Reason for Seasons

Do now: Why do you think we have different temperatures during the different seasons?

Most people think it’s the distance between us and the sun Most people think it’s the distance between us and the sun? Are you one of them? Let’s see if you are right!

What are the two motions Earth makes in space? Rotation - spinning Revolution – going around the sun.

Tilt An axis is an imaginary line that runs through the center of the Earth and is what we rotate around. We are tilted 23.5 degrees from being straight up and down. This means we have a tilt of 23.5 degrees.

What do Tim and Moby have to say about the seasons? The Four Seasons

1. Tilt of Earth’s Axis During the summer, we (in the northern hemisphere) are tilted towards the Sun

This means we will have warmer temperatures in the northern hemisphere This means we will have warmer temperatures in the northern hemisphere. It’s summer! The northern hemisphere is leaning towards the sun making it summer.

Why does leaning affect temperature? It has to do with direct and indirect insolation (rays of sun). When the sun is straight overhead it makes the rays more concentrated and therefore hotter. The more spread out the rays the less concentrated and so the colder the temperatures.

More concentrated – 90 degree angle of insolation. Less concentrated – lower angle of insolation.

In places the sun shines on directly (with a 90 degree angle) the temperatures will be hotter – where it is not direct or indirect (a lower angle) it will be colder.

2. Revolution The earth does not stay in one place but the tilt always stays the same. This is called parallelism. This means that different places on Earth will receive direct or indirect rays of insolation depending on the time of year.

On Dec. 21, 231/20S receives the direct ray. On June 21st – 231/20N receives the direct ray. On Sept. 22 and March 20 the equator receives the direct rays since neither the northern or the southern hemispheres are leaning towards the sun.

Earth Revolution Animation

A closer look at the 1st day of each of the seasons.

Summer Solstice June 21st – longest day of the year! Earth’s axis is tilted towards the Sun Sun has the longest path in the sky

15 hours of daylight and 9 hours of darkness Sun rises in the NE and sets NW Sun’s direct rays hit the Tropic of Cancer (23 ½ oN)

The Sun’s direct rays hit the Tropic of Cancer on June 21st.

Summer Solstice Notice how the number of hours of daylight increases as you go higher in latitude. The Artic Circle is experiencing 24 hours of daylight.

How many hours of daylight is the Antartic Circle experiencing on the summer solstice? 0 hours – it experiences 24 hours of darkness.

Winter Solstice December 21st – shortest day of the year Earth’s axis is tilted away from the Sun Sun has the shortest path in the sky

9 hours of daylight and 15 hours of darkness Sun rises in the SE and sets in the SW Sun’s direct rays hit the Tropic of Capricorn (23 1/2o S)

Equinoxes March 21st and Sept. 23rd 12 hours of daylight and 12 hours of darkness (equal)

Sun rises exactly East and sets exactly West Sun’s direct rays hit the Equator

Equinoxes

Diagram summarizing seasons

Summary of the cause of seasons: The Earth’s axis is tilted causing parts of the Earth to be tilted towards or away from the sun. The Earth’s axis does not change – parallelism (the axis remains parallel to its previous position as Earth revolves around the Sun). Earth revolves around the sun changing its position. So does the distance between the Earth and the Sun cause the seasons? NO! We are actually closer to the sun in the winter!!

Daily path of the sun. When we diagram the path of the sun on the first day of each of the seasons it would look like this. What do you notice about the summer’s path? Winter path? Summer path is long. Winter path is short.

The Noon Sun At noon, the Sun has reached its highest point in the sky for that day In N.Y. (lat. ~42o), the Sun will NEVER be directly overhead at your zenith Noon

The highest angle (73. 5o) the Sun ever reaches in N. Y The highest angle (73.5o) the Sun ever reaches in N.Y. is on the Summer Solstice Only at latitudes between 23 ½ o N & 23 ½ o S can the noon Sun be directly overhead

Where does the Sun rise and set on the Summer Solstice? Rises NE Sets NW

Where does the Sun rise and set on the Equinoxes? Rises E Sets W

Where does the Sun rise and set on the Winter Solstice? Rises SE Sets SW

Which season does the Sun have the longest path in the sky? Summer

At noon in the northern hemisphere, towards which direction would an observer look to find the Sun? SOUTH

73.5o 50o 26.5o New York Notice the angle of the sun changes 23 ½ degrees higher or lower as you go from one season to the next.

Must know season facts!! If you know this chart you have the basics! Day Begins Place receiving 900 angle Sunrise/sunset Length of day and night Summer Solstice Fall equinox Winter Spring Northeast/ Northwest 15 day 9 night June 21 Tropic of Cancer Sept 21 East West 12 day 12 night Equator Southeast Southwest 9 day 15 night Tropic of Capricorn Dec 21 March 21 Equator East West 12 day 12 night

Watching the daily path of the sun. http://astro.unl.edu/naap/motion3/animations/sunmotions.swf http://www.eram.k12.ny.us/education/components/docmgr/default.php?sectiondetailid=17500&fileitem=633&catfilter=333