E. Earth and Space Science: Space Exploration · Space Exploration: The Sun, the Solar System and the Universe

Investigating the Moon, Eclipses, Tides and Meteor Showers

E2.6 A1.2 A1.3About 75 minutes

Learning goals

I can use a model to explain the phases of the Moon.
I can explain why eclipses happen and why they do not happen every month.
I can explain spring and neap tides.
I can explain the causes of meteor showers.
I can design and evaluate a physical model of the Sun–Earth–Moon system.

Words to know

synodic month
The time from one new moon to the next, about 29.5 days.

sidereal month
The time the Moon takes to orbit Earth once relative to the stars, about 27.3 days.

umbra
The darkest, central part of a shadow, where the light source is completely blocked.

penumbra
The lighter, outer part of a shadow, where the light source is only partly blocked.

annular eclipse
A solar eclipse in which the Moon is too far from Earth to cover the Sun completely, leaving a bright ring.

spring tide
An extra-large tidal range that happens near new and full moon, when the Sun and Moon line up.

neap tide
A smaller tidal range that happens near the quarter moons, when the Sun and Moon pull at right angles.

meteor shower
Many meteors appearing to come from one point in the sky, when Earth passes through a stream of debris left by a comet or asteroid.

Remember from Grade 6

In Grade 6 you learned that the Moon's phases come from seeing different amounts of its sunlit half, that solar eclipses happen at new moon and lunar eclipses at full moon, and that the Moon's gravity causes tides. In Grade 9 you will dig into the details: why eclipses are rare, why the Moon always shows the same face, why some tides are bigger, and where meteor showers come from. You will also build and test a model.

A question to start

New moon happens about once a month. A solar eclipse can only happen at new moon. So why don't we get a solar eclipse every month? Keep this puzzle in mind; you'll be able to answer it by the end of the lesson.

The phases of the Moon, in detail

The Sun always lights half of the Moon.
As the Moon orbits Earth, we see different fractions of that lit half.
Waxing means the lit part we see is growing (new → full); waning means shrinking (full → new).
Crescent means less than half lit; gibbous means more than half lit.
One cycle of phases (new moon to new moon) is a synodic month: about 29.5 days. The Moon actually completes one orbit relative to the stars in about 27.3 days (a sidereal month), but because Earth has moved along its orbit around the Sun, the Moon needs about two more days to line up with the Sun again.
The Moon rises about 50 minutes later each day, on average. A full moon rises around sunset; a first-quarter moon is high in the south around sunset.
The Moon always shows us the same face. It spins on its axis exactly once per orbit, a result of Earth's gravity over billions of years (this is called tidal locking). There is a far side we never see from Earth, but it is not always dark: it gets sunlight during our new moon.

The Moon's position in its orbit (inner circles, lit half facing the Sun) and the phase we see from Earth (labels). The right-hand side of each Moon is always the lit side, but we see different amounts of it.

Eclipses, in detail

Every shadow from a large light source has two parts: the dark umbra and the lighter penumbra.

Solar eclipse (at new moon): the Moon's shadow falls on Earth.

People in the small umbra see a total eclipse: the sky darkens and the Sun's corona appears.
People in the larger penumbra see a partial eclipse.
The Moon's orbit is an ellipse. When the Moon is near its farthest point, it looks slightly too small to cover the Sun: we get an annular eclipse with a bright 'ring of fire'.

Lunar eclipse (at full moon): the Moon passes into Earth's shadow. Everyone on the night side of Earth can see it. During totality, the Moon often turns coppery red, because Earth's atmosphere bends some sunlight into the shadow, and the air scatters away blue light and lets red pass, the same reason sunsets are red.

Why not every month? The Moon's orbit is tilted about 5° compared with Earth's orbit around the Sun. Most months, the new or full moon passes a little above or below the Sun–Earth line, so the shadows miss. Eclipses happen only when a new or full moon occurs near the two points where the Moon's orbit crosses Earth's orbital plane.

Solar and lunar eclipses (not to scale). In a solar eclipse, the Moon's small umbra touches a narrow strip of Earth. In a lunar eclipse, the Moon passes through Earth's much larger umbra.

Tides, in detail

The Moon's gravity pulls more strongly on the side of Earth facing it and less on the far side. This stretches Earth's oceans into two bulges: one facing the Moon and one on the opposite side. As Earth rotates through the bulges, most coasts get two high tides and two low tides about every 24 hours and 50 minutes.

The Sun also creates tides, about half as strong as the Moon's.

Spring tides (nothing to do with the season): at new and full moon, the Sun and Moon line up and their tides add together. High tides are higher and low tides are lower.
Neap tides: at first and third quarter, the Sun and Moon pull at right angles. The tidal range is smaller.

The shape of a coastline matters too. The Bay of Fundy, between New Brunswick and Nova Scotia, has some of the highest tides in the world: the difference between high and low tide can be about 16 metres in some places.

Meteor showers

Comets (and some asteroids) leave trails of dust along their orbits. When Earth passes through a trail each year, many tiny particles hit our atmosphere and burn up as meteors. Because the particles travel in parallel paths, the meteors seem to come from one point in the sky, called the radiant.

