Why we have seasons, eclipses and tides
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The Earth is closest to the Sun in early January, in the middle of the British winter. So distance cannot be what makes summer warm. Seasons, eclipses and tides all come from the way the Sun, Earth and Moon are arranged, and each has a well-known wrong explanation to get out of the way first.
Seasons: the tilt, not the distance
The Earth's axis is tilted by about 23.4°, and it keeps pointing the same way in space (towards the Pole Star) all year. So as the Earth orbits, the Northern Hemisphere leans towards the Sun for part of the year and away from it six months later.
When your hemisphere leans towards the Sun, two things happen:
- The Sun climbs higher at midday. Its light hits the ground more directly, so the same beam is concentrated on a smaller area and heats it more.
- Days are longer than nights, giving more hours of heating.
Together these give summer. When your hemisphere leans away, the Sun stays low and days are short: winter. That is why summer in London comes at the same time as winter in Sydney.
| Date (approx.) | Name | In the Northern Hemisphere |
|---|---|---|
| 20 March | March (spring) equinox | Day and night roughly equal |
| 21 June | June (summer) solstice | Longest day |
| 22–23 September | September (autumn) equinox | Day and night roughly equal |
| 21 December | December (winter) solstice | Shortest day |
The same tilt sets the positions of the tropics and the polar circles, covered in latitude and longitude. Even at midsummer the Sun is never directly overhead in the UK; that happens only between the tropics.
Try it: in a darkened room, with an adult, shine a torch straight down onto squared paper and count the squares it lights. Now tilt the torch. The same light spreads over more squares, so each square gets less. That is winter sunshine.
Eclipses: three bodies in a line
An eclipse happens when one body moves into the shadow of another.
Solar eclipse
The Moon passes directly between the Sun and the Earth, and its shadow falls on part of the Earth. This can only happen at new moon.
The Sun is about 400 times wider than the Moon but also about 400 times further away, so the two look almost exactly the same size in our sky. That coincidence lets the Moon just cover the Sun. In a total eclipse the sky darkens and the Sun's faint outer atmosphere, the corona, appears. Only a narrow strip of the Earth sees totality; places either side see a partial eclipse.
On 12 August 2026 a total eclipse crossed Greenland, Iceland and northern Spain, while the UK saw a partial one. The next total eclipse, on 2 August 2027, crosses southern Spain and North Africa.
Safety. Never look directly at the Sun, even when most of it is covered, and never through binoculars, a telescope or a camera viewfinder. It can permanently damage your eyes, and it does not hurt while it happens. Sunglasses are not safe. Use certified eclipse glasses, or a pinhole projector that casts an image of the Sun onto card, and always do this with an adult.
Lunar eclipse
The Earth passes between the Sun and the Moon, and the Earth's shadow falls on the Moon. This can only happen at full moon, and it is completely safe to watch.
The Moon rarely disappears. It usually turns a coppery red, because a little sunlight is bent through the Earth's atmosphere into the shadow, and the atmosphere lets red light through more easily than blue.
Why not every month?
The Moon's orbit is tilted by about 5° to the Earth's orbit. Most months, the new and full moons pass a little above or below the Sun–Earth line, and the shadows miss. Eclipses only happen when a new or full moon falls near the points where the two orbits cross.
The Earth's shadow is also not the cause of the Moon's ordinary phases; if it were, every full moon would be an eclipse. See the phases of the Moon.
Tides: the Moon's uneven pull
Most coasts have two high tides and two low tides roughly every day. The main cause is the Moon's gravity.
The Moon pulls hardest on the side of the Earth nearest to it, less on the centre, and least on the far side. That difference stretches the oceans into two bulges, one facing the Moon and one on the opposite side. As the Earth turns, a coastline passes through both bulges, so it gets two high tides. Because the Moon also moves along its orbit, the pattern arrives about 50 minutes later each day.
The Sun affects tides too, but less, because it is so much further away.
- Spring tides come around new moon and full moon, when Sun, Earth and Moon are in line. The pulls combine: high tides are higher and low tides lower. They happen twice a month all year round and have nothing to do with the season.
- Neap tides come around first and last quarter, when the Sun and Moon pull at right angles. The range between high and low water is smallest.
Worked example: predicting the tide
High tides come roughly every 12 hours 25 minutes. A harbour has high tide at 06:00 on Monday.
- Next: 06:00 + 12 h 25 min = 18:25 on Monday.
- Then: 18:25 + 12 h 25 min = 06:50 on Tuesday.
- Tuesday's morning high tide is about 50 minutes later than Monday's.
Real times differ because coastlines and the seabed shape the water's movement, so always use the official tide table before going on a beach.
Check yourself
True or false? Correct the false ones.
- A lunar eclipse can only happen at full moon.
- The Earth is closest to the Sun in July.
- Neap tides have the smallest range between high and low water.
- Dark sunglasses make it safe to look at a partial solar eclipse.
- When it is summer in the UK, it is winter in Australia.
(1 true. 2 false: early January. 3 true. 4 false: sunglasses do not protect your eyes. 5 true.)
For a quick self-test on all three topics, try this seasons, eclipses and tides gap-fill on ModernScholar. Teachers who want a labelled diagram for the class can build a simple sheet using EduPrintables' free sheet builder.
Frequently asked questions
Why are July and August usually warmer than June in the UK?
Land and sea take time to warm up. After the longest day they are still gaining more heat than they lose, so the warmest weeks come later. This delay is called seasonal lag.
How often is there a total solar eclipse?
Somewhere on Earth, roughly every 18 months on average. Any one place sees one only rarely, often centuries apart, because the path of totality is so narrow.
Are there tides on lakes?
The Moon pulls on lakes too, but even large lakes are too small for a noticeable tide. The effect shows clearly only on oceans and the seas connected to them.
Sources and further reading
- What causes the seasons? – NASA Space Place – a clear explanation of the tilt, written for young readers.
- Tides – NASA Science – how the Moon and Sun create tides, including spring and neap tides.
- Eclipse viewing safety – NASA Science – how to watch a solar eclipse without harming your eyes.
- Tides and water levels – NOAA Ocean Service – a fuller tutorial on how tides work.
