The Reason for the Seasons

The explanation of the beautiful changes that occur on Earth from season to season lies in the movement and position of the Earth in space. Let’s go into space to examine these positions and movements and discuss their role in seasonal changes. What causes most areas on Earth to experience seasons?

Here is the Earth in space. Realize that the Sun and Earth are not to scale. Although this Sun is slightly larger than the Earth, it’s not nearly big enough. If the Earth being projected on this screen were around 6 inches across, the Sun would be over 50 feet in diameter.

Do you know what the red line running through the Earth represents? That’s right. The red line is the equator. The equator is an imaginary line that separates the Earth into two halves. These halves are known as hemispheres.

The area above the equator is known as the Northern Hemisphere. The area below the equator is known as the Southern Hemisphere.

In order to understand the seasons, you must first understand that the Earth is rotating in space. Rotation is the spinning of objects in space. At any given time, half of the Earth is facing the Sun. That half of the Earth is experiencing day time. The other half is facing away from the Sun. They are experiencing night time.

As Earth rotates/spins, any location on Earth moves through sunlight/day and darkness/night. If Earth’s rotation causes day and night, how long does it take Earth to rotate around one time? How long does it take to get from the beginning of one day to the beginning of the next day? It takes 24 hours to get through the light and dark before starting a new day. 24 hours in a day isn’t a number that someone just made up. 24 hours is how long it takes the Earth to completely rotate (spin around) one time.

The blue arrow running from the North Pole to the South Pole through the center of the Earth is Earth’s axis of rotation. The axis of rotation is an imaginary line that an object in space spins around.

The main reason for the seasons is that the Earth’s axis of rotation is tilted. In other words the Earth isn’t straight up and down as pictured below. It is positioned and rotates at a tilt. Watch the Earth and click. This is how the Earth is actually positioned.

In this picture, you can see that the North Pole and the Northern Hemisphere are TILTED AWAY FROM the Sun. What season do you think the Northern Hemisphere is having when it is TILTED AWAY FROM the Sun? That’s right. The Northern Hemisphere is having winter.

If we look at the Earth a little closer, we can see why. First, you will notice that being tilted AWAY means more of the Northern Hemisphere is shadowed. This means longer nights and shorter days. Have you ever noticed that it gets dark much earlier in the winter? dark=night time Light=daytime

Obviously, when you’re NOT facing the Sun, it is generally cooler than it is during the day when you ARE facing the Sun. This means cooler, winter temperatures. Another cause for cooler temperatures is the ANGLE of the light that is hitting the Northern Hemisphere. Direct sunlight is hotter than light at an angle. Look at where most of the direct sunlight is hitting the Earth. dark=night time Light=daytime

The most direct sunlight is hitting in the SOUTHERN hemisphere. The Northern Hemisphere gets light at an angle. Light at an angle is less concentrated and cooler. This also contributes to the Northern Hemisphere’s cooler temperatures in the winter.

In summary, winter happens in the Northern Hemisphere when it is TILTED AWAY from the Sun. This causes shorter days with less sun. It also means that the sunlight hits at an angle which is less concentrated and cooler.

What month would it be if the Northern Hemisphere is experiencing winter? That’s right, it would be December. That’s when the winter season begins in the NORTHERN Hemisphere. But what about the Southern Hemisphere?

Notice that in December, when the Northern Hemisphere gets less sunlight, the SOUTHERN Hemisphere actually gets MORE sunlight. The sunlight is also striking them more directly. So what season does the SOUTHERN Hemisphere have in December? December is summer time if you live in the Southern Hemisphere.

This time of year is known as the December Solstice. A solstice is a time when sunlight is at a maximum in one hemisphere and a minimum in the other. Since the Northern Hemisphere is getting minimum sunlight, December is known as their WINTER solstice. The Southern Hemisphere gets maximum sunlight, so December is their SUMMER solstice.

If the Earth stayed in this position, the Northern Hemisphere would have a permanent winter. Of course, the Southern Hemisphere would have never-ending summer. However, the Earth doesn’t stay in this position. The Earth revolves/orbits around the Sun. A revolution/orbit is the movement of one object in space going around another because of gravity.

The Earth revolves around the Sun once every year (365 days). As it revolves, it stays tilted in the same direction in space. From this view, it stays tilted to the left. Now watch what happens as Earth revolves to the opposite side of the Sun.

Notice that the Northern Hemisphere is still tilted to the left. Being tilted to the left also means that the Northern Hemisphere is now TILTED TOWARD the Sun. What season does the Northern Hemisphere have when they are TILTED TOWARD the Sun? That’s right they are having summer.

If we look at the Earth a little closer, we can see why. First, you will notice that being tilted TOWARD the Sun means more of the Northern Hemisphere is in light. This means longer days and shorter nights. Have you ever noticed that it stays light much later in the summer? Light= day time Dark = night time

Obviously, when facing the Sun, it is generally warmer than it is during the night when you are NOT facing the Sun. This means warmer, summer temperatures. Another cause of warmer temperatures is the ANGLE of the light that is hitting the Northern Hemisphere. Direct sunlight is hotter than light at an angle. Look at where most of the direct sunlight is hitting the Earth. dark=night time Light=daytime

The most direct sunlight is now hitting in the NORTHERN Hemisphere. The Southern Hemisphere gets light at an angle. The direct light striking the Northern Hemisphere is more concentrated and warmer. This also contributes to the warmer temperatures in the summer.

