[INTRO MUSIC] Winter, spring, summer, fall, Seasons, I just love them all. Other than inspiring me to make up goofy poetry,
why wouldn't you love the seasons? There's always something to look forward to! You already know that summer
means long days of fun in the sun, and winter means shorter days. Not to mention building snowmen and
making lots of 'Frozen' references! Okay, so clearly, the Sun and
seasons are linked - but how? You already know that the sun is pretty important -
it IS the center of the solar system after all. You also know that our planet, Earth, revolves around
the sun, making its orbit once every 365 days. And remember, Earth isn't taking that lap
while it's standing straight up and down, instead it's tilted on its axis, the invisible
line around which our planet spins. Put together the Earth's tilt on its axis,
and the orbit it makes around the sun, and you get the yearly pattern we call seasons. Let's see how. Since the Earth is tilted, for part of the year, one
of the hemispheres — which is half of the earth — is leaning toward the sun, and the other
part of the year it's leaning away. Let's follow the northern hemisphere
once around the Sun to see how this works. In June, the northern hemisphere
is tilted towards the Sun. This means that it's getting a lot of direct
sunlight - light that's hitting it straight on. If you are sat directly underneath a bulb,
you know that things can get pretty hot, and that's exactly what's happening
to the northern hemisphere. It's summertime, and the living is easy.
Temperatures are warm, and days are long. In December, though, the northern
hemisphere is tilted away from the Sun. It's getting indirect sunlight, meaning
light is hitting it at an angle. Indirect sunlight means cooler temperatures,
shorter days, and, for lots of folks, hot cocoa and bundling up, since it's winter. But how can the angle of the Sun's light
make a difference between hot and cold? Well, try this little trick with a flashlight. Get a flashlight and dim the
lights in your room a little bit. If you turn the flashlight on and point
it straight down onto your desk, you'll see a small, bright, concentrated circle of light. That's kind of how sunlight hits the northern
hemisphere during the summer: bright and intense. Now move the flashlight down at an
angle and point it at the top of your desk. See how the light isn't as bright
and is less intense where it falls? That's like the sunlight we get in winter. But what about spring and autumn? During these two seasons, the Earth's orbit causes the northern hemisphere to be neither
tilted toward the Sun, nor away from it. So temperatures during the spring and fall are
more moderate: not too hot and not too cold, since the entire globe is getting about the
same amount of direct sunlight at once. Now, let's take a look at how the amount of sunlight affects temperatures in the northern hemisphere over the course of a whole year. An easy way to show this yearly
pattern is by using a graph. This graph shows the average high temperature in each
month for one year in Toronto, Canada - where I live. Looking at the graph, we see that during
December, January, and February — when the northern hemisphere is
getting very little direct sunlight — temperatures are low. And in the months of June, July, and August — when the tilt of the Earth on its axis is
causing Toronto to get direct sunlight — the temperatures are much higher. Proof positive that something is going
on here, and that something is this: The season that you're experiencing
right this very minute is caused in part by the amount
of direct sunlight you're getting. So, seasons are caused by the Earth's tilt on its
axis as it cruises around the Sun in its orbit. When one hemisphere gets more
direct sunlight, it's summer there. Temperatures are warmer, days
are longer, and nights are shorter. And when it gets more indirect sunlight, it's winter. Temperatures are cooler, days
are shorter, and nights are longer. And now you know what causes
summer, spring, autumn, and winter. [OUTRO MUSIC]
