Annual Motions:Annual Motions:What causes the What causes the

seasons?seasons?

Earth ScienceEarth Science

Cornell NotesCornell Notes

1)1) RotationRotation

2)2) RevolutionRevolution

3)3) Earth’s orbitEarth’s orbit

Astronomy Basics…Astronomy Basics…

RotationRotation – – when a planet when a planet spins on its spins on its axis. It takes axis. It takes 24 hours for 24 hours for Earth to spin Earth to spin on its axis on its axis once.once.

Astronomy Basics…Astronomy Basics… RevolutionRevolution – The – The

time it takes when time it takes when an object goes an object goes around a sun or around a sun or planet. planet.

It takes 365 days for It takes 365 days for Earth to revolve Earth to revolve once around the once around the Sun.Sun.

It takes 30 days for It takes 30 days for the moon to revolve the moon to revolve around Earth.around Earth.

Astronomy Basics…Astronomy Basics… Earth’s orbitEarth’s orbit – –

The Earth’s orbit The Earth’s orbit is is EccentricEccentric - -(oval-shaped).(oval-shaped).

PerihelionPerihelion – – closest to the closest to the sun.sun.

AphelionAphelion – – farthest from the farthest from the sun.sun.

Basic Concepts…Basic Concepts…(We will refer to the (We will refer to the NORTHERN HEMISPHERE.)NORTHERN HEMISPHERE.)

1) Not caused by the Earth1) Not caused by the Earth’’s distant from the sun!!!s distant from the sun!!! AphelionAphelion – Earth is 94.5 million miles away from the – Earth is 94.5 million miles away from the

sun (IN JULY).sun (IN JULY).

PerihelionPerihelion – Earth is 91.5 million miles away from – Earth is 91.5 million miles away from the sun (IN JANUARY).the sun (IN JANUARY).

2) Caused by the Earth2) Caused by the Earth’’s tilt of the AXIS: s tilt of the AXIS: 23.5°23.5°..

The EarthThe Earth’’s Axis remains at a s Axis remains at a fixedfixed place (as it orbits around the place (as it orbits around the sun), so that, at one point, the northern hemisphere of Earth is sun), so that, at one point, the northern hemisphere of Earth is tilted towardtilted toward the sun, while at another point six months later, the the sun, while at another point six months later, the northern hemisphere is northern hemisphere is tipped awaytipped away from the sun. from the sun.

3) Tropic of Cancer3) Tropic of Cancer

23.5 N Latitude23.5 N Latitude

3) Tropic of Capricorn3) Tropic of Capricorn

23.5 S Latitude23.5 S Latitude

How does the angle affect our How does the angle affect our seasons?seasons?

As the winter solstice approach, the amount of As the winter solstice approach, the amount of daylight is decreased. Why? During the daylight is decreased. Why? During the winterwinter, the , the sunsun’’s rays s rays will not hit our area directlywill not hit our area directly. The sun . The sun appears to be at a appears to be at a lowerlower altitude. altitude.

In contrast, during the In contrast, during the summersummer, the sun , the sun hits our area more directly and the sun hits our area more directly and the sun appears to be at a appears to be at a higherhigher altitude. altitude.

Summer Solstice:

• Sun hits the N. Hemisphere directly (Tropic of Cancer 23.5 N).

• June 21 – 22.

• longest amount of daylight

• high angle of insolation.

Autumnal Equinox:

• Autumn/Fall

• Sept 21.

• hits equator directly

• average angle of insolation.

• equal amts of day and night/

Winter Solstice:

• winter

• Dec 21.

• shortest day.

• sun hits @ S. Hemisphere directly at 23.5 S. (Tropic of Capricorn).

• low angle of insolation.

Vernal Equinox:

• Spring

• March 21.

• hits equator directly

• average angle of insolation.

• equal amts of day and nightSummer Solstice

Autumnal Equinox

Winter Solstice

Vernal Equinox

Angle of insolation

- The angle at which the sun's rays strike a particular location.

Only latitudes between the tropic of cancer and Tropic of Capricorn can receive direct rays. The maximum angle of insolation is 90 degrees.

• The angle of insolation varies with:

• 1) Season - The maximum angle of insolation, maximum duration, and maximum intensity occur during the summer.

• 2) Latitude - As latitude increases, the angle of insolation decreases

• 3) Time of day- From morning to noon the angle of insolation increases and the from noon to evening the angle of insolation decreases.

Q: When is the best time to get a tan during the summer? 12 noon or 6 pm? Why?

• As the angle of insolation increases, the sun gets higher in the sky. As the sun gets higher in the sky the length of the shadow decreases.

Effect 1:

a) Winter – Our shadows are longer at noon (angle in low).

b) Summer – Our shadows are shorter at noon (angle is high).

Effect 2:

a) Winter – The duration (amount of time) of daylight is shorter.

b) Summer – The duration of daylight is longer.

Effect 3:

a) Winter – intensity (heat) of insolation is low.

b) Summer – intensity of insolation is higher.