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Physics 06256.1 Earth and the Solar System

Solar System in 3D

Orbit the Sun in three dimensions. Compare orbital radii and periods, and see why outer planets take longer.

  • Orbits and gravity
  • Orbital period
  • v = 2πr / T

Loading the Solar System…

Predict

Neptune orbits at about 30 AU. Roughly how long is its year compared with Earth’s?

Experiment

  1. 1Select Mercury, then Neptune, and compare their orbital radii and periods.
  2. 2Work out Earth’s orbital speed from v = 2πr / T using the panel values.
  3. 3Find two planets whose periods differ by roughly a factor of ten.

Explain

Planets stay in orbit because the Sun’s gravitational attraction acts towards the centre of the orbit, continuously changing the planet’s direction without changing its speed.

Further out, the gravitational field is weaker, so the orbital speed is lower — and the path is longer. Both effects lengthen the year, which is why Neptune takes 165 Earth years rather than 30.

Orbital speed is v = 2πr / T: the circumference of the orbit divided by the time to go round once.