We have tides not currents in the Channel (and in the seas around the world).
They are caused by the gravitational pull of the Sun and the Moon as the Earth rotates. The tides vary in height and strength depending on the position of the Moon in its orbit around the Earth relative to the Sun.
When the Sun and the Moon are on the same axis and in line you get Spring tides. Maximum effect is with a New Moon (and High Spring tides) - this is when the Moon is in line with the Sun and they are both on the same side of the Earth.This happens in 28 (and a bit) day cycles.
Low Spring tides happen when you have a Full Moon. This is when the Moon is on the opposite side of the Earth to the Sun. Neap tides are when the Moon is at 90° to the axis of the Earth and the Sun (every 14 days).
Hence the lunar cycle is:
- New Moon - High Springs
- 7 days later - Neap tides
- 7 days to Full Moon - Low Springs
- 7 days to - Neap tides
After 28 days, the cycle is back to New Moon. The cycles are regular therefore the time of High and Low water is the same for the lunar month - for example:
- High tides at Dover on the Spring tide is about 0100 hours and 1300 hours CTU (old GMT)
- High tides at Dover on the Neap Tide is about 0700 hours and 1900 hours CTU
The higher the tide the larger the amount of water that is moved by the gravitational pull. This means the tidal flow is stronger as the greater amount of water has to be moved from one place to another during the same time period (6 hours - give or take a bit between High and Low water).
As the days progress the time of High water gets later - hence we have a different start time for our swims depending on the position of the Moon and the tidal height. The full explanation is a little more complicated but this is a good start to understanding what is happening.
For more information about the tides, you may wish to visit the following links: