Download Orbits, Asteroids, and Comets

Orbits, Asteroids, and Comets
The Geometry of Orbits
• Planets revolve in an ellipse around the sun
– An ellipse has two fixed points called foci that
are on either side of the center of the axis
– The sun lies at one focus and is not the center
of Earth’s orbit
The Geometry of Orbits
• If the two foci are located
near the ends of the axis, an
ellipse is long and narrow
• If the foci move closer
together, the shape of the
ellipse becomes circular
Calculating Eccentricity
(elongation) of an ellipse
Eccentricity = distance between the foci
length of the major axis
e = d/L
Gravity and the Planets
• Gravity is the force that holds the planets
and other objects in the solar system in
their orbits
• Any object that orbits another object in
space is known as a satellite
– Earth is a satellite of the sun
– The moon is Earth’s satellite
Gravity and the Planets
• If a satellite has
an elliptical orbit,
gravity causes the
speed to change
– The satellite will
move faster when
it’s near its
primary and
slower when it’s
farther away
Gravity and the Planets
• The closer a planet is to the sun, the faster
it moves in its orbit
– Mercury, the planet closest to the sun, travels
about 1.6 times as fast as Earth and 10 times
the speed of Pluto
Eccentricity
• Eccentricity is a number between 0 and 1 that
tells you how round or elliptical orbit is.
– An eccentricity of 0 means that the orbit is round
• Planets have very round orbits
– An eccentricity of 1 means that the orbit is
elliptical
• Comets have very elliptical orbits
Kepler’s Planetary Laws of Motion
• Used a lifetime of data kept by another
astronomer (Brahe) to explain the motion of
the planets
• Two main ideas:
– Objects orbiting the Sun move faster when they
are near
• a) Does the distance to the center of the Sun stay the
same in any orbit?
• b) Which orbit has the least variation in distance from
the Sun throughout the orbit? Which has the most?
• c) Earth’s orbit has an eccentricity of about 0.017.
Compare this to the ellipse with the lowest
eccentricity that you drew. Why does it make sense
to describe Earth’s orbit as “nearly circular”?
• d) The Hale-Bopp comet has an eccentricity of
0.9950. How would you describe this orbit? Why
does it make sense that we only see this comet every
2,500 years?