Seasons, Phases of
the Moon and
Eclipses
Chapter 2.2 & 2.3
Reasons for the Seasons
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Earth’s spin is called rotation
It’s orbit around the sun is called revolution
The Sun appears to rise in the east and set in the west
because the Earth rotates west to east (counter
clockwise)
The Earth also orbits CCW as well
Earth takes one year to orbit the Sun at 107,000 km/h
The seasons are caused by the Earth’s tilt
NOT by change in distance from the Sun.
Reasons for the
Seasons
• Earth’s orbital distance does vary
throughout the year- but only 3% farther
at its farthest point than at its nearest.
• Earth’s axis remains pointed at Polaris
(also called the North star) at a 23.5
degree axis tilt.
Reasons for the Seasons
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Summer Solstice- around June 21st, when the
Northern Hemisphere receives the most direct
sunlight.
Winter Solstice- around December 21st, when the
Northern Hemisphere receives the least direct
sunlight
Spring (vernal) Equinox- around March 21st, when
N. hemisphere goes from being slightly tipped away
to slightly toward the Sun.
Fall (autumnal) Equinox- around September 22nd,
when N. hemisphere does from being slightly tipped
toward to slightly away from the Sun.
Reasons for the
Seasons
• What is precession?
• How long is a cycle of Earth’s
precession?
• How long will it be before the Earth’s axis
will point at Vega instead of Polaris?
The Moon
The Moon orbits the Earth about once every 27.3 days
It’s average distance form Earth is 380,000km (240,000 miles)
The Moon
• Waning means “decreasing”
• Waxing means “increasing”
• First quarter Moon occurs when the
Moon is one quarter through its cycle.
• A third quarter Moon must mean....
• Gibbous- hump-backed opposite of the
Crescent moon
The Moon
• Synchronous Rotation: the Moon must
rotate once on its axis in the same time
that it makes a single orbit of Earth.
• This is why we always see the same side
of the moon.
Eclipses
• Lunar eclipse- when earth lies directly
between the Sun and the Moon,
therefore Earth’s shadow falls on the
Moon.
• Solar eclipse- when the Moon lies
directly between the Sun and the Earth,
so the Moon’s shadow falls on the Earth.
Eclipses
• Since the Moon’s orbit is slightly inclined
to the ecliptic plane, the Moon spends
most of its time either above or below it.
• The ecliptic plane is the plane of Earth’s
orbit around the Sun.
• When the Moon crosses the ecliptic
plane it is called a node.
Eclipses
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Two conditions must be met for an eclipse to occur:
1. The phase of the Moon must be full (lunar eclipse)
or new (solar eclipse) since those are the only
phases when the Sun, Earth and Moon can lie in
a straight line.
2. The new or full moon must occur during one of
the periods when the nodes of the Moon’s orbit
are aligned with the Sun and the Earth.
Eclipses
• A central umbra is where sunlight is
completely blocked
• A penumbra surrounds the umbra where
light is only partially blocked.
Eclipses
• A total lunar eclipse occurs when the
Sun, Earth and Moon are almost
perfectly aligned.
Eclipses
• A partial lunar eclipse will occur when
alignment is a bit less perfect and only
part of the Moon will pass through the
umbra.
Eclipses
• A penumbral Lunar eclipse occurs if the
Moon passes through the Earth’s
penumbra only.
Total Solar Eclipse
• Moon is closer to Earth in its orbit
• Umbra stretches up to 270km in
diameter
• Prenumbral shadow up to 7000km in
diameter.
• People will only view a total solar eclipse
if they are within the umbra.
Partial & Annular Solar
Eclipse
• Partial are seen as a result of the
penumbral shadow
• When Moon is farther from Earth in its
orbit the umbral shadow may not reach
Earth
• A small region of Earth directly behind
the umbra will see an annular eclipse
and those in the surrounding penumbral
see a partial eclipse.