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How Solar Eclipses Occur

Introduction

Understanding the Mechanism of Solar Eclipses

　What kind of phenomenon is a solar eclipse? How does it occur?
　In this classroom exercise, the solar eclipse phenomenon is observed from two vantage points: the surface of the Earth and outer space. A web cam is set up on a globe to represent a ground-based observer.
　The picture from the web cam appears on the screen of a computer, and this image is projected onto a large screen so it can be observed by the whole class.

Experimental Method

Things to Prepare

　Globe; stick; Plasticine; Light source to represent Sun (e.g. flashlight); Web cam with filter; Computer; Projector

Creative Inc. WEBCAM5

To add a sense of familiarity, the 'observer' was given a face. The light source representing the Sun was too bright for the web cam, so a 415nm band pass filter was attached as 'sunglasses'

Preparation of the Earth, Moon & Sun Model

Roll the plasticine into a ball to use as the Moon

1. Attach the plasticine Moon to the end of the stick can to allow it to be supported from a distance.
   
2. Set up the light source representing the Sun at one end of the classroom and use it to illuminate the globe of the Earth.
   
3. Using the stick, mount the Moon near the Earth.
   
4. Depending on the position of the plasticine Moon, it sometimes casts a shadow on the globe, and sometimes not. In this way, the movement of the Moon's shadow on the Earth's surface can be simulated.

Observing with the Web cam

　The web cam represents an observer.
　Look at the image from the web cam when the 'observer' in the plasticine Moon's shadow and compare it with the image obtained when it is in full 'sunlight'.
　The image from the web cam appears on the computer screen. Hooking the computer up to a projector, it will be possible for the whole class to observe the different web cam images.

Experimental Results

Model Solar Eclipse as Observed by the Web cam

　The light source representing the Sun is visible as a bright disk, and the plasticine Moon is a dark disk in the foreground.

Simulation of Total Solar Eclipse
As viewed from the centre of the 'Moon's' shadow on the globe, the 'Sun' is almost entirely hidden.

Simulation of Partial Solar Eclipse
As viewed from the edge of the 'Moon's' shadow on the globe, the 'Sun' is partially obscured, but still visible.

Discussion

　Think about the relative positions of the Earth, Moon and Sun which are required for a solar eclipse to be visible on the the Earth's surface.

How Solar Eclipses Occur

Total Solar Eclipse 2002 Dec 4 Photographed from Ceduna, Australia 　As viewed from space, when the Sun, Moon and Earth form a straight line, the shadow of the Moon falls on the Earth

　At this time, an observer on the surface of the Earth in the region of the Moon's shadow will see the Sun being progressively covered by the Moon, as if being eaten up.