The Moon and the Tides, Grade 7: Phases, Gravity, Spring and Neap

There are two high tides on opposite sides of Earth at the same time, and the usual explanation of the second one is not a mechanism at all.

You will read that the far-side bulge happens because the land is stretched on the side nearest the Moon and "squashed" on the side furthest from it. Squashing of the land is not why, and it does not mean anything physical.

The real reason is simpler and a Grade 7 can hold it: the Moon does not pull everything equally.

Why there are two bulges, properly

The Moon's pull gets weaker with distance. So it pulls three things by three different amounts.

What Distance from the Moon Pulled
The water on the near side Closest Hardest
The solid Earth in the middle In between Less
The water on the far side Furthest Least of all

So the near-side water is pulled away from Earth, and Earth is pulled away from the far-side water. Both end up as bulges. Nothing has been squashed.

The sentence to give a class: on the near side the water is pulled towards the Moon more than the planet is, and on the far side the planet is pulled towards the Moon more than the water is. Two bulges, one cause, no squashing.

One drawing to watch: you may see the bulge shown as a single teardrop pointing at the Moon, with a second figure a page later correctly showing two symmetric bulges. The second one is right.

Where this fits in the curriculum

Subject Natural Sciences
Grade 7
Term 4
Strand Planet Earth and Beyond
Topic The relationship of the Moon to the Earth
Status Class activities. Term 4 carries no formal assessment
Marks The topic revision is 20. Our worksheet is 40

The missing fact: the Moon is tidally locked

You will read that we see "the half of the Moon that is on the inside of the line of its orbit". That is not meaningful geometry.

We always see the same face of the Moon because the Moon turns exactly once on its axis for every one orbit of Earth. That is called tidal locking, and it is the single most interesting fact in this topic. It usually does not appear at all.

Why it happens: Earth's gravity raised a bulge in the solid Moon long ago, and the friction of that bulge being dragged around slowed the Moon's spin until the two periods matched. The same process is slowly lengthening Earth's day.

The demonstration takes thirty seconds and settles it. Have one learner stand still as Earth. A second walks a circle around them, keeping their face pointed at Earth the whole way. By the time they get back they have turned around once, and Earth never saw their back. Then do it again without turning, facing the same wall throughout, and Earth sees every side. The first version is the Moon; the second is not.

Two numbers to get right

What you will read What is actually the case
"It takes the Moon about 29,5 days to orbit Earth" 29,5 days is the time from one New Moon to the next, which is the synodic month. The orbit itself takes 27,3 days. The two differ because Earth has moved along its own orbit in the meantime, so the Moon has to go a little further to line up with the Sun again
"There are about 167 moons in our Solar System" Well over 400 now, and Saturn alone has 274 confirmed. The count rises every year as surveys find more. Our own Grade 8 solar system page publishes the current figures
"Every 24 hours the tide rises and falls twice" The tidal day is about 24 hours and 50 minutes. That is why high tide arrives roughly 50 minutes later each day, and why a tide table is a table rather than a fixed time

That 50 minutes is the detail a coastal class will already have noticed, and it is worth the minute it takes. The Moon has moved on in its own orbit while Earth turned, so Earth has to turn a bit further to bring the same place back under it.

Phases of the Moon

The Moon makes no light of its own. Half of it is always lit by the Sun. What changes is how much of the lit half we can see from where we are standing.

Phase Where the Moon is What you see
New Moon Between Earth and the Sun Nothing. The lit side faces away
First quarter A quarter of the way round Half lit
Full Moon Earth between the Moon and the Sun The whole lit face
Last quarter Three quarters round The other half lit

A southern hemisphere note worth having: a waxing crescent here is lit on the right as you look at it, where in Europe it is lit on the left. Diagrams drawn for the northern hemisphere have the lighting mirrored. Check any phase strip you hand out against the sky outside.

Gravity, and what this topic gets right

The numbers in this topic are good, which is worth saying because the tide mechanism is not.