The Northern Hemisphere’s summer happens when they are tilted TOWARD the Sun. This causes longer days with more sun. It also means that the sunlight that they get hits more directly and is more concentrated and warmer.

What month would it be when the Northern Hemisphere is tilted TOWARD the Sun and is experiencing summer? It would be June. June is when summer starts in the Northern Hemisphere.

What season is it in the Southern Hemisphere in June? It would be winter. That’s because they have shorter days, and receive their sunlight at an angle. Remember, sunlight at an angle is less concentrated and cooler.

June is the summer solstice for the Northern Hemisphere. That would make it the winter solstice in the Southern Hemisphere.

That covers summer and winter, but what about spring and fall? Spring and fall occur during what is called an equinox. An equinox is when the amount of sunlight in the Northern and Southern Hemispheres is EQUAL. During an equinox, both hemispheres experience 12 hours of daylight and 12 hours of night. Let’s examine the Earth’s position in space during an equinox.

Here is the Earth during an equinox. Notice that the North pole is STILL tilted in the same direction (to the left). The Sun is in an entirely different (perpendicular) direction.

This means that the Northern Hemisphere isn’t tilted toward or away from the sun. This causes equal sunlight. Both hemispheres have a dark half. In this case, it is the half that is facing you.

However, both hemispheres also have a light half. It is the half that is facing the Sun. The half that is facing away from you. The other equinox would be on the exact opposite position of Earth’s revolution.

The Northern Hemisphere is still tilted to the left. This means that the Earth’s tilt isn’t toward or away from the Sun.

Both hemispheres have a light half (now facing towards you). The dark half of each hemisphere is facing away from the Sun (and in this position, away from you).

Both of these positions create equal amounts of sunlight and darkness for both hemispheres. It also makes the angle of sunlight the same for both hemispheres. This results in seasons that have weather and temperatures that are moderate. These are the spring and fall seasons. These seasons look very similar depending on what direction you are viewing them from.

One equinox occurs in September. What season does the Northern Hemisphere have in September? The other equinox occurs in March. What season does the Northern Hemisphere have in March? The Northern Hemisphere would be having fall. The Southern Hemisphere would have spring. The Northern Hemisphere would be having spring. The Southern Hemisphere would have fall.

Now that we have identified the positions that cause the four seasons, let’s put them together in the proper order. The December Solstice The Northern Hemisphere’s winter The Southern Hemisphere’s summer Now watch as the Earth revolves around the Sun.

Here is the Earth three months later. The March Equinox The Northern Hemisphere’s spring The Southern Hemisphere’s fall Now watch as the Earth revolves around the Sun.

Here is the Earth after another three months. 6 months from where we started. The June Solstice The Northern Hemisphere’s summer The Southern Hemisphere’s winter Now watch as the Earth revolves around the Sun.

Here is the Earth after another three months. 9 months from where we started. The September Equinox The Northern Hemisphere’s fall The Southern Hemisphere’s spring Now watch as the Earth revolves around the Sun.

After another three months, the Earth has completed it’s revolution. We are back to the December Solstice.

R V E E I W Q E U T S I O N S

Question 1 What do you call the motion of one object in space around another? A.solsticeB. revolution C.equinoxD. hemisphere

Your missed the link. Click the arrow and try again.

Sorry, that is incorrect. Try Again

That is Correct!

Question 2 When both the Northern and Southern Hemisphere get EQUAL amount of sunlight. A.solsticeB. revolution C.equinoxD. hemisphere

Your missed the link. Click the arrow and try again.

Sorry, that is incorrect. Try Again

That is Correct!

Question 3 This word means half of the Earth. A.solsticeB. revolution C.equinoxD. hemisphere

Your missed the link. Click the arrow and try again.

Sorry, that is incorrect. Try Again

That is Correct!

Question 4 When one hemisphere is getting maximum sunlight and the other is getting minimum. A.solsticeB. revolution C.equinoxD. hemisphere

Your missed the link. Click the arrow and try again.

Sorry, that is incorrect. Try Again

That is Correct!

When the Earth is in this position, what season would it be in the Northern Hemisphere? What month is it? Is it an equinox or a solstice? The Northern Hemisphere would be having summer because they are tilted TOWARD the Sun. The Northern Hemisphere has summer in June. June is a SOLSTICE because the N. Hemisphere gets max. sunlight and the S. Hemisphere gets minimum.

What two seasons occur when the Earth is in these positions? During what months do these positions occur? Are these equinoxes or solstices? In both of these positions, one hemisphere experiences spring and the other experiences fall. The months are March and September. They are both equinoxes because the tilt of the Earth is perpendicular to the Sun.

When the Earth is in this position, what season would it be in the Northern Hemisphere? What month is it? Is it an equinox or a solstice? The Northern Hemisphere would be having winter because they are tilted AWAY FROM the Sun. The Northern Hemisphere has winter in December. December is a SOLSTICE because the N. Hemisphere gets minimum sunlight and the S. Hemisphere gets maximum sunlight.

The End If you liked this presentation and want to preview more powerpoint presentations, games, and activities, just click the links below. A Red-Nosed Reindeer Mystery- Christmas Powerpoint- Exploring the Scientific Method Fun Hands-On Experiments that Emphasis the Scientific Method (listed below) Bubble Trouble Getting Into the Swing of It Squeezing the Most Out of Science MANY Other Products