Tides vary on a monthly cycle as well; the tide rises highest when the moon is full or new. Is that because the moon has more mass when it's full? No, it only looks that way. Spring tide, as the highest part of the monthly tidal cycle is called, happens when the moon and the sun are pulling together. When the moon is full (or new) its gravitational pull is aligned with that of the sun, giving the water an extra tug. During a half-moon, the difference between high tide and low tide is least; this is called neap tide.