The Reason for the Seasons

#1 Here’s Mars… Where would you guess that it is COLDEST on Mars? If you guessed here… …and here… …you’re right. But why? And how did you know?

#2 The key is that Planets are…

#3 …and sunlight comes in from just one direction ½ of Jupiter lit by the Sun light

#4 It’s all about CONCENTRATION! 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, giving it a lot of energy (HEAT!), and focusing this energy over a small area.

#5 But at the north & south pole? That same picture of Venus the direct ray of light Here comes a ray from the sun toward the south pole of Venus… notice how much bigger an area the ray covers…this means it’s LESS concentrated!

#6 So direct light heats planets up more than slanted light! Around the middle of the planet, the light is very focused and hits directly, heating that area up greatly. But at the top and bottom of the planet, the light comes in at a slant, meaning it warms those areas much less.

#7 So that explains why… The best planet ever! Up here we have…

#8 …and… The best planet ever! Around here we have…

#9 …and… The best planet ever! Down here we have… I’m hot

#10 This helps, but it 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. In Texas, it’d be about 90 degrees every day of the year with occasional rain. Not bad, but definitely BORING.

#11 This is the key, so repeat after me… It’s… the…

#12 The Earth doesn’t sit perfectly straight up and down A “false” view of Earth rotating on its axis. This just isn’t right!

#13 Our Axis is TILTED! About 23.5 degrees

#14 A more accurate view… Notice the…

#15 Remember from earlier… …That the middle of the planet gets the most direct sunlight (heat). HOT medium COLD

#16 So one half of the Earth “tilts” toward the sun! Yep! Which hemisphere of Earth (Northern or Southern) is having Summer & Winter in this diagram? Direct light = Summer! Slanted down here = Winter!

#17 What’s this mean for Texas? When our Northern Hemisphere tilts toward the Sun, we actually get nearly the most direct sunlight of anywhere on the planet! No wonder we get over 100 degrees so often in July & August!

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

#19 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! Watch the animation…pay attention to the direction Earth’s axis points as it orbits the Sun.

#20 So it’s a combination of the TILTed axis and our revolving!

#21 The Earth reaches 4 important points in its orbit Summer Solstice Late June TX tilted toward Sun Longest day, shortest night Begins Summer

#22 Three months later… Autumnal Equinox Late September TX tilt balanced 12 hrs day, 12 hrs night Begins Autumn/Fall

#23 Three months later… Winter Solstice Late December TX tilted away from Sun Shortest day, longest night Begins Winter

#24 Three months later… Vernal Equinox Late March TX tilt balanced 12 hrs day, 12 hrs night Begins Spring

#25 And this cycle continues over and over… …completing every 365 ¼ days, which we call a “year.” By our best estimation, this process has happened approximately… Yeah, we’re old.

#26 So, In Summary… If somebody asked you, “Why do we have seasons on Earth?” you could now answer with great confidence… Fact 1: Round planets heat up wherever they get direct sunlight and are cooler at the tops and bottoms. Fact 2: The Earth’s axis is TILTED 23.5 o. Fact 3: As Earth revolves around the Sun, the tilt causes different parts of it to receive more or less sunlight for a few months, causing the weather and daylight to change.

#27 and… THAT is the reason for the seasons!