annual motions - reasons for seasons

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

seasons?seasons?

Earth ScienceEarth Science

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

1)1) Not caused by the Earth’s distance from Not caused by the Earth’s distance from the sun!!!the sun!!!

Aphelion – Earth is 94.5 million Aphelion – Earth is 94.5 million miles away from the sun (IN JULY).miles away from the sun (IN JULY).

Perihelion – Earth is 91.5 million miles Perihelion – Earth is 91.5 million miles

away from the sun (IN JANUARY).away from the sun (IN JANUARY).

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

The Earth’s Axis remains at a The Earth’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 sun’s rays sun’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?

Angle of Insolation…

* 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.

Annual Motions:

The “Reasons” for Seasons.

– ecliptic

– summer solstice

– winter solstice

– autumnal equinox

– vernal equinox

Objectives• Identify the relative positions and motions of Earth, the

Sun, and the Moon.

• Describe the phases of the Moon.

• Explain eclipses of the Sun and Moon.

Vocabulary

The Sun-Earth-Moon System

– synchronous rotation

– solar eclipse

– perigee

– apogee

– lunar eclipse

The Sun-Earth-Moon System• The relationships between the Sun, Moon, and

Earth are important to us in many ways.


– The Sun provides light and warmth, and it is the source of most of the energy that fuels our society.

– The Moon raises tides in our oceans and illuminates our sky with its monthly cycle of phases.

– Every society from ancient times to the present has based its calendar and its timekeeping system on the apparent motions of the Sun and Moon.

Daily Motions• The Sun rises in the east and sets in the west,

as do the Moon, planets, and stars as a result of Earth’s rotation.


• We observe the sky from a planet that rotates once every day, or 15° per hour.

Daily Motions

Earth’s Rotation


– There are two relatively simple ways to demonstrate that Earth is rotating.

1. A Foucault pendulum, which has a long wire, a heavy weight, and will swing in a constant direction, appears from our point of view to shift its orientation.

2. Flowing air and water on Earth are diverted from a north-south direction to an east-west direction as a result of Earth’s rotation in what is known as the Coriolis effect.

If you sit and watch the Foucault pendulum for an hour, you will "see" that the plane of the swing of the iron ball slowly shifts anti-clockwise by about 8.4 degrees per hour.

This is an optical illusion. The building is actually shifting "under" the Foucault Pendulum!

The Foucault Pendulum (support + wire + iron ball) is attached to this building. The building is attached to the Planet Earth, which rotates on its own axis about once every 24 hours.

The Earth also goes around the Sun, once every year. The Sun, in turn, goes around the centre of our galaxy, The Milky Way, once every 250 million years. These are all local motions.

Daily Motions

Earth’s Rotation


– The length of a day as we observe it is a little longer than the time it takes Earth to rotate once on its axis.

– Our timekeeping system is based on the solar day, which is the time period from one sunrise or sunset to the next.

Annual Motions• The annual changes in length of days and

temperature are the result of Earth’s orbital motion about the Sun.


• The ecliptic is the plane in which Earth orbits about the Sun.

Annual Motions

The Effects of Earth’s Tilt


– Earth’s axis is tilted relative to the ecliptic at approximately 23.5°.

– As Earth orbits the Sun, the orientation of Earth’s axis remains fixed in space.

– At one point, the northern hemisphere of Earth is tilted toward the Sun, while six months later it is tipped away from the Sun.

– As a result of the tilt of Earth’s axis and Earth’s motion around the Sun, the Sun is at a higher altitude in the sky during summer than in the winter.

Annual Motions (Not on notes page).

The Effects of Earth’s Tilt


Altitude is measured in degrees from the observer’s horizon to the object. There are 90 degrees from the horizon to the point directly overhead, called the zenith of the observer.

Annual Motions

Solstices


– As Earth moves from position 1, through position 2, to position 3, the altitude of the Sun decreases in the northern hemisphere.

– Once Earth is at position 3, the Sun’s altitude starts to increase as Earth moves through position 4 and back to position 1.

Annual Motions

Solstices (Not on Notes Page)


– The summer solstice occurs around June 21 each year when the Sun is directly overhead at the Tropic of Cancer, which is at 23.5° N.

– The summer solstice corresponds to the Sun’s maximum altitude in the sky in the northern hemisphere.

*The latitude of Virginia Beach is 36.852N. The longitude is -75.978W.

90°

Annual Motions

Solstices (Not on Notes Page)


– The winter solstice occurs around December 21 each year when the Sun is directly overhead at the Tropic of Capricorn which is at 23.5° S.

– The winter solstice corresponds to the Sun’s lowest altitude in the sky in the northern hemisphere.

Annual Motions

Equinoxes


– When the Sun is directly overhead at the equator, both hemispheres receive equal amounts of sunlight.

– The autumnal equinox occurs around September 21, halfway between the summer and the winter solstices when the Sun is directly over the equator.

Annual Motions

Equinoxes


– The vernal equinox occurs around March 21, halfway between the winter and the summer solstices when the Sun is directly over the equator.

– For an observer at the Tropic of Cancer or Tropic of Capricorn, the Sun is 23.5° from the point directly overhead during the equinoxes.

Annual Motions

Equinoxes


For a person standing at the x at 23.5º N, the Sun (at noon) would appear in these positions on the winter solstice, the vernal equinox, and the summer solstice. On the autumnal equinox, the Sun would be at the same altitude as on the vernal equinox.

annual motions - reasons for seasons

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