4…and sunlight comes in from just one direction Half of Jupiter lit by theSunlight
5It’s all about the angle at which the light hits the planet! Venus, lit by the SunLook at a ray of sunlighthitting the middle of theplanet.The ray hits this part ofthe planet directly at a 90 degree angle. It focuses the sun’s energy (heat)over a relatively small area.
6But at the north & south pole? Here comes a second ray from the sun toward the south pole of Venus. Notice it hits Earth’s surface at a angle that is much less than 90 degrees.the direct ray of lightNotice how much LARGER an area the ray covers…this means it’s LESS concentrated (or more spread out).
7So direct light heats planets up more than indirect light! YUP!Around the middle of the planet, the sun’s rays hit Earthdirectly, 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 comesin 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.
11This helps, but it doesn’t explain the seasons! So far we only know why the hottest part of each planetis around the middle…the sunlight hits that part most directly.If that was all that happened, though, we WOULDN’T havedifferent seasons…just the same boring weather all year long.In Texas, it’d be about 90 degrees every day of the year with occasional rain. Not bad, but definitely BORING.
12This is the key, so repeat after me… It’s…the…TILT
13The Earth doesn’t sit perfectly straight up and down A “false” view ofEarth rotating on itsaxis.This just isn’t right!
16Remember from earlier… …That the middle of the planet gets the most direct sunlight(heat).COLDmediumHOT
17So 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 havingSummer & Winter in this diagram?
18What does this mean for Texas? When our Northern Hemisphere tilts toward the Sun, weactually get nearly the most direct sunlight of anywhere on theplanet! No wonder we get over 100 degrees so often in July &August!
19So…how do we get Winter here??? That happens when our (Northern) hemisphere tilts away fromthe sun.Summerdownhere!
20And the final piece to the puzzle? So, the TILT is a big part of it, but something else mustcause us to tilt either away or toward the Sun at differenttimes during the year……and that last bit is something you’ve known for years!The Earth revolves aroundthe Sun!Watch the animation…payattention to the directionEarth’s axis points as itorbits the Sun.
21So it’s a combination of the TILTed axis and our revolving around the sun! Yeah!!!
22Complete the table4132A 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.
24Misconceptions about the Seasons 1. We experience seasons because Earth is closer to the sun in the summer and farther in the winter.WRONGActually, 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?
26Why do we experience day and night? The Earth is…Rotating
27Day versus NightWhen where you are is pointed toward the Sun, it is day. Then the Earth rotates you away from the Sun, and it is night.SunlightDaytimeNighttime
28Day and Night as the Earth is rotating on its AXIS
29Northern Hemisphere Summer Southern Hemisphere Winter Hemispheres experiencing summer receive MORE DIRECT RAYS from the sun than hemispheres experiencing winter. This direct light as its tilted towards the sun keeps the hemisphere ILLUMINATED for a greater amount of time during the day.
30Length of Day in Hemispheres 4123#1 June Day lengthEngland (Northern Hemisphere) - 15 hours and 38 minutesAustralia (Southern Hemisphere) - 10 hours and 24 minutes
31The Earth reaches 4 important points in its orbit 123Position 1: June SolsticeAbout June 21stTX tilted toward SunWe have our longest day, shortest nightBegins Summer
32Three months later… Position 2: September Equinox About September 23rd 4123Position 2: September EquinoxAbout September 23rdTX tilt in between “toward” and “away”12 hrs day, 12 hrs nightBegins Fall
33#23 Three months later… Position 3: December Solstice 4123Position 3: December SolsticeAbout December 22ndTX tilted away from SunShortest day, longest nightBegins Winter
34Three months later… Position 4: Vernal Equinox About March 21st 3Position 4: Vernal EquinoxAbout March 21stTX tilt half way between “toward” and “away”12 hrs day, 12 hrs nightBegins Spring
35So, In Summary…If somebody asked you, “Why do we have seasons on Earth?” youcould now answer with great confidence…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 differentparts of it to receive more or less direct sunlight for a few months,causing the weather and daylight to change.