Physics Is Life
Me

Unit 3, lesson 17. About 15 minutes.

Planets, the Sun and tides

Why does the sea come up the beach and go back again, twice a day?

Play

Getting the experiment ready…

Explain

Planets go around the Sun.

The Moon pulls on the sea.

The Sun's gravity keeps all the planets in orbit. The further away a planet is, the longer it takes to go around.

Earth takes one year. Jupiter takes almost 12!

The Moon's gravity pulls on our oceans. It makes the sea rise and fall on the shore. These are the tides.

Kepler's third law: the square of a planet's orbit time is proportional to the cube of its distance from the Sun. With years and astronomical units, T² = a³.

Tides happen because the Moon's gravity is stronger on the near side of Earth than the far side, so the ocean bulges on both sides.

Worked example: Jupiter is at 5.2 AU. So T = 5.2^(3/2) ≈ 11.9 years.

  1. The Sun pulls the planets
  2. Further planets are slower
  3. The Moon pulls the sea
Want the full story?

Orbits are really ellipses, not perfect circles, and the sim draws them as circles. Earth spins under the two ocean bulges, so most coasts get two high tides each day. The Sun adds its own smaller tide, and real tides also depend on the shape of the coast.

The Formula

years to go round the Sun = distance from the Sun, to the power of one and a half

T=a3/2T = a^{3/2}
TT
the time for one orbit, measured in Earth years (years)
aa
the distance from the Sun, measured in astronomical units, where 1 AU is the Earth–Sun distance (AU)

Try the numbers

Time for one orbit: 1 years

Try it at home

You need a chair, some space to walk.

You need: a chair, some space to walk.

Walk around a chair, like Earth around the Sun.

  1. Put a chair in the middle to be the Sun.
  2. Walk slowly around it in a circle, close in.
  3. Now walk around it further away, at the same speed.

What's happening: the far path is much longer, so it takes more time. Far planets also move more slowly, so their year is much longer.

Physics is life

Your body

Your birthday comes once every orbit.

You go around the Sun once for every birthday. How many orbits have you done?

Daily life

Fishing boats need tide times.

Sailors, fishers and surfers plan their day with tide tables.

Nature

Crabs live by the tides.

Many sea animals time their lives by the tides, feeding when the sea comes in.

Space

Planets take years to go around.

Neptune takes about 165 Earth years to go around the Sun once.

Quick check

Question 1 of 3

What keeps planets in orbit?

Words to know

Tide: The daily rise and fall of the sea, caused mainly by the Moon's gravity.

Tide
The daily rise and fall of the sea, caused mainly by the Moon's gravity.
Planet
A large round world that orbits a star.
Orbit
The curved path of one thing around another, held by gravity.

Meet the scientist

Next up

What is energy? Forms of energy