Vocabulary
 Rotation
 Lunar eclipse
 Revolution
 Nutation –will cover
 Precession
 Barycenter-will cover
 Perihelion
 Elliopsoidal Model-will
 Aphelion
 Perigee
 Apogee
 Solar eclipse
cover
Revolution
 Period of REVOLUTION= one complete elliptical
orbit around the sun or planet (moon).
 Earth=365.26 Why do we need a leap year?
 Mercury=88 days
 Pluto=248 Earth years
 Planets revolve in a counterclockwise direction
around the sun.
Rotation
 The period of ROTATION= movement of planet
around its axis.=1 day/night cycle.
 Earth=24 hours
 Mercury=59 Earth days
 Pluto=6 Earth days
Tilt
 The Earth is tilted on a 23.5 o angle on it’s axis
 http://www.josleys.com/show_gallery.php?galid=313
 The tilt of Earth is the reason day and night are not 12
hours each and why we have seasons.
 The side of the planet tilted towards the sun, is
receiving more direct sunlight and longer
days=summer.
 The side of the planet tilted away from the sun, is
receiving less direct sunlight and shorter days=winter.
Tilt
 http://www.learner.org/jnorth/tm/ReasonsBack.html
 Any planet that is tilted on its axis will have seasons.
 Seasons are marked by specific dates:
 Summer solstice June20/21 –sun is at highest point in
the sky and longest day of the year.
 Winter solstice Dec.21/22-the Northern hemisphere is at
full tilt away from the sun and shortest day of the year.
 Autumnal equinox Sept. 22/23-day and night are equal
 Vernal equnox March 20/21 day-day and night are equal.
Precession
 Change in the direction of the axis but there is not
change in the 23.5o tilt.
 Changes the stars near the pole-Polaris (current) or
Vega.
 Does not affect seasons.
Barycenter
 http://spaceplace.nasa.gov/barycenter/
 The balance between the masses of the planets as they
orbit.
 In the case of the moon and the Earth-the moon doesn’t
orbit the Earth’s exact center, but a point 1,710 km below
the surface. This is where the masses balance each
other.
 Causes the sun to wobble-it is not stationary in the solar
system. As planets tug on it with their gravity it
wobbles, but never goes far from its barycenter.
Nutation
 A wobble around the Earth’s axis 1/2o change/18 year
period.
 Due to the moon and tides.
Ellipsoid Earth
 The Earth’s diameter is wider at the equator than at the
poles-not a perfect sphere.
 Caused by the rotation of the Earth and the centrifugal
force perpindicular to it.
 The earth has a molten core, tectonic plates, and a thin
crust that can move; therefore there is a slight
flattening.
Phases of the Moon
 Oreo activity
 The amount of the moon lit up by the sun.
 ½ of the moon is lit at all times. The amoutn visible
depends on the location of the moon with respect to
the sun and Earth.
 The same side of the moon faces the Earth because its
period of rotation about its axis and its revolution
around the Earth are about the same.
Eclipses
 Solar eclipse-when the moon moved between the
Earth and the sun.
 New moon phases
 Lunar eclipse-the moon is eclipsed when it moves it
moves within the Earth’s shadow.
 Full moon phases
Solar eclipse
Lunar eclipse
Why don’t we always see
them?
 Moon’s orbit is inclined 5% so most of the phases its
shadow misses Earth.
 Must cross the plane of ecliptic for an eclipse to take
place.