2nd Grade Science
Patterns in Day and Night
Unit 1
Unit 1: Patterns in Day and Night
The Sun
Have you ever gone outside on a really sunny day and felt hot? What do you do when that happens? We normally get under some shade to cool off, like an umbrella, or a tree. Why is that?
It's because sunlight heats the Earth's surface, by transferring (or giving) it energy. This energy is also known as solar radiation. Life wouldn't exist without this energy. By getting in the shade, we avoid getting hit by direct sunlight, which would normally "heat" our skin. Boom = problem solved.
So, how big is the sun? How bright is it?
In this picture, the horse closer to us seems bigger, right? In reality, both of these horses are around the same size, but because of their position in the picture, they appear to be very different sizes.
Size, or how big something is, is relative - meaning you need to compare an object to another object to figure out how big it actually is. Things that are closer to us appear bigger, because they take up more of our "field of view", or what we see. That's why the Sun appears so big to us, as it is our closest star. But when we look at other stars in our galaxy, we realize the Sun is actually medium sized.
The same applies to brightness. Objects closer to you appear brighter, so the Sun seems very bright. However, it's really just medium in brightness.
This star, Alpha Centauri, appears super bright. This is because it's much closer to Earth than the others in our view!
Earth's Movement
The Earth moves in two different ways at the same time.
1) It rotates, or spins, around an "axis". This axis is an invisible line that goes through the North and South poles. Because of this, we say the Earth's rotation is tilted. In one day, the Earth completes a rotation, or a full spin.
2) It revolves, or orbits (moves in an almost circle) around the Sun. The Sun stays in place, at the center of our solar system. In one year, the Earth revolves one whole time around the Sun.
The Earth rotating around an imaginary red axis. This axis is actually invisible in real life.
The Earth revolving around the Sun, while rotating around its axis at the SAME time!
Day and Night
Because the Earth is a sphere (shaped like a ball, round), sunlight can only shine on one half of the Earth at a time while it's rotating. This creates day and night!
- The warmer, brighter side of the Earth that receives sunlight, or the sunny side, experiences daytime.
- The cooler, darker half of the Earth that doesn't get sunlight experiences night.
This cycle of day and night starts over every day because it takes the Earth one full day to complete a rotation, or spin.
Be careful! During the day, the Sun may SEEM to move in the sky. This is not true. Because the Earth revolves around the Sun, we see the Sun from different points during our revolution. The Sun doesn't move, we do.
Play this game to explore space and day and night. Be patient, it might take some time to load.
Shadows
Sunlight shining on the Earth's surface also creates something else we know - shadows. But how are they formed?
Sunlight shines on opaque (not clear) objects. Since those objects aren't clear, meaning light can't pass through them, they block light. Remember the shade under a tree we talked about earlier? When sunlight hits a tree, an opaque object, the tree blocks the light. This creates a shadow, which we stand in to keep us cool.
Shadows don't only happen outside though. They happen wherever there's a light source, like a lamp or a flame.
If you stand on different surfaces, like a hilly road instead of a flat sidewalk, you might notice your shadow looks different. Go outside and try this yourself!
Watch this super fun video and see if you can answer the questions below. Try to answer the questions on your own, then click on them to see if you were right!
When could Peep and his friends not see their shadows? Why do you think that happened?
When a cloud blocked the Sun and its light, there was no longer sunlight to create shadows.
Can you think of another time during the story that their shadows disappeared? Why couldn’t they see them? How were they able to see them again?
When they took a nap under a shady tree, their shadows disappeared because the tree blocked the light from hitting them. Once they left the shade, the sunlight hit them again and created shadows.
How did Peep and Chirp change their shadows? What did they make their shadows look like?
Chirp holds up a bundle of twigs and Peep notices Chirp's shadow looks like Tom. They get an idea and decided to scare Tom. Peep, Chirp, and Quack work together to make a cat shadow with their bodies. Tom sees on the fence in front of him the shadow of a cat being growled at by Nellie and thinks she is growling at him and runs away!
Shadows can be longer or shorter depending on the time of day. In the morning and afternoon, the Sun seems to be lower in the sky. We talked about this earlier - the Sun isn't moving, the Earth is! Because the Sun is "lower" in the sky, sunlight is cast at a larger angle, and your shadow is longer. At noon/12 pm, the Sun is almost vertical in the sky. Since the Sun is right above you, your shadow is shorter. If you don't believe me, go outside and check!
Seasons
Remember that the Earth orbits (or revolves around) the Sun. It's because the Earth moves this way that we have seasons: Summer, Fall, Spring, and Winter!
The Northern Hemisphere (top half of the Earth) is always in the opposite season of the Southern Hemisphere (bottom half of the Earth). Remember, this is because the Earth is tilted on an imaginary axis!
Notice something really interesting here: because of the Earth's tilt, the North is either tilted towards or away from the Sun, depending on what point in the revolution the Earth is.
If you don't get it yet, don't worry! We're not done. But first, watch Bill Nye the Science Guy explain why seasons happen.
Depending on what part of the year (and part of revolution) the Earth is on, the top half of the Earth is either pointed towards the Sun, or away. On the left, the top half (Northern Hemisphere) is pointed closest to the Sun. This means it gets more sunlight. This time of year, we are in Summer! Now, look on the right! Later in the year, the North is pointed away from the Sun, and the South is pointed closest. This means the North (us) is in winter because it's getting the least sunlight, and the South is in summer!
Here's the Earth at 4 different points in its revolution, or orbit. Look: when the North is tilted towards the Sun (left), we experience Summer. When the North is tilted away (right), we experience Winter. At the points in between, we experience Spring and Fall/Autumn.
The Moon
We finally did it. On July 20th, 1969, we made it to the Moon.
Astronaut Neil Armstrong famously said, "That’s one small step for man. One giant leap for mankind." And it was! America's trip to the Moon was just the start of something special. But before we got there, we had to know some basic (but super important) facts about the Moon. Now, it's time for you to learn them too!
Just like the Earth revolves around the Sun, the Moon revolves around the Earth...all at the same time! Pretty awesome, right?
It takes about 28 or 29 days for the Moon to complete one whole revolution around Earth. Because of this, we use months to tell time, which are around 28 to 31 days long! The words "month" and "moon" actually come from the same word!
