Reasons for the Seasons

Why do we have Seasons? Things to think about… Do all places have seasons? Do all places have the same seasons as us? Do all places have the same seasons at the same time?

Brain Pop Seasons: http://www.brainpop.com/science/weather/seasons/

Average Daily Temperatures (°C) in Tourist Cities (from http://www.infoplease.com/ipa/A0004587.html ))     January April July October Cape Town (South Africa) 20 19 15 18 Caracas (Venezuela) 24 27 25 26 London (United Kingdom) 6 13 22 14 Mexico City (Mexico) 21 Montreal (Canada) -5 10 Moscow (Russia) -6 8 7 Nairobi (Kenya) 23 Paris (France) San José (Costa Rica) Seoul (Korea) 1 17 28 Singapore 29 31 30 Stockholm (Sweden) Sydney (Australia) Tokyo (Japan) 9  

Making Sense of Seasons The key is that our planet is… ROUND

…and sunlight comes in from just one direction Half of Jupiter lit by the Sun light 7

It’s all about the angle at which the light hits the planet! Venus, lit by the Sun Look at a ray of sunlight hitting the middle of the planet. The ray hits this part of the planet directly at a 90 degree angle. It focuses the sun’s energy (heat) over a relatively small area. 8

But at the north & south pole? Here comes a second ray from the sun toward the south pole of Venus. Notice it hits the surface at a angle that is much less than 90 degrees. the direct ray of light Notice how much LARGER an area the ray covers…this means it’s LESS concentrated (or more spread out). 9

Direct light heats planets up more than indirect light! Around the middle of the planet, the sun’s rays hit Earth directly, so the energy is more concentrated. There is more energy per unit area and the Earth heats up a relatively greater amount. But at the top and bottom of the planet, the light comes in at a less direct angle, meaning the energy is less concentrated. It is spread over a greater area and therefore does not warm the planet as much. 10

So that explains why… Up here we have… 11

…and… Around here we have… 12

…and… Down here we have… I’m cold 13

But this doesn’t explain the seasons! So far we only know why the hottest part of each planet is around the middle…the sunlight hits that part most directly. If that was all that happened, though, we WOULDN’T have different seasons…just the same boring weather all year long. 14

Tilt - a - World The Earth doesn’t sit perfectly up and down.

An Axis in Time… The Earth spins around it’s own axis. (Which is like an imaginary line running from the North pole straight through to the south pole.) We spin once around our axis every 24 hours, which accounts for day and night!

The axis also has a role in the seasons. The axis is tilted, at an angle of 23.5 degrees, which is the secret to the seasons!

This is important… It’s… the… TILT 18

The Earth’s Axis TILT About 23.5 degrees 19

A more accurate view… Notice the… TILT 20

Remember from earlier… …That the middle of the planet gets the most direct Sunlight (heat). COLD medium HOT 21

So one half of the Earth “tilts” toward the sun! What does the tilt do? Direct light = Summer! Indirect down here = Winter! Which hemisphere of Earth (Northern or Southern) is having Summer & Winter in this diagram? 22

What does this mean for us? When our Northern Hemisphere tilts toward the Sun, we receive more direct sunlight and so the weather is warmer. Places like Texas actually get nearly the most direct sunlight of anywhere on the planet! 23

So…how do we get Winter? That happens when our (Northern) hemisphere tilts away from the sun. Summer down here! 24

And the final piece to the puzzle… So, the TILT is a big part of it, but something else must cause us to tilt either away or toward the Sun at different times during the year… …and that last bit is something you’ve known for years! The Earth revolves around the Sun! 25

A hemisphere is experiencing summer if it is tilted directly toward the sun. A hemisphere is experiencing winter if it is tilted directly away from the sun. 26

Seasons Animation http://esminfo.prenhall.com/science/geoanimations/animations/01_EarthSun_E2.html 27

Misconceptions about the Seasons 1. We experience seasons because Earth is closer to the sun in the summer and farther in the winter. Actually, the opposite is true. We are closer to the sun in the winter (91million miles away) than we are in the winter (94 million miles away). If this were true, wouldn’t Hawaii experience snow? 28

Notice the distance during the summer! 29

Brain Pop: Solstices and Equinox http://www.brainpop.com/science/earthsystem/solsticeandequinox/

The Earth reaches 4 important points in its orbit 1 2 3 Position 1: June Solstice About June 21st Tilted toward Sun We have our longest day, shortest night Begins Summer 31

Three months later… Position 2: September Equinox About September 23rd 4 1 2 3 Position 2: September Equinox About September 23rd Tilt in between “toward” and “away” 12 hrs day, 12 hrs night Begins Fall 32

Three months later… Position 3: December Solstice About December 22nd 4 1 2 3 Position 3: December Solstice About December 22nd Tilted away from Sun Shortest day, longest night Begins Winter 33

Three months later… Position 4: Vernal Equinox About March 21st 3 Position 4: Vernal Equinox About March 21st Tilt half way between “toward” and “away” 12 hrs day, 12 hrs night Begins Spring 34

So, In Summary… Fact 1: Round planets heat up wherever they get direct sunlightand are cooler at the tops and bottoms. Fact 2: The Earth’s axis is TILTED 23.5o. Fact 3: As Earth revolves around the Sun, the tilt causes different parts of it to receive more or less direct sunlight for a few months, causing the weather and daylight to change. 35

Questions  Explain why some places on Earth have relatively constant temperatures all year. Use examples of places in your explanation. There are specific dates when seasons are said to begin. Explain why these dates have been chosen to start the seasons.