Download Exploring the Solar System

Exploring the Solar System
Chapter 25.1
Models of the Solar System
• Ancient observers noticed
– The sun appears to rise in the east and set in the
west
– The moon rises and sets every day
– Stars move across the sky in a set pattern
– Bright, star like objects seemed to wander among
stars…planets (from Greek work for wanderers)
Geocentric Model
• Geo = Earth
• Most ancient Greeks thought the Earth was at
the center of the universe
• In a geocentric model, the Earth is stationary
while objects in the sky move around it
• They believed that the stars and planets were
on a giant sphere that revolved around the
Earth
Heliocentric Model
• Helio = sun
• In a heliocentric model, Earth and the other
planets revolve around the sun.
Planetary Orbits
• 1600 Johannes Kepler, a German
mathematician, analyzed the
paths of the planets.
• Three laws of planetary motion.
Kepler’s 1st Law of Planetary Motion
• 1st law: The path of the planets
about the sun is elliptical in
shape, with the center of the
sun being located at one focus.
(The Law of Ellipses)
Kepler’s 2nd Law of Planetary Motion
• 2nd law: An imaginary line drawn
from the center of the sun to the
center of the planet will sweep out
equal areas in equal intervals of
time. (The Law of Equal Areas)
• The planets move faster when
they are closest to the sun, and
slower when they are farthest
from the sun.
Kepler’s 3rd Law of Planetary Motion
• 3rd law: The ratio of the squares of the periods
of any two planets is equal to the ratio of the
cubes of their average distances from the sun.
(The Law of Harmonies) p2 = a3
Kepler’s 3rd Law of Planetary Motion
• It means that if you know the period of a
planet's orbit (P = how long it takes the planet
to go around the Sun), then you can
determine that planet's distance from the Sun
(a = the semi-major axis of the planet's orbit).
Kepler’s 3rd Law of Planetary Motion
• It also tells us that planets
that are far away from the
Sun have longer periods
than those close to the
Sun. They move more
slowly around the Sun.
Gravity
• Newton realized the sun
must exert a gravitational
force on the planets that
keeps them in orbit.
• Without gravity from the
sun the planets would fly
off into space.
• Gravity and inertia
combine to keep the
planets in orbit around
the sun.
Our solar system
• Our solar system consists of the sun, the
planets, their moons, and a variety of smaller
objects that revolve around the sun.
• Distances in our solar system very large so
astronomers use astronomical units to
describe them.
• 1 astronomical unit (AU) = average distance
from Earth to the sun. (149,598,000 km
Review
• Can you name Kepler’s Three Laws of
Planetary motion?
• What keeps the planets in orbit around the
sun?
• What is an astronomical unit?