Download Precession

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=0qHjtp4cd
CA
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.