Kepler's First Law
Earth and Space

Drawing ellipses to represent orbits

Practical Activity for 14-16 PRACTICAL PHYISCS

Class practical

A simple method for drawing an ellipse, using the two foci.

Apparatus and Materials

For each student or group of students

  • Drawing board
  • Drawing pins, 2
  • Loop of thread, about 40 cm long
  • Pencil

Health & Safety and Technical Notes

Read our standard health & safety guidance


Procedure

  1. Stick two drawing-pins into a board (though not pressed too far in) about 10 cm apart and slip the loop of thread over them. Insert a pencil point in the loop and position it so that the thread is tight. Move the pencil round the pins, always keeping the thread tight and thus trace out an ellipse.
  2. Students could be encouraged to draw ellipses with the drawing pins closer or farther apart.

Teaching Notes

  • Drawing ellipses is a useful skill for students to master.
  • The Earth's orbit around the Sun is not perfectly circular. It follows a slightly elliptical path, with the Sun at one focus.
Kepler's First Law
is expressed by the relation r_1+r_2=2a
formalises Orbits

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