1-3 The Sun, Earth and Moon

For a lot of the following phenomena, there is a lot more than what is shown in this simplified explanation. However, this will give you a good idea about the basic ideas behind what causes all of these things to happen.

Rotation vs. Revolution

The words rotation and revolution sound very similar, and many people use them as if they were the same thing. But they are not!

Rotation

When we talk about an object rotating, we are describing how it spins on its axis of rotation. For the Earth, imagine a metal rod so large that it is driven through the planet, going in the North Pole and out the South Pole.

You can see the axis sticking out of the Earth, coloured red. Obviously this rotation is much too fast: the Earth rotates on its axis once per day, in about 24 hours.

Revolution

When an object revolves, we are describing how it moves around another, larger, object. In the Earth's case, it revolves around the Sun.

You can see the axis here as well -- note how it always points the same way in space -- and it goes around the Sun once per year, about 365 days.

The solid black line in the above animation is the Earth's orbit, the path that it takes on its journey around the Sun. If you imagine this orbit as being on a flat surface, we call this the Plane of the Ecliptic. This comes from the word for flat surface in geometry, which we call a plane.

Moon Phases

The Moon orbits around the Earth, as you probably know. (The Moon's orbital plane is tilted about 5° to the Ecliptic.) Since the Moon doesn't give off its own light, it is called a non-luminous object. The Sun, which is a star, gives off its own light; stars are called luminous objects. (Think about the word "illuminate," or the French word for a light, lumière.)

You have seen a Full Moon, with the entire side of the Moon facing us being lit up:

Sometimes we see the Moon with less than its full face lit up. This part of the Moon's cycle, or phase, is a crescent moon:

Depending on where the Moon is in its orbit around the Earth, you will see one of these different phases:

The cycle of phases of the Moon all comes from the positions of the Earth, Sun and Moon in space. In the diagram below, we are looking down on the North Pole of the Earth, and light from the Sun is coming in from the left.

The side of the Earth or Moon that is facing the Sun is lit up by the Sun's light. If you're on Earth and you're facing the Sun, it's daytime; if you're on the other side it's nighttime.

Let's say the Moon is at the "3 o'clock" position in the above diagram. And let's say you're standing on Earth where the red dot is in the diagram below.

The side of the Moon that you see is the side facing the Earth. Since that side is fully illuminated, you will see a Full Moon.

Now let's move the Moon to the "12 o'clock" position. Your view of the Moon will again be of the side facing Earth, but there's a difference now:

The side that you can see has its left side illuminated, and the right side is dark. We call this phase Last Quarter.

If the Moon has moved to the "9 o'clock" position, the side facing us is the totally dark side:

To look up at the Moon, we need to be on the daylight side of Earth; sometimes you can see the Moon in the daytime, so that's not a huge issue. The problem is that the side facing Earth is not illuminated at all. This part of the Moon's cycle is called New Moon, and it's often considered the "start" of the cycle.

Finally, if we put the Moon at the "6 o'clock" position, we get the reverse of the 12 o'clock position:

The observer is going to see the right side illuminated. (If it helps, either turn your screen or your head upside-down to see this better.) This phase is called First Quarter.

There are phases in between New, First Quarter, Full and Last Quarter, as the composite image showing all the phases suggests. A Crescent Moon occurs either just before or just after a New Moon, and a Gibbous Moon is just before or after a Full Moon. The word waxing means "getting larger," and the word waning means "getting smaller." (These are very unusual words in the English language and we don't use them often, except for Moon phases.)

Eclipses

Everything that isn't luminous casts a shadow.

This is true in space as well. We can see the shadow of Jupiter's large moon Io on the tops of its clouds:

The Earth and Moon in space cast shadows as well. You might think that every New Moon would result in the Moon "getting in the way" of light coming from the Sun, but this doesn't happen all the time, since the orbital planes of the Earth around the Sun, and the Moon around the Earth, are tilted 5.2° relative to each other. (The diagram below greatly exaggerates this, and it is not to scale.)

As such, it's very rare that the shadow from the Moon falls on Earth, or that the Moon passes into Earth's shadow. But, sometimes things line up and you get an eclipse.

Lunar Eclipse

A lunar eclipse happens when the Moon passes through the shadow of the Earth. This can only happen during a Full Moon.

