closest to the sun in early January and farthest from the sun in early July.  The changing distance from the sun does not cause a change in the seasons.  The cause of seasons is the tilt of Earth’s axis as it revolves around the sun.  Because the axis always points the same way, half the year the Northern hemisphere is towards the sun, and the other half it is away from the sun.  This causes changes in the length of the daylight and the angle at which the sun’s rays strike the Earth.

Seasonal Changes

 Summer  Northern hemisphere is tilted towards the sun  The sun strikes that part of Earth more directly, making days last longer and producing a hot season.  Winter  Northern hemisphere is tilted away from the sun  Sun’s rays come in at a low angle in the north and the days are shorter.  Areas north of the equator get less heat and experience a cold season.  The opposite occurs in the southern hemisphere.  During Spring and Fall neither hemisphere is really tilted toward the sun.  Both hemispheres experience moderate temperatures.

Proof of Earth’s Revolution  Proof that Earth revolves around the sun comes from observations of stars.  Constellations: patterns in the night sky that remind people of animals or characters in myths.  During the year different constellations become visible which suggests that Earth’s position in relation to the sun changes. The two most popular are: Ursa Major (The Great Bear ↑, which contains the Big Dipper) and Orion, the Hunter.

Questions? Chapter 6: Earth’s Motions in Space