<P> Because the gravitational field created by the Moon weakens with distance from the Moon, it exerts a slightly stronger than average force on the side of the Earth facing the Moon, and a slightly weaker force on the opposite side . The Moon thus tends to "stretch" the Earth slightly along the line connecting the two bodies . The solid Earth deforms a bit, but ocean water, being fluid, is free to move much more in response to the tidal force, particularly horizontally . As the Earth rotates, the magnitude and direction of the tidal force at any particular point on the Earth's surface change constantly; although the ocean never reaches equilibrium--there is never time for the fluid to "catch up" to the state it would eventually reach if the tidal force were constant--the changing tidal force nonetheless causes rhythmic changes in sea surface height . </P> <P> When there are two high tides each day with different heights (and two low tides also of different heights), the pattern is called a mixed semi-diurnal tide . </P> <P> The semi-diurnal range (the difference in height between high and low waters over about half a day) varies in a two - week cycle . Approximately twice a month, around new moon and full moon when the Sun, Moon, and Earth form a line (a configuration known as a syzygy), the tidal force due to the sun reinforces that due to the Moon . The tide's range is then at its maximum; this is called the spring tide . It is not named after the season, but, like that word, derives from the meaning "jump, burst forth, rise", as in a natural spring . </P> <P> When the Moon is at first quarter or third quarter, the Sun and Moon are separated by 90 ° when viewed from the Earth, and the solar tidal force partially cancels the Moon's tidal force . At these points in the lunar cycle, the tide's range is at its minimum; this is called the neap tide, or neaps . Neap is an Anglo - Saxon word meaning "without the power", as in forđganges nip (forth - going without - the - power). </P>

When do spring tides on earth take place