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In this video lesson, learn about solar energy and how it affects temperatures on Earth. You will also learn what causes changes in day length and why some parts of Earth experience seasonal differences throughout the year.

What Is Solar Energy?

If you ask the question, ‘What causes the seasons?’, the most common response is that Earth’s orbit takes it farther away from the sun, so we get less heat. This sounds like a logical explanation, but it’s not true! The correct answer to this question is also the reason that the Equator is always warmer than the polar regions as well as why the amount of daylight changes throughout the year.The root of all these phenomena is solar energy, or the energy that Earth receives from the sun. The amount of energy depends entirely on the angle that the sun’s rays hit Earth’s surface.

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Earth’s Shape Affects its Temperature

Let’s start with the general temperature on Earth. Imagine this: if you hold a flashlight at a right angle to a surface, you get a uniform, bright circle. Now, tilt that flashlight to a 45; angle and that same beam of light gets spread over a larger area. The beam of light is both larger and less intense.The same happens at the Equator and the polar regions. The Equator gets sunlight at a direct right angle, so all the solar energy gets concentrated in this area.

But, because the earth is round, the polar regions receive the sun’s rays as if they were the tilted flashlight, so the same solar energy is spread over a larger area, and the result is a lower temperature. Just think: if Earth were flat, the polar regions would be just as warm as the tropics, because they would receive the same intensity of solar energy!

Earth’s Tilt Causes the Seasons

The angle of the sun’s rays not only creates temperature differences across Earth, but it’s also responsible for the seasons in temperate regions. Remember how I said most people think the earth is farthest from the sun during winter months?Well, the earth is actually closer to the sun when the Northern Hemisphere is experiencing winter but is just tilted away from the sun! This means that in the Northern Hemisphere, the sun’s rays act more like the tilted flashlight during the winter and more like the right angle flashlight during the summer. The opposite is true for the Southern Hemisphere, which is why they have their summer during our winter and their winter during our summer.

Earth’s Tilt Affects Day Length

The tilt of the earth not only creates seasons but also affects the amount of daylight we receive each day. You may have noticed that days are shorter in winter and longer in summer. Depending on where you live, this may or may not be a big difference.For example, daylight in the tropics along the Equator is roughly the same all year long, but in the polar regions, there may be as much as 24 hours of daylight during the summer and 24 hours of darkness during the winter. This occurs above the Arctic Circle in the Northern Hemisphere and below the Antarctic Circle in the Southern Hemisphere.Interestingly enough, even though daylight is different across Earth each day, the planet actually receives the same total amount of sunlight each day! And, there are some really special days when things are either the most extreme or the same across Earth.The most extreme days are called the solstices, and these are when the tilt of the earth is greatest relative to the sun.

During the summer solstice, which is around June 21st, the area above the Arctic Circle receives its 24 hours of daylight and the area below the Antarctic Circle experiences 24 hours of darkness. The winter solstice, around December 21st, is just the opposite: the area above the Arctic Circle is in 24 hours of darkness, and the area below the Antarctic Circle has 24 hours of daylight.The days when everything is the same across Earth are called the equinoxes, which are when all across Earth, the hours of daylight and darkness are of equal length. This occurs because, instead of being tilted toward or away from the sun, Earth is at a right angle to the sun.

There are two equinoxes: the vernal equinox, which is around March 20th, and the autumnal equinox, which is around September 22nd. Equinox literally means ‘equal night,’ and this equal amount of day and night is true for the tropical and polar regions (and everything in between them) on these two days.

Solar Energy Warms Earth

The sun not only creates interesting changes on Earth throughout the year but also makes it possible for life to exist! The sun’s rays carry a lot of energy, which provides a lot of heat. These rays penetrate our atmosphere and hit Earth’s surface. Some of this heat is reflected and sent back into space, and some of it is absorbed by the earth. The energy that is re-emitted toward space from Earth’s surface is called terrestrial radiation. Believe it or not, this is the energy that keeps Earth warm and hospitable.

Think about it this way – it’s much warmer at the base of a mountain than up at the top, right? This is because there is more heat near the surface of the earth. The temperature on Earth depends not only on how much solar energy is coming in but also on how much energy is allowed to go back into space.When the sun is shining brightly, we have a net energy gain on Earth because it is absorbing more energy than it is sending back toward space. At night, the opposite is true because Earth is still emitting energy as heat, but it is not absorbing any solar radiation.

This is also why cloudy days are cooler than sunny days – the planet can’t absorb solar energy that isn’t able to reach Earth’s surface.Solar energy is important because it creates the greenhouse effect. Like the glass that traps heat in a greenhouse, greenhouse gases trap heat under Earth’s atmosphere, keeping our planet warm enough to live on. Without the greenhouse effect, Earth would be well below freezing and not much would be able to survive.The problem with greenhouse gases is when too much heat gets trapped on Earth, the planet warms too much, and we get the enhanced greenhouse effect. This leads to global climate change, which, as we learned about in another lesson, has dangerous implications for the earth and its organisms.

Lesson Summary

Solar energy is the energy that comes from the sun.

This energy comes through our atmosphere and warms Earth’s surface, which makes for a nice and hospitable planet. This is a delicate balance, though, because if too much solar energy gets trapped under greenhouse gases, the earth warms too much and the enhanced greenhouse effect leads to global climate change on Earth.The angle of the sun’s rays determines our seasons, day length and regional temperatures. Like a flashlight, when the sun’s rays hit the earth at a right angle, solar energy is concentrated on a small area and temperatures are warmer. Because the earth is round, the polar regions are like tilting the flashlight to a 45; angle: the same amount of solar radiation is spread over a larger area and temperatures are cooler.The tilt of the earth also affects the angle of the sun’s rays and causes the seasons in temperate regions.

During the summer solstice, the earth is actually farthest from the sun, but the Northern Hemisphere is tilted toward the sun, so solar energy is more concentrated in this region. During the winter solstice, the earth is closer to the sun, but the Northern Hemisphere is tilted away from the sun, and so solar energy is less concentrated in this region. Since the Southern Hemisphere is tilted in the opposite direction, the seasons are reversed.

The equinoxes are the two days each year when daylight and darkness occur for equal periods of time for all regions of Earth. Even in the polar regions, where 24 hours of daylight and 24 hours of darkness occur during the solstices, equinoxes provide 12 hours of daylight and 12 hours of sunlight. And, despite the fact that different parts of Earth receive different amounts of daylight throughout the year, each day the earth receives the same total amount of solar energy – it’s just more concentrated in some places than others!

Learning Outcomes

After you’ve reviewed this video lesson, you may be able to:

  • Understand what determines seasons, day length and temperatures on Earth
  • Tell what happens during the summer and winter solstices
  • Give details about what happens during the equinoxes and identify when they occur
  • Interpret enhanced greenhouse effect, and describe how it occurs and its impact on Earth

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