Context: Tides are the daily rise and fall of sea level at any given place. The pull of the Moons gravity on Earth is the primary cause of tides and the pull of the Suns gravity on Earth is the secondary cause (Figure 1.1). The Moon has a greater effect because, although it is much smaller than the Sun, it is much closer. The Moons pull is about twice that of the Suns. To understand the tides it is easiest to start with the effect of the Moon on Earth. As the Moon revolves around our planet, its gravity pulls Earth toward it. The lithosphere is unable to move much, but the water is pulled by the gravity and a bulge is created. This bulge is the high tide beneath the Moon. On the other side of the Earth, a high tide is produced where the Moons pull is weakest. These two water bulges on opposite sides of the Earth aligned with the Moon are the high tides. The places directly in between the high tides are low tides. As the Earth rotates beneath the Moon, a single spot will experience two high tides and two low tides approximately every day. High tides occur about every 12 hours and 25 minutes. The reason is that the Moon takes 24 hours and 50 minutes to rotate once around the Earth, so the Moon is over the same location every 24 hours and 50 minutes. Since high tides occur twice a day, one arrives each 12 hours and 25 minutes. What is the time between a high tide and the next low tide? The gravity of the Sun also pulls Earths water towards it and causes its own tides. Because the Sun is so far away, its pull is smaller than the Moons. Some coastal areas do not follow this pattern at all. These coastal areas may have one high and one low tide per day or a different amount of time between two high tides. These differences are often because of local conditions, such as the shape of the coastline that the tide is entering. The gravitational attraction of the Moon to ocean water creates the high and low tides. The tidal range is the difference between the ocean level at high tide and the ocean level at low tide (Figure 1.2). The tidal range in a location depends on a number of factors, including the slope of the seafloor. Water appears to move a greater distance on a gentle slope than on a steep slope. If you look at the diagram of high and low tides on a circular Earth above, youll see that tides are waves. So when the Sun and Moon are aligned, what do you expect the tides to look like? Waves are additive, so when the gravitational pull of both bodies is in the same direction, the high tides are higher and the low tides lower than at other times through the month (Figure 1.3). These more extreme tides, with a greater tidal range, are called spring tides. Spring tides dont just occur in the spring; they occur whenever the Moon is in a new-moon or full-moon phase, about every 14 days. Neap tides are tides that have the smallest tidal range, and they occur when the Earth, the Moon, and the Sun form a 90o angle (Figure 1.4). They occur exactly halfway between the spring tides, when the Moon is at first or last quarter. How do the tides add up to create neap tides? The Moons high tide occurs in the same place as the Suns low tide and the Moons low tide in the same place as the Suns high tide. At neap tides, the tidal range is relatively small. The tidal range is the difference between the ocean level at high tide and low tide. Studying ocean tides rhythmic movements helps scientists understand the ocean and the Sun/Moon/Earth system. This QUEST video explains how tides work, and visits the oldest continually operating tidal gauge in the Western Hemisphere. Click image to the left or use the URL below. URL: 
 Question: these tides have the smallest tidal range.
Output: neap tides