Solar eclipse occurs when the moon is in line between the earth and the sun (see Figure 1). The moon cast a shadow on the earth's surface and obscurs some parts on the sun. The porportion of the sun being blocked depends on the position of the observer on the earth. When only the moon's penumbral shadow strikes the earth, a partial eclipse of the sun is observed. However, if the moon's dark umbral shadow sweeps across earth's surface, a total eclipse of the sun is seen.

Figure 1

Sometimes the moon is farther away from the earth and its umbral shadow is not long enough to reach the earth. The moon appears smaller than the sun and cannot completely cover it (see Figure 2). Instead, the 'antumbral' or negative shadow reaches the earth. If you are within this shadow, you will see an eclipse where a ring or 'annulus' of bright sunlight surrounds the moon at the maximum phase. Those within the penumbra would observe partial annular eclipse.

Figure 2

The different stages in a total solar eclipse

There are 5 stages in a total solar eclipse:

1. "Eclipse Begins" - instant of first external tangency between the moon and the sun, this is the start of partial solar eclipse;
2. "Total Eclipse Begins" - instant of first internal tangency between the moon and the sun, this is the start of total solar eclipse;
3. "Maximum Eclispe" - when the centre of the moon is closest to the centre of the sun;
4. "Total Eclipse Ends" - instant of last internal tangency between the moon and the sun, this is the end of the total solar eclipse;
5. "Eclipse Ends" - instant of last external tangency between the moon and the sun, this is the end of a partial solar eclipse.

During an annular eclipse, there would be "Annular Eclipse Begins" and "Annular Eclipse Ends" instead of the "Total Eclipse Begins" and "Total Eclipse Ends" while in a partial solar eclipse, there would be no "Total Solar Eclipse Begins" and "Total Solar Eclipse Ends".

Click here for relevant diagram:

Next Solar Eclipse visible in Hong Kong