Drawing ellipses to represent orbits
A simple method for drawing an ellipse, using the two foci.
Apparatus and materials
For each student or group of students
Drawing pins, 2
Loop of thread, about 40 cm long
Sheet of paper
Health & Safety and Technical notes
a 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.
b Students could be encouraged to draw ellipses with the drawing pins closer or farther apart.
1 Drawing ellipses is a useful skill for students to master.
2 The Earth's orbit around the Sun is not perfectly circular. It follows a slightly elliptical path, with the Sun at one focus.