Fact Status
Earth's mass is about 6 x 1024 kg Correct
The Sun holds more than 99,8 % of the mass of the Solar System Correct, and better than the Grade 8 figure of "about 99 %"
The Moon's gravity is about one sixth of Earth's Correct
Earth's orbit around the Sun is almost circular Correct, and it contradicts the elliptical orbit drawn in the Sun topic. See our Sun and the seasons page
The planetary mass table, from Mercury at 0,055 to Jupiter at 317,83 Earth masses Correct throughout

Gravity as a force is handled on our Grade 9 types of forces page, where it is measured with a force meter and weight is separated from mass. Here the job is different: gravity as the thing that holds the Moon in orbit and raises the tides.

Spring and neap tides

When What happens
Spring tide New Moon and Full Moon The Sun and Moon pull in line, so the bulges add up. Highest highs and lowest lows
Neap tide First and last quarter The Sun and Moon pull at right angles, so they partly cancel. The smallest range

Spring tides have nothing to do with the season. The name comes from the water springing up, and they happen twice a month all year round. It is the single most common misunderstanding in this topic and it is purely the word.

And the low tides matter as much as the highs. A spring low is the lowest water of the month, which is why that is when the rock pools are worth walking.

The shore, which is the best biology in the topic

This part is genuinely good and genuinely local, and worth more time than it usually gets.

Animal What it does with the tide
Plough snail Rides the swash up and down the beach, using its foot as a sail to follow the water and find stranded food
White mussel Buries itself and filters the water through two siphons, one in and one out
Sand hopper Lives in the kelp wrack thrown up at the high-water mark

West coast sea temperatures of 6 to 14 °C are correct for the Benguela, which is why our west coast feels so much colder than the east. And kelp is correctly called a seaweed here, which is worth noting because the Grade 7 biosphere topic earlier gave it "stems" and "leaves". Kelp is an alga: it has a holdfast, a stipe and a blade.

One claim in the research skills section that is wrong

"Plagiarism is a serious criminal offence."

It is not a crime. Plagiarism is academic and professional misconduct. It can get you expelled, fired or stripped of a qualification, all of which are serious, but nobody is arrested for it.

Copyright infringement is the separate legal matter, and in most cases it is a civil dispute rather than a criminal one.

Worth correcting because the real consequences are quite bad enough without overstating them, and a learner who finds out the claim was false may discount the whole warning.

If it does not work

Problem Cause Fix
A learner cannot see why there are two bulges The squashing explanation Three distances, three different pulls. Earth is pulled away from the far-side water
The class thinks spring tides happen in spring The word Twice a month, all year. The water springs up
The phase diagram does not match the sky It was drawn for the northern hemisphere Check it against the Moon outside. A waxing crescent is lit on the right here
The walking demonstration shows nothing The learner is not keeping their face to the centre Face Earth the whole way. Then repeat facing one wall and compare
Two sources give different moon counts The number rises every year Record the year. Over 400 now, 274 at Saturn alone
A learner writes that the Moon orbits in 29,5 days The synodic month was quoted as the orbit 27,3 days to orbit. 29,5 from New Moon to New Moon

How the 40 marks are made up

Part Marks
Phases, and why we always see one face 10
Gravity, orbits and the numbers 8
The tides, and the two bulges 12
Spring and neap tides 6
Life on the shore 4

Term 4 carries no formal assessment. The topic revision is 20 marks and it adds up correctly. Our worksheet is 40 and the split above is ours.

Free worksheet and marking memo

Both free, no sign up, straight to the PDF.

Related practicals

What you need to run it

Item Price What it is for
World globe, 30 cm R400 The Earth in the walking model, and it is the same globe the seasons topic needs

Almost everything here is free. The tidal locking demonstration needs two learners and a floor. The phases need a ball, a lamp and a dark room. The globe is the only thing worth buying, and it does double duty across both Term 4 topics.

One honest note. The obvious product for this topic is a Sun, Earth and Moon orbitor, which turns all three on a geared arm. Ours is out of stock. It is a R1 950 item and we are not going to link you to something you cannot buy. Two learners walking a circle teach tidal locking better than any gear train anyway, because the learner has to do the turning themselves.

The rest of the astronomy range is in astronomy and the charts are in educational wall charts.