2.2.4. Tides Based on Height

The height of rising water (high tide) varies appreciably depending upon the position of sun and moon with respect to the earth:

1. Spring tides: On full moon and new moon days, the Sun, the Moon and the Earth are almost in the same line. On these days, the Sun and the Moon exert their combined gravitational force on the Earth. Thus on these two days the high tides are the highest and are known as

spring tides. The height of a spring tide is about 20 per cent more than the normal high tide. They occur twice every month.

2. Neap tides: On half Moon days (i.e. first and last quarter phases of the Moon), the Sun and the Moon are at right angles to the centre of the Earth. The tide producing forces of the Moon and the Sun, work in opposite directions and they partly cancel each other's force. In such cases, the high tide is lower than the normal and low tide is higher than the normal. The difference is about 20 per cent. This is known as the neap tide.