Predictable Phenomena

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Slide 1

Today we are going to talk about predictable phenomena on Earth.

Slide 2

The phenomena we’re particularly interested in are eclipses and the phases of the moon. Eclipses occur when an astronomical object such as the Sun or the moon is temporarily obscured from view.
We’re going to talk about two main types of eclipses: lunar eclipses, which occur when the moon passes into the earth’s shadow, and solar eclipses, which occur when the moon passes between the Earth and the Sun.

Finally, we’re going to discuss the phases of the moon. These are the different shapes that the moon appears to take on as it orbits the Earth.

Slide 3

Let’s start by talking about eclipses.

Slide 4

Remember that the eclipses we’re interested in occur whenever something passes in front of the Sun, blocking out some of its light. The eclipses that we see on Earth are caused either by the Earth blocking out the light of the Sun as it reflects on the moon, or the moon directly blocking out the light of the Sun. Remember, the moon doesn’t really block out the Sun, it just blocks our view of it from the Earth.
The eclipses we can see from the Earth can be classified into two main types: lunar eclipses, which occur when the moon passes into the earth’s shadow, and solar eclipses, which occur when the moon passes between the Earth and the Sun. Don’t forget that you should never look directly at a solar eclipse as you can damage your eyes. Let’s look at lunar eclipses in a little more detail.

Slide 5

Lunar eclipses occur when the moon passes directly behind the Earth and into its shadow. The Earth blocks the Sun’s light, so that it cannot be reflected fully off the moon. Of course, this can only happen when the Sun, the Earth and the moon are lined up in a special way, and the moon is full. We’ll talk more about what that means when we talk about the phases of the moon. So, eclipses don’t occur very often. Only 228 of them will occur during the 21st century. The moon appears to glow red during a lunar eclipse because the Earth’s atmosphere filters the Sun’s light before it is reflected off the moon.

Slide 6

When the Sun’s light shines on the Earth, it forms a shadow shaped like a cone. This shadow has three distinct parts. The umbra is the inside portion of the shadow that lies behind the Earth (when viewed from the sun). It is the darkest part of the shadow cone. This part of the shadow is v-shaped. Another part of the shadow, called the antumbra appears past the point where the tip of the v of the umbra occurs. It is lighter than the umbra. Finally, the penumbra is the lighter, outer part of the shadow cone.

Slide 7

There are three different types of lunar eclipses. The type of eclipse depends on where the moon lies with respect to the Earth’s shadow cone. A total lunar eclipse occurs when the entire moon passes into the umbra (the darkest part of the Earth’s shadow). The moon can appear to be red during a total lunar eclipse because the Earth’s atmosphere acts like a camera filter, blocking out some colours of light. When this happens, we call the moon a “Blood moon”

Slide 8

If the Moon does not fully move into the umbra, we end up with a partial lunar eclipse. Although the Earth moves between the sun and the full moon, the three bodies do not line up perfectly. A small part of the moon’s surface is covered by the Earth’s umbra. It looks like someone has taken a dark bite out of the moon.

Slide 9

If the moon only passes through the penumbra (the lighter outside part of the Earth’s shadow), we end up with a penumbral lunar eclipse. You can only just see penumbral eclipses. The Earth blocks some of the Sun’s light from reaching the moon’s surface. You might, if you’re very observant, notice a dark shading on the moon’s surface.

Slide 10

The other main type of eclipse is the solar eclipse. In a solar eclipse, the moon casts a shadow on the Earth, making the Sun go dark. The new moon moves between the Sun and the Earth, blocking the Sun’s light and casting a shadow on part of the Earth.

Slide 11

There are four types of solar eclipses. In a total solar eclipse, the moon completely covers the Sun. Total solar eclipses can only occur when the moon is near its closest point to the Earth. You can only see a total solar eclipse if you are at the part of the Earth where the moon’s umbra falls. As the moon’s umbra is not very big, total solar eclipses cannot be simultaneously seen at many points on the Earth.

Slide 12

In a partial solar eclipse, only part of the Sun is covered by the moon. It looks like the moon has taken a bite out of the Sun – it looks a bit like Pacman. You can only see a partial solar eclipse if you are at the part of the Earth where the moon’s penumbra falls.

Slide 13

Annular eclipses occur when the moon is near the farthest point of its orbit from the Earth. The moon looks smaller, and you can see a ring or light (or an “annulus”) from the Sun around the moon.

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A hybrid eclipse is a combination between an annular eclipse and a total eclipse. This is the rarest type of solar eclipse. Observers in one place on Earth will see a total eclipse, and observers in another place on Earth will see an annular eclipse. Both events occur at the same time. The duration of these phases of the eclipse is always short.

Slide 15

Now we’re going to have a look at the phases of the moon. In this picture you can see that the moon appears to have a number of different shapes. However, the moon doesn’t really change shape. It just changes its appearance.

Slide 16

As the moon rotates around the Earth, its appearance changes. This is because we see different sides of the moon as it rotates around the Earth. Half of the moon is completely dark. The different shapes we see are the portions of the moon that are directly lit by the sun.

Slide 17

When we look at the moon, we see one of four main shapes. It can be full, a quarter, a crescent, or gibbous as shown in this diagram (point to the parts with the mouse as you go). If you look at the gibbous shape carefully, you’ll notice that it is formed by taking a crescent-shaped bite out of the full moon.

Slide 18

We combine these terms with two new words to give the eight phases of the moon. When the moon is waxing, it is growing. The way the moon appears at each phase is different, depending on where you are on the Earth. In the Southern Hemisphere, it gets bright from the left when it is waxing. When the moon is waning, it is shrinking. In the Southern Hemisphere, it gets dark from the left.

Slide 19

Altogether, the moon has eight phases. Starting with the new moon on the left-hand side of the picture, we move anti-clockwise to the waxing crescent moon (the moon is growing lighter from the left), then we see the first quarter (notice that there’s more light on the left), then waxing gibbous (even more light from the left) to the full moon, which appears to be the largest. Next the moon starts to appear smaller, growing darker from the left. There’s a waning gibbous moon, followed by the last quarter, which is darker, and the waning crescent moon, which is darker still. Finally, we get back to the new moon.

Slide 20

Let’s just do a brief recap of the concepts we’ve discussed in this video. First, eclipses. Eclipses occur whenever something passes in front of the Sun and blocks its light. There are two main types of eclipses. Lunar eclipses, in which the Earth moves between the Sun and the full moon, and solar eclipses, in which the moon comes between the Sun and the Earth. There are three types of lunar eclipses: total, partial and penumbral eclipses, and four types of solar eclipses: total, partial, annular and hybrid eclipses. The rarest of these types is the hybrid eclipse.

Slide 21

Next, we talked about the phases of the moon. Remember that the moon doesn’t change shape at all. It only appears to change shape as the moon revolves around the earth and different parts of it are visible from the Earth. Just like the Earth, the moon has a dark side and a light side. There are eight different phases of the moon. The moon has a different appearance, depending on where you are on the Earth. The phases of the moon, as they appear in the Southern Hemisphere are shown on the top row of the diagram. These are (read in order as they appear on the slide, starting with the new moon). In the Northern hemisphere, the phases of the moon appear to be reflected as seen on the bottom row of the diagram. Closer to the equator, the moon appears to shrink and grow vertically, or in an up and down direction.