When the line joining the Earth and Moon is at right angles to that joining the Earth and Sun, the gravitational force on the Earth and oceans is minimum and so the gravitational bulge is a minimum also.
The tidal range at this time is minimum and these ‘lower tides’ are called neap tides. Neap tides are associated with ‘Half Moon’, but inertia also affects this; so neap tides occur around 2 days after the first and second ‘Half Moon’.
- Tides change from spring tides to neap tides and back to spring tides over a 14-day period.
- The heights of a spring and neap tides vary throughout the year, the highest springs being associated approximately with the equinoxes in the spring and autumn; hence the term ‘spring’ tides.
- The lowest neaps are associated approximately with the solstices, the origin of the term neap being unclear but probably originating from the old English ‘nepflod’ used to describe the lowest high waters.