The moon, as it is closer to the earth, exerts twice the gravitational pull of the sun on the earth.
When the sun and the moon are in a line as on a new moon (Amavasya) or a full moon day (Purnima) both of them pull together at the same time in the same direction.
This combined pull produces an extra large tide. It is called a spring tide.
In its first quarter (Asthmi Shukla Paksha) and the third quarters (Asthmi-Krishna Paksha) the gravitational force of the two heavenly bodies is at right angle.
At this time, the two pulls are opposing each other and are not acting in the same direction.
In other words they cancel or neutralize each other’s effect.
It produces a weak tide which is called a neap tide.
Effects of Tides—–
Tides act as link between the port and the open sea.
The tidal current clear away the river sediments and slows down the growth of delta.
It increases the depth of water which helps ships to move safely to the ports.
It also acts as a source for producing electricity.