22.1 Early Astronomy
Ancient Greeks
 Geocentric Model
• In the ancient Greeks’ geocentric model, the
moon, sun, and the known planets—Mercury,
Venus, Mars, and Jupiter—orbit Earth.
 Heliocentric Model
• In the heliocentric model, Earth and the other
planets orbit the sun.
22.1 Early Astronomy
The Birth of Modern Astronomy
 Johannes Kepler
• Kepler discovered three laws of planetary motion:
1. Orbits of the planets are elliptical.
2. Planets revolve around the sun at varying
speed.
3. There is a proportional relationship between
a planet’s orbital period and its distance to
the sun.
22.1 Early Astronomy
The Birth of Modern Astronomy
 Johannes Kepler
• An ellipse is an oval-shaped path.
• An astronomical unit (AU) is the average
distance between Earth and the sun; it is about
150 million kilometers.
Planet Revolution
22.1 Early Astronomy
The Birth of Modern Astronomy
 Sir Isaac Newton
• Although others had theorized the existence of
gravitational force, Newton was the first to
formulate and test the law of universal
gravitation.
 Universal Gravitation
• Gravitational force decreases with distance.
• The greater the mass of an object, the greater is
its gravitational force.
Gravity’s Influence on Orbits
22.2 The Earth–Moon–Sun System
Motions of Earth
 Rotation
• Rotation is the turning, or spinning, of a body on
its axis (day/night).
22.2 The Earth–Moon–Sun System
Motions of Earth
 Revolution
• Revolution is the motion of a body, such as a
planet or moon, along a path around some point
in space (once/year).
• Perihelion is the time in January when Earth is
closest to the sun.
• Aphelion is the time in July when Earth is
farthest from the sun.
17.1 Atmosphere Characteristics
Earth-Sun Relationships
 Earth’s Motions
• Earth has two principal motions—rotation and
revolution.
 Earth’s Orientation
• Seasonal changes occur because Earth’s
position relative to the sun continually changes
as it travels along its orbit (23.5 degree tilt).
Tilt of Earth’s Axis
17.1 Atmosphere Characteristics
Earth-Sun Relationships
 Solstices and Equinoxes
• The summer solstice is the solstice that occurs
on June 21 or 22 in the Northern Hemisphere
and is the “official” first day of summer.
• The winter solstice is the solstice that occurs on
December 21 or 22 in the Northern Hemisphere
and is the “official” first day of winter.
17.1 Atmosphere Characteristics
Earth-Sun Relationships
 Solstices and Equinoxes
• The autumnal equinox is the equinox that
occurs on September 22 or 23 in the Northern
Hemisphere.
• The spring equinox is the equinox that occurs
on March 21 or 22 in the Northern Hemisphere.
17.1 Atmosphere Characteristics
Length of Daylight
 The length of daylight compared to the
length of darkness also is determined by
Earth’s position in orbit.
Solstices and Equinoxes
22.2 The Earth–Moon–Sun System
Motions of Earth
 Precession, Nutation, and Barycenter
• Precession- change in direction of the axis, but
without change in tilt. This changes the stars
near the Pole, but does not affect the seasons.
Over a period of 26,000 years.
• Nutation- wobbling around the precessional
axis. This is a change in angle- ½ degree one
way or another. This occurs over an 18 year
period. And is due to the Moon exclusively.
This would very slightly increase or decrease
the amount of seasonal effects.
Precession
Precession and Nutation
22.2 The Earth–Moon–Sun System
Motions of Earth
 Precession, Nutation, and Barycenter
• Barycenter- the point between two objects
where they balance each other.
• For example, it is the center of mass where two
or more celestial bodies orbit each other.
22.2 The Earth–Moon–Sun System
Two bodies
with the same
mass orbiting a
common
barycenter.
22.2 The Earth–Moon–Sun System
Motions of Earth
 Precession, Nutation, and Barycenter
• When a moon orbits a planet, or a planet orbits
a star, both bodies are actually orbiting around
a point that lies outside the center of the larger
body.
• For example, the moon does not orbit the exact
center of the Earth, but a point on a line
between the Earth and the Moon approx. 1,710
km below the surface of the Earth, where their
respective masses balance. This is the point
about which the Earth and Moon orbit as they
travel around the Sun.
22.2 The Earth–Moon–Sun System
Two bodies with a
major difference in
mass orbiting a
common barycenter
internal to one body
(similar to the Earth–
Moon system)
22.2 The Earth–Moon–Sun System
Motions of Earth
 Precession, Nutation, and Barycenter
• This means that the Sun is not stationary in our
solar system. It actually moves as the planets
tug on it, causing it to orbit the solar system’s
barycenter. The Sun never strays too far from
the solar systems barycenter.
22.2 The Earth–Moon–Sun System
Two bodies with an
extreme difference
in mass orbiting a
common barycenter
internal to one body
(similar to the Sun–
Earth system)