Perseids: peak around August 12 each year; debris from Comet Swift–Tuttle.
Geminids: peak around December 13–14; debris from an asteroid called 3200 Phaethon.

The best viewing is after midnight, away from city lights, with no bright moon.

Worked example

Example 1: Predicting a phase

Tonight there is a first-quarter moon. What phase will it be in about one week? About two weeks?

One cycle is about 29.5 days, so each quarter of the cycle is about 7.4 days.
First quarter + about 1 week → full moon.
First quarter + about 2 weeks → third quarter.

About full in one week and third quarter in two weeks.

Worked example

Example 2: Explaining a rare event

Use evidence to explain why total solar eclipses are seen from any one place only rarely, even though they happen somewhere on Earth about every year or two.

A solar eclipse needs a new moon near a crossing point of the Moon's tilted orbit, which happens only a couple of times a year.
Even then, the Moon's umbra is small when it reaches Earth, so totality is visible only in a narrow path (often about 100 to 200 km wide).
That path is in a different place each time, and much of Earth is ocean.

Because the umbra covers only a narrow path that moves to different places, any one spot sees totality only rarely. (Parts of Ontario saw totality on April 8, 2024, as you learned in Grade 6.)

Hands-on activity

Design challenge: a working Sun–Earth–Moon model

You need: a lamp with the shade removed (the Sun), a ball about the size of a tennis ball or a styrofoam ball on a pencil (the Moon), your own head (Earth), a dark room. Optional: a second small ball and a hula hoop or string circle to model the 5° tilt.

Grown-up note: bare bulbs get hot; do not touch the bulb and keep paper away from it.

Phases: stand about 2 m from the lamp. Hold the Moon at arm's length, slightly above your head. Turn slowly counterclockwise (to your left) in a full circle, watching the lit part of the ball. Sketch the 8 phases.
Solar eclipse: turn so the Moon is between your eyes and the lamp. Adjust until the ball blocks the bulb. Where is the Moon's shadow?
Lunar eclipse: turn so your head is between the lamp and the Moon. Watch the ball enter your head's shadow.
Why not every month? Now hold the Moon a little above your head's level for the whole orbit (a tilted orbit). Do you still get eclipses each time?
Evaluate your model: list two things it shows well and two things it shows badly (think about sizes, distances, and the Sun's brightness).

What to look for: the ball shows all 8 phases in the right order; eclipses happen only when the ball, head and lamp line up exactly; with the tilted orbit, the shadows usually miss, which explains why eclipses are rare. A good evaluation notes that the model's Moon is much too close and too big compared with real distances, and the lamp is not to scale.

Just for fun

Riddle: Why did the Moon skip dinner?

Because it was already full. (Don't worry, give it two weeks and it'll be new.)

Your turn

What is the difference between waxing and waning?

Why is the synodic month (29.5 days) longer than the sidereal month (27.3 days)?

Why do we always see the same side of the Moon?

What is an annular eclipse?

Why are there not eclipses every month?

When do spring tides happen, and why are they larger?

What causes the Perseid meteor shower?

More practice: warm-up

Which phase comes after a waxing gibbous moon?

At which phase can a lunar eclipse happen?

What is the dark central part of a shadow called?

About how many high tides do most coasts get per day?

Around what date do the Geminids peak?

More practice: on your own

Why does the Moon turn red during a total lunar eclipse?

Why can everyone on the night side of Earth see a lunar eclipse, but only a narrow strip sees a total solar eclipse?

A full moon rose at 7:00 p.m. About when will the Moon rise tomorrow?

Why are meteors in a shower seen best after midnight?

What is a neap tide, and at which phases does it happen?

Challenge

If the Moon's orbit were not tilted at all, how often would we have solar and lunar eclipses?

Astronauts standing on the Moon during a lunar eclipse (on Earth) would see what event in their sky?

The Moon is slowly moving away from Earth (a few centimetres per year). Predict what will happen to total solar eclipses in the very distant future, and explain.

Check your understanding

Quiz

What causes the phases of the Moon?

A solar eclipse can only happen at:

Why don't eclipses happen every month?

Spring tides happen when:

A meteor shower happens when:

About how long is one cycle of the Moon's phases?

For parents and teachers

Big idea: E2.6 asks students to conduct investigations to explain the causes of astronomical phenomena observable from Earth. Grade 6 introduced phases, eclipses and tides; Grade 9 adds the detail (umbra and penumbra, the 5° tilt, spring and neap tides, synodic vs sidereal months, meteor showers) and asks students to build and evaluate a model (A1.2, A1.3).

Common difficulties:

Thinking phases are caused by Earth's shadow. Only lunar eclipses involve Earth's shadow.
'Spring tides happen in spring.' The name means 'springing up'.
Why eclipses are rare: the 5° tilt is the key.

Safety: supervise any lamp with a bare bulb. Never look at the Sun, even during a partial eclipse, without certified eclipse glasses.

How to help: keep a two-week Moon journal together: sketch the Moon and note the time and direction each evening. Check the dates of the next Perseid or Geminid shower and plan a safe viewing night.