Earth, Sun, Moon and the Seasons

Topic 4 and 6

Discussion Question

Review Question

Actual Motions of the Earth

• Completes 1 rotation in 24 hrs– 15/hour

• Spins on an imaginary line called an axis– Runs through poles– Axis is tilted

Completes 1 revolution in 365 days

~ 1/day

Revolves around the Sun

Perihelion = closest to the Sun

Aphelion = further from the Sun

Animation

Celestial Sphere• An imaginary sphere with a very large

radius • The earth is located at the center of the

sphere• Poles of sphere along with the poles of

Earth

Apparent Motion of Celestial Objects

• Celestial object: A natural object that is located outside of Earth’s atmosphere– Stars, Planets, Moons

• Appear to move across the sky– Result of Earth’s motions– travels E to W Daily @ 15 /hr– Yearly motions result of Earth’s revolution

• Apparent Motion: illusion in which a stationary object appears to move

Why do celestial objects appear to travel at a rate of 15 /hr across our sky?

Apparent Motion of Celestial Objects

Celestial Object Motion Position Description

Planets

- Daily EW 15/hr

- “Eastward amongst stars”

- All planets revolve around the Sun- Some revolutions are slower than Earth’s- Retrograde Motion: Planets sometimes appear to make loops and move back (western among stars)

Sun

- Daily EW 15/hr

- Forms an Arc -Length of path changes daily

-Lowest position at sunrise and sunset-Reaches highest pt at Local Solar Noon- Altitude depends on LAT and time of year-Greater length of Arc = More Sunlight-Summer = Longest path-Winter = Shortest path

Moon

- Daily EW 15/hr

- Rises 50 mins later each day

- Moon has to “catch up” to Earth’s revolution- Moves “eastward amongst stars”- Cyclic and predictable

Animation

Sun's Path and Latitude

Daily Path of the Sun

Only between 23.5 N and 23.5S can the noon sun be directly overhead at 90 angle (Zenith)

Path of the Sun

Solar Noon• The time of day when the Sun is at its highest

altitude in the sky • Altitude of Solar Noon changes daily

– Depends on latitude and season– Cyclic event

Sunrise photos taken at same local 3x in the year

North of East Due EastSouth of East

Is solar noon at the same time as clock noon each day?

Determine the Noon Angle of the Sun 1. Determine the location’s latitude.2. Determine its difference from latitude when

noon Sun is at the Zenith (90°).3. Subtract difference from 90.

Example:Location’s Latitude = 35oN Date: June 21st Difference from Tropic of Cancer (35 - 23.5) = 11.5o

Angle of Noon Sun (90 – 11.5) = 78.5o

Location Date Altitude of the Noon Sun

0°

Winter Solstice 66.5°

Equinoxes 90°

Summer Solstice 66.5°

90° N

Winter Solstice Not visible

Equinoxes 0°

Summer Solstice 23.5

90° S

Winter Solstice 23.5

Equinoxes 0°

Summer Solstice Not visible

23.5° N

Winter Solstice 43°

Equinoxes 66.5°

Summer Solstice 90°

Location Date Altitude of the Noon Sun

23.5° S

Winter Solstice 90°

Equinoxes 66.5°

Summer Solstice 43°

41° N

Winter Solstice 25.5°

Equinoxes 49°

Summer Solstice 72.5°

30° S

Winter Solstice 83.5°

Equinoxes 60°

Summer Solstice 36.5°

At our Latitude (41N)

Season First Day Position of Arc

Summer Solstice ~ June 21st - Sun rises & sets N. of E- Longest path

Autumn Equinox ~ Sept. 23rd - Sun rises & sets due E- Middle Path

Winter Solstice ~ Dec. 21st - Sun rises & sets S. of E- Shortest path

Vernal (Spring) Equinox

~ March 21st - Sun rises & sets due E- Middle Path

Interactive

Why does the altitude of the

noon Sun differ by 23.5°?

