Science notes, unit 10

Eclipse

Textbook Science Year 6 (DLP, Semakan 2017), pages 165–178

What this chapter covers

You can explain how an eclipse of the Moon happens and why it only occurs at full moon (10.1.1)
You can explain how an eclipse of the Sun happens and why it only occurs at new moon (10.1.2)
You can explain why eclipses do not happen every month, using the tilt of the Moon's orbit (10.1.1, 10.1.2)
You can link eclipses to the properties of light, and name the umbra and the penumbra (10.1.3)
You can tell a total eclipse from a partial eclipse and watch an eclipse of the Sun safely (10.1.3)
You can predict the conditions on Earth during an eclipse (10.1.4)
You can use a model and sketches to describe eclipses (10.1.5)

Sometimes the Sun seems to vanish in the middle of the day, or the full moon turns red. These are eclipses. In this unit you will find out how they happen, why they are rare, and how to watch them safely.

10.1Eclipse of the Moon and eclipse of the Sun

Eclipse of the Moon

An eclipse of the Moon happens when Earth is between the Sun and the Moon in a straight line. Earth blocks some or all of the sunlight, so the shadow of Earth falls on the Moon.

Sun, Earth and Moon in a straight line, with the shadow of Earth falling on the Moon — Eclipse of the Moon: Sun, Earth, Moon in a straight line. The Moon is inside the shadow of Earth.

It can only happen at the full moon phase.
During the eclipse the Moon often turns reddish-brown.

Notes follow Science Year 6 (DLP), pp.166-167. Figures are redrawn, not to scale.

Eclipse of the Sun

An eclipse of the Sun happens when the Moon is between the Sun and Earth in a straight line. The Moon blocks some or all of the sunlight, so the shadow of the Moon falls on Earth's surface.

Sun, Moon and Earth in a straight line, with the shadow of the Moon falling on Earth — Eclipse of the Sun: Sun, Moon, Earth in a straight line.

It can only happen at the new moon phase. At new moon the Moon's surface facing Earth is dark, so we cannot see it.
It can only be seen in the daytime.

	Eclipse of the Moon	Eclipse of the Sun
Who is in the middle?	Earth	Moon
Whose shadow?	Earth's shadow on the Moon	Moon's shadow on Earth
Moon phase	Full moon	New moon
Time we see it	Night	Daytime

Do not mix up the two. In an eclipse of the Moon, the shadow lands on the Moon. In an eclipse of the Sun, the Moon's shadow lands on us.

Why not every month?

The Moon goes round Earth about once a month, so there is a full moon and a new moon every month. But there is not an eclipse every month. The Moon's orbit is tilted about 5° from Earth's orbit round the Sun. So at most full moons the Moon passes above or below Earth's shadow, and at most new moons its shadow passes above or below Earth.

Side view: the Moon's orbit is tilted so the Moon passes above the shadow of Earth — The tilt of the Moon's orbit (drawn bigger than it really is) lifts the Moon above Earth's shadow.

Each kind of eclipse can happen about twice a year.

Is there always an eclipse of the Moon at full moon? Explain.

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No. The Moon's orbit is tilted about 5°, so the Moon is often above or below Earth's shadow and the Sun, Earth and Moon are not in a straight line.

10.2Eclipses and the properties of light

Sunlight travels in a straight line and cannot pass through opaque objects such as Earth and the Moon. That is why they make shadows. Every shadow in an eclipse has two parts.

Umbra is the darkest part of a shadow. Penumbra is the partly dark area around the umbra.

The shadow of Earth with a darker umbra and a lighter penumbra, and the Moon at three positions A, B and C — The Moon at A is in the penumbra, B is at the edge of the umbra, C is inside the umbra.

Where the Moon (or the observer) is	What is seen
Moon completely in Earth's umbra	Total eclipse of the Moon
Moon only partly in Earth's umbra	Partial eclipse of the Moon
On Earth inside the umbra of the Moon's shadow	Total eclipse of the Sun (and a partial one just before and after)
On Earth inside the penumbra of the Moon's shadow	Partial eclipse of the Sun only

The Moon's shadow reaching Earth, with a small dark umbra and a wide light penumbra — Point P is in the umbra, so P sees a total eclipse of the Sun. Point Q is in the penumbra, so Q sees only a partial eclipse.

Did you know? The reddish-brown Moon in a total eclipse is lit by sunlight that bends through Earth's atmosphere. It is the same effect that makes sunrise and sunset look red.

Where must you stand to see a total eclipse of the Sun?

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In the path of the umbra of the Moon's shadow. In the penumbra you only see a partial eclipse.

Model and sketch

In the textbook activity, a torch is the Sun, a clay sphere is Earth and a pin on a plastic circle is the Moon. Move the pin into the line between the torch and the sphere in a dark room and look at the shadow on a white card. When you sketch, label the Sun, Earth, Moon, umbra and penumbra, and draw straight light rays.

10.3Conditions on Earth during an eclipse

Eclipse of the Sun: the surroundings become darker, like evening, and the temperature falls. Animals react as if night is coming: birds return to their nests and some animals become quiet.
Eclipse of the Moon: the level of the sea tide rises.

Safety. Never look straight at the Sun, even during an eclipse. It can damage your eyes permanently. Use special filter glasses or a pinhole camera. It is safe to look at an eclipse of the Moon with the naked eye.

Four small bright circles with a bite taken out, images of a partly eclipsed Sun formed on a card — A pinhole camera lets sunlight form a safe image on a card.

Pinhole camera: cut a square hole in a card, cover it with aluminium foil, and prick a few small holes in the foil. Hold a second white card behind it so sunlight through the holes forms an image on that card. Look at the card, never at the Sun.

Name two things that happen on Earth during a total eclipse of the Sun.

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It becomes darker and the temperature drops. Animals such as birds go back to their nests.