Download Kepler`s First Law

Astronomy 1020-H
Stellar Astronomy
Spring_2016
Day-8
Course Announcements
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SW-2 … will download scores on Monday.
SW-3 & 4 will be due soon-ish.
1st set of Dark Sky observing nights:
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Mon. Feb. 8 – 7:30pm at the observatory.
Tues. Feb. 9 is the weather backup if both are cancelled.
Why does the sky change over
the course of a year?
As we orbit the sun the direction opposite the sun changes and we only see
the stars when the sun is not up
Color-Card Question
 Basic Motions & Ancient Astronomy:
 Seasonal Sky Analogy
 Apparent Motion of the Sun
Kepler’s First Law: The orbit of a planet about
the Sun is an ellipse with the Sun at one focus.
Kepler’s Second Law: A line joining a planet and
the Sun sweeps out equal areas in equal
intervals of time.
According to Kepler’s second law, a
planet with an orbit like Earth’s would:
A. move faster when further from the Sun.
B. move slower when closer to the Sun.
C. experience a dramatic change in orbital speed
from month to month.
D. experience very little change in orbital speed
over the course of the year.
E. none of the above.
Kepler’s THIRD LAW
 The size of the orbit determines the
orbital period
 planets that orbit near the Sun orbit with shorter
periods than planets that are far from the Sun
3
a
=
AU
2
P
years
Kepler’s THIRD LAW
 The size of the orbit determines the orbital period
 planets that orbit near the Sun orbit with shorter periods than
planets that are far from the Sun
p = ~ 12 years
p = 1 year
Kepler’s THIRD LAW
The size of the orbit determines the orbital period
planets that orbit near the Sun orbit with shorter periods
than planets that are far from the Sun
MASS DOES NOT MATTER
Both have p = 1 year
Which of the following best describes what would
happen to a planet’s orbital speed if it’s mass were
doubled but it stayed at the same orbital distance?

A. It would orbit half as fast.

B. It would orbit less that half as fast.

C. It would orbit twice as fast.

D. It would orbit more than twice as fast.

E. It would orbit with the same speed.
Color-Card Question
 Renaissance Astronomy
 Kepler’s 3rd Law
Newton’s First
Law of Motion
• A body remains at rest or moves in a straight line at
a constant speed unless acted upon by an outside
(net) force.
• A rockets will coast in space along a straight line at
constant speed.
• A hockey puck glides across the ice at constant
speed until it hits something
Newton’s Second
Law of Motion
• (net)Force = mass x acceleration or
Fnet = m x a
• Acceleration is the rate of change in velocity – or
how quickly your motion is changing.
• Three accelerators in your car!!