Rule of twelfths

In places where there is only one high and one low water per day, the rule can be used by assuming the steps are 2 hours.

The calculation can be simplified by adding twelfths together and reducing the fraction beforehand: If midwinter sunrise and set are at 09:00 and 15:00, and midsummer at 03:00 and 21:00, the daylight duration will shift by 0:30, 1:00, 1:30, 1:30, 1:00 and 0:30 over the six months from one solstice to the other.

The rule is a rough approximation only and should be applied with great caution when used for navigational purposes.

The rule assumes that all tides behave in a regular manner, this is not true of some geographical locations, such as Poole Harbour[5] or the Solent[6] where there are "double" high waters or Weymouth Bay[5] where there is a double low water.

The rule assumes that the period between high and low tides is six hours but this is an underestimate and can vary anyway.