Light from the Sun bends as it passes through Earth's atmosphere. Since the bluish-coloured light bounces down towards the Earth (which is why the sky is blue), this leaves the reddish-orange end of the rainbow to pass through, bent inwards. So, the Earth's shadow isn't a uniform black, but more of a dark reddish-orange.

This is the light that hits the Full Moon's surface during a lunar eclipse, giving it an eerie colour. Sometimes the media call it a "blood moon" because of that colour.

The penumbra is the partial shadow of something, but the umbra is the total shadow where the entire source is blocked.

For a video showing these parts as the Moon passes through the shadow, go here.

Solar Eclipse

While being very pretty, lunar eclipses aren't the dramatic ones: a solar eclipse, where the Moon's shadow passes over the Earth, are pretty spectacular.

Here you can see the shadow on the Earth's surface, as seen from the International Space Station.

From the Earth, looking up towards the Sun during a total solar eclipse, the Sun's visible surface is blocked by the Moon, which just happens to be about the same size in the sky. This allows us to see the Sun's atmosphere, the corona, which is normally outshone by the Sun itself.

Seasons

We are closest to the Sun in January.

"But if we're closer to the Sun," you might ask, "shouldn't we be warmer in January, not colder?"

This is because (a.) we are only a tiny bit closer in January than in July, and (b.) the difference in temperature is caused by the angle at which the Sun hits the Earth's surface.

Consider two identical flashlights: one shines light straight down at a table in a dark room, and the other shines light down at an angle to the table.

If we look at the shape that the light makes on the table, we can see that the area A is much bigger than the area B:

If the same amount of light energy is spread over a larger area, any one square unit of table is going to receive less energy in area B than in area A.

Since the light from the Sun heats the Earth, if a given area on Earth's surface receives less energy, it's not going to heat up as much.

The graphic below shows how the light from the Sun hits different parts of Earth's surface differently, at different times of the year.

(Allentown is a city in the United States which has a similar latitude to ours, so the angles are going to be pretty similar.)

It also helps to realize that the Earth's axis always points the same way in space.

So, in the Northern Hemisphere in June, that part of the Earth gets more direct/concentrated sunlight than the Southern Hemisphere. That makes it summer in Canada and winter in Australia.

The opposite situation happens in December: the North Pole points away from the Sun, and the Northern Hemisphere gets more-angled sunlight, causing that part of the Earth to heat up less. That makes it winter in Canada and summer in Australia.

This video gets a little silly at times, but its explanation is quite clear.

Tides

Between Nova Scotia and New Brunswick is a part of the Atlantic Ocean called the Bay of Fundy.

Far up into the Bay, the water level rises and lowers twice a day, very dramatically. This photo shows the same boats in the water, about six hours apart.

Tides are the regular rising and lowering of water levels on Earth's surface. These are somewhat noticeable along ocean coasts, but at the Bay of Fundy the size and shape of the Bay greatly increases the change in water level.

But, what causes tides?

The Earth and Moon pull on each other, through gravity.

All objects that have mass can pull on other objects, using gravity. The Earth and the Moon pull on each other with the same force, but since the Moon is less massive, the gravitational pull affects the Moon more than the Earth: it's pulled into an orbit around the Earth. (This is simplified a bit but that's the main idea.)

However, since gravity is a two-way street, the Moon pulls back on the Earth. It has less of an effect, but one thing it does is that it "sloshes" the water around on Earth's surface, causing a tidal bulge in the Earth's ocean's level.

You can see here that there are in fact two tidal bulges on Earth. One is caused by the Moon's gravitational pull, but the one on the other side of the Earth is caused by a variety of forces, summarized here as "inertia." It's not important right now that you know what causes this second bulge; just know it's there.

While these tidal bulges stay in the same place relative to the Moon, keep in mind that the Earth is rotating all time. Since there are two bulges, any one point on Earth is going to rotate through one of them approximately every 12 hours. (It's a little more than that, because the Moon revolves around the Earth at the same time.)

Incidentally, there are tides on the Great Lakes... but they are only a few centimetres in height, so you would never notice them! This article nicely describes what would actually cause water levels on these lakes to change.

Practice

The Basics

1. What causes seasons on Earth?

2. Explain the difference between a solar eclipse and a lunar eclipse.

3. How is a New Moon different from a Full Moon?

Extensions

1. Uranus has an axis which has an unusual tilt: its poles essentially point straight at, or straight away from, the Sun at its extremes. What effect would this have on its seasons?

2. Is it safe to look at a lunar eclipse without eye protection? What about a solar eclipse?