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THE SKY
CHAPTER 2 (Part 4)
Sun Cycle and Seasons
Objectives
• To be able to interpret and apply the term
“brightness” to stars (finishing this objective).
• To be able to describe how the sky moves
with reference to the Earth’s rotation.
• To be able to predict the seasons and
describe what causes them.
• To be able to synthesize information on
astronomical cycles to predict Earth’s climate.
Precession
• Earth spins like a giant top, and like a
giant top, its weight tips it.
• Earth is tipped 23.5 degrees from
vertical.
• Earth’s large mass and rapid rotation
keep its axis of rotation pointed toward
a spot near the star Polaris.
• Due to its rotation, Earth has a slight
bulge around its middle – gravity from
the sun and moon pull on the bulge
twisting Earth on its axis.
• The result is that the twisting of the axis
of rotation combines with the rotation
of the Earth causing precession.
• Earth’s axis precesses, taking about
26,000 years for one cycle.
Precession
• Video showing how Precession works.
• Can be found at
http://www.youtube.com/watch?v=0qHjtp4cd
CA
Cycles of the Sun
• Earth’s rotation on its axis defines the day but
its revolution around the sun defines its year.
Annual Motion of the Sun
• If the Sun were not so strong, you could see it
rise in the morning in front of the stars and
move across the sky.
• Between sunrise and sunset, it moves a
distance roughly equal to its own diameter.
• As Earth moves around its nearly circular
orbit, the sun rises in front of different stars.
• So, in March, the sun is in front of Aquarius
(people say it is in Aquarius).
Annual Motion of the Sun
The apparent path of the sun against the background stars is called the
ecliptic.
Annual Motion of the Sun
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Earth circles the sun in 365.25 days.
This appears to circle the sky in that period.
It moves 1 degree eastward in 24 hours.
This “movement” is what gives us seasons.
The Seasons
• If Earth rotated upright in its orbit, it would
not experience seasons; however it is tipped
23.4 degrees on its axis.
• Earth goes through a cycle of seasons (except
at the equator) because of changes in the
amount of solar energy reaching Earth.
Tilt of the Earth
Note where the shadows fall.
You can see where the north
pole is perpetually dark
when the south pole is
perpetually light and vice
versa.
The directness and thus
energy of the sun’s rays
change.
Tilt of the Earth
This picture also shows the
spring and autumnal
equinoxes (March and
September) and the winter
and summer solstices
(December and June).
Seasons in the southern
hemisphere are the opposite
of ours.
Misconception
• The seasons don’t occur because Earth moves
closer to or farther away from the sun.
• In the northern hemisphere we are actually
1.7% closer to the sun during the winter
(seasons occur because Earth’s axis is not
perpendicular to its orbit).
Festivals
• All over the world and among the majority of
cultures (not so much around the equator),
festivals are planned around the solstices and
equinoxes.
– Ex. Christmas (planned to replace “pagan”
celebrations), New Years, “We Xipantu” (sunrise of
the new sun) the Mapuche new year celebration
in July, etc.
– Rosh Hashanah marks the Jewish new year in
September.