Direction of sunrise and sunset changes during the year

What is “Manhattanhenge” and when does this happen?

INSOLATION• Portion of the Sun’s radiation that reaches Earth’s

surface• Amount varies depending on Latitude and Season

(Cyclic)

INcoming SOLar radiATION =

Intensity of Insolation

- Rate at which solar energy is received by a given area per time

Angle of Insolation

- Angle at which solar energy reaches Earth- Higher angle = greater intensity - Only locations between 23.5 N and 23.5 S receive a 90 degree noon sun angle at some point during the year

Duration of Insolation

- Amount of time that sunlight is reaching Earth’s surface- Greatest in Summer, least in Winter -Always 12 hrs at equator -Polar regions have greatest yearly Range (0-24 hours)

Which number shows Earth at perihelion position? The aphelion position?

1

42

3

Interactive

- Lowest Angle- Lowest Intensity

- Shortest Duration

- Highest Angle- Highest Intensity- Longest Duration

Animation

At our latitude (41N)

During which month is the longest day of the year in NYC? The shortest? How long is the duration of Insolation for each day?

Maximum duration in NY (15 hours) on summer solstice.Average duration (12 hours) on equinoxes.Least duration in NY (9 hours) on winter solstice.

Animation

At our latitude (41N)

**Most amount of hours during summer****Least amount of hours during winter**

Factors affecting Absorption and Reflection

• In Atmosphere– Ozone (03 ) absorbs UV (ultraviolet) light – Clouds and Aerosols reflects and scatters

insolation• Examples of Aerosols: ice crystals, water droplets,

dust, bacteria, volcanic ash

• On Earth’s Surface– Land: better absorber and radiator– Water: better reflector of sunlight– Ice/snow: reflect radiation

Describe a surface that would absorb insolation?

Reflect insolation?

Why do we have seasons?• Result from cyclic variations of the angle of

insolation and the intensity and duration of insolation that occurs during the year

• Generally follows the north-south shift in the direct rays of insolation– Caused by tilt of Earth’s axis occurs between 23.5 N and

23.5 S

Video

What can you infer this to mean based on the word cyclic?

Reasons for the seasons• The Tilt of Earth’s axis• Parallelism of axis• Revolution of Earth

The Tilt of Earth’s axis• Earth’s axis is tilted 23.5 • Angle of the tilt determines the intensity of the

seasons

• Parallelism of the Axis – The tilt is always pointed in the same direction regardless of its position in its orbit

• Results of Angle and Parallelism– Angle of insolation varies based on location on Earth– Duration of insolation varies throughout the year

Angle Intensity Results

Greater than 23.5 Greater - Warmer Summers- Cooler Winters

Less than 23.5 Less - Cooler Summers- Warmer Winters

Parallelism of the Axis

• Earth’s Axis points in the same direction in space at all location in its orbit around the sun

Earth’s Revolution around the Sun• The Earth travels along a cyclic path around the Sun• Earth’s tilt is not always pointed towards the Sun

– Varies due to Parallelism of the Axis

Significant DatesDate Name Significance

December 21st Winter Solstice

- First day of winter- N. Pole tilted away from the Sun

- Direct sunlight @ Tropic of Capricorn (23.5 S)- Southern Hemisphere experiences greater duration of

insolation

March 21st Vernal

Equinox

- First day of spring- Earth not tilted towards Sun

- Direct sunlight @ the equation (0)- Equal duration of insolation

June 21st Summer Solstice

- First day of summer- N. pole tilted toward the Sun

- Direct sunlight @ Tropic of Cancer (23.5 N)- Northern Hemisphere experiences greater duration of

insolation

September 23rd Autumnal Equinox

- First day of fall (autumn)- Earth not tilted towards Sun

- Direct sunlight @ the equator (0)- Equal duration of insolation

Animation

Explain why the N. Pole does not receive any sunlight for 6

months?