The flood tides are the highest, and the ebb tides are the lowest. These are called spring tides. They occur twice during every revolution of the moon-once at full, and once at new moon. When the sun and moon are 90 degrees apart, each produces a tide on the portion of the earth directly under it, diminishing somewhat produced by the other body. High tide, then, occurs under the moon, while the high tide caused by the sun, becomes by comparison, a low tide. Such tides are called neap tides.