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A neap tide is a relatively low high tide, or a generally low tidal range between low and high tides. During the moon's first and third quarter, periods of neap tides are experienced, due to the way in which the sun and moon act on the Earth to create tides. The opposite of a neap tide is a spring tide, a strong tide which is extremely high or very low, creating a large tidal range. Because the tidal cycle is well known, scientists can predict when spring and neap tides will occur, and how high they will be.
Tides are caused by an interaction of factors. The spinning motion of the Earth plays a role, as do the gravitational pulls of the moon and the Sun. In a neap tide, the sun and moon are at right angles to each other, pulling in opposite directions. These pulls effectively balance each other out, keeping the water level relatively stable. While low and high tides do occur during a neap tide, the range in height between them is very minimal.
By contrast, in a spring tide, the Sun, moon, and Earth are in syzygy, which means that they are all lined up in a row. Since the Sun and moon are pulling together, they generate a much greater force, creating correspondingly higher and lower tides. During spring tides, people can take advantage of the low tide to gather various organisms from rocks which are normally covered by water, and ships may also time departures and arrivals to coincide with the tide.
The sun and moon orient themselves to create a neap tide when the moon is one quarter full, which occurs twice during the lunar cycle. Spring tides occur when the moon is full and dark. People who want to get a rough idea of what to expect from the tides can check and see where in the lunar cycle they are to determine if there will be a neap or spring tide. Additional information about when high and low tides will occur can be gathered from organizations which provide weather and tide data.
People may want to be aware that the topography of the Earth influences tides, which is one reason why tidal levels are not the same all over the Earth, or even in a similar region of the Earth. Tide charts generally indicate the area they apply to, and some have conversion tables which can be used in neighboring regions to determine when the high and low tides will occur, and what their heights will be.