Gravity and Orbits

How & why things go round

The earth’s orbit …

… is an ellipse.

Earth

Sun

Orbits

• An ellipse is a squashed circle• In the solar system, all the planets

have elliptical orbits• The sun in nearly in the centre of

these orbits

Stars

• The Sun is a star• The stars that we see change

position in the sky very slowly• They are in fixed patterns –

constellations• The planets move more rapidly

across the sky

Planets and Stars• The Sun and other stars give out

light• The planets look like stars to the

naked eye• Planets reflect light from the sun

back to us

Sun Earth

PlanetLight

Comets• They have very elliptical orbits• The sun is not at the centre of the

orbit• The ‘tail’ of the comet always

points away from the sun

Sun

Comet and Tail Orbit

Comets

• As they get closer to the sun the tail increases in size

• They get brighter as the tail reflects more light

• They are only visible when close to the sun

Gravity

• The earth, Sun and moon are attracted by a force called gravity

• As the distance between bodies increases, the force of gravity decreases rapidly

• Smaller bodies rotate round larger bodies because they move faster and have smaller mass

Gravity and centripetal force

Gravity – pulls towards the centre

Centripetal force in the direction of travel