Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds
 

Eclipses

Demonstration

Drawing diagrams of eclipses of the Sun and the Moon.

Apparatus and materials

None required.

Health & Safety and Technical notes


Remind students that they must never look directly at the Sun (except through approved solar eclipse viewers). This could blind them. (Eclipse viewers too must be carefully checked for damage before each use – hold each one in front of lamp – check no cracks or pinpoints of light.)

Procedure


eclipse of the sun diagram to show how it happens

 
a Sketch an eclipse of the Sun for the class as shown (not drawn to scale); the Sun is much too near and Moon is too big and too near.
eclipse cones of Moon and Earth

 
b Sketch the same situation, but to scale, showing the shadow cones of the Moon and the Earth.

 

Moon's orbit around Earth

 
c Sketch another diagram to scale, but reducing the scale so that the Sun, Moon and Earth are in the picture. The small circle is the Moon’s orbit. The Earth, at the centre of that circle, is too small to show. On this scale the Earth is a dot 1/800 centimetre in diameter. The Moon is much too small to show. (The diameter of the Moon's orbit is half the diameter of the Sun. The Earth's diameter is 109 times smaller than the Sun's diameter.)

 

Teaching notes


1 Eclipses have always excited interest and sometimes fear. Early astronomers concluded from eclipses that the Moon shines only by reflected sunlight and that the Earth is round.
 
2 The Moon produces a cone-shaped shadow, with an angle of half a degree to its axis at the apex. In an eclipse of the Sun, the tip of the shadow cone only just reaches the Earth .
 
In an eclipse of the Moon, the shadow of the Earth, which itself will have narrowed by one Moon diameter out at the distance of the Moon, just covers 2.5 Moon diameters as the Moon passes through it.
 
From that, and the half degree angle subtended by the Moon, you can show that the Moon must be about 60 Earth radii away, a distance now known precisely to the nearest metre.

This experiment was safety-checked in April 2007