THE SKY

CHAPTER 2 (Part 3)Precession

Starter

• NASA PODCAST – “STORIES OF THE SOLAR SYSTEM: VENUS”

• WE WILL BE TALKING ABOUT ASTRONOMICAL INFLUENCE ON EARTH. TAKE NOTE OF VENUS’ CLIMATE AND HOW IT IS DIFFERENT FROM EARTH’S DESPITE HOW MUCH THE TWO PLANETS SHOULD HAVE IN COMMON.

Objectives

• To be able to interpret and apply the term “brightness” to stars (finishing this objective).

• To be able to describe how the sky moves with reference to the Earth’s rotation.

• To be able to predict the seasons and describe what causes them.

• To be able to synthesize information on astronomical cycles to predict Earth’s climate.

Precession• Earth spins like a giant top, and like a

giant top, its weight tips it. • Earth is tipped 23.5 degrees from

vertical.• Earth’s large mass and rapid rotation

keep its axis of rotation pointed toward a spot near the star Polaris.

• Due to its rotation, Earth has a slight bulge around its middle – gravity from the sun and moon pull on the bulge twisting Earth on its axis.

• The result is that the twisting of the axis of rotation combines with the rotation of the Earth causing precession.

• Earth’s axis precesses, taking about 26,000 years for one cycle.

Precession

• Video showing how Precession works.• Can be found at

http://www.youtube.com/watch?v=0qHjtp4cdCA

Precession Summing Up Questions

• What role do the sun and moon play in causing precession? In your answer, do not forget Earth’s equatorial bulge.

• The sun’s and moon’s gravitational force acts on Earth’s equatorial bulge, causing the spinning Earth’s axis to sweep around in a conical motion like the motion of a top’s axis.

Precession Summing Up Questions

• Earth’s axis of rotation precesses with a period of how many years?

• Answer: 26,000 years• What does that mean with respect to Polaris, the

north star?• Answer: It will cease being the north star• What bright star will replace Polaris as the north star?• Answer: Vega – in 12,000 yrs the pole will be within 5

degrees of Vega.

Review Question on Rotation

• Earth’s rotation on its axis produces what cycle?

• Answer: It produces the cycle of day and night.