The Moon’s gravitational pull on Earth plays a major role in ocean tides. Twice a month, when the Sun and Moon line up, their combined gravitational forces make exceptionally high and low tides.
Aboriginal and Torres Strait Islander people have observed this correlation for centuries and incorporated it into their fishing and hunting knowledge systems. This can be used to predict when fish are more active and easier to catch.
Tidal cycles
The moon phases do not have a direct effect on the tides, but they can affect how water moves along a coastline. Weather systems can also have an effect, which can lead to high or low tides that aren’t expected.
The most obvious effect of the moon on tides is its gravitational pull on Earth. As the Moon orbits, it pulls on the ocean’s surface, causing it to bulge out on the side facing the Moon and to recede on the opposite side of the Earth.
These two tidal bulges account for the alternation in daily high tide heights that happen when the Moon is at full and new moon. When the Moon is at first quarter or third quarter, however, these two effects do not combine to produce large spring tides, instead producing small neap tides.
This is because the tidal force on the near side of the Moon is stronger than that on the far side, and when it’s at full moon or new moon, both forces are aligned so that their influences combine to create a spring tide. On the other hand, when the Sun and the Moon are at 90deg to each other (first quarter or third quarter), their influences don’t add together so the high tides and low tides aren’t as extreme.
Neap tides
Three or four times a year the new or full moon (called perigee) coincides with the point at which the moon is closest to the Earth in its orbit. During these events the tidal effects of the sun and moon are much stronger than normal, and the tides can rise higher than average.
These high and low tides are called spring tides, though they have nothing to do with the season of spring. They occur twice each lunar month, all year long.
The tidal range of high and low water levels during a tidal cycle is determined by the effect of the Sun’s gravity on the oceans. The range of the tidal cycle changes each month as the Sun’s pull on the Earth slightly deforms the oceans.
The tidal forces of the sun and moon are at their minimum in the quarter phases of the moon, producing very small ranges of tidal highs and lows, known as neap tides. These neap tides are moderate, occurring twice each month during the first and third quarter of the moon, and producing lower high and higher low tides than the normal tidal range.
High tides
High tides are caused by the moon’s gravitational pull on the water in the ocean. That pull causes the ocean’s water to bulge out on the side closest to the moon and the side farthest from the moon.
When the Sun and the Moon are in line with the Earth, their gravity can make exceptionally high tides called spring tides. But when the moon and the Sun are not in alignment, their gravitational forces cancel each other out, causing much smaller ranges of tides called neap tides.
Tides also vary from place to place, and they can be affected by weather systems, such as storms. Strong offshore winds can push water away from coastlines, causing a more dramatic high tide.
Tides also depend on the shape and depth of the ocean, as well as the distance between continents. These factors can make the time between the onset of a high tide and the tidal bulge more noticeable, called the tidal lag.
Low tides
The moon’s gravitational pull can cause ocean water to rise and fall. These changes are called tides.
There are two high and two low tides every day in most places around the world. These tides are caused by the difference in strength of the Moon’s gravitational force on Earth’s near side compared to its strength on Earth’s far side.
This small difference squeezes the oceans into a stretched-out, elliptical shape on opposite sides of Earth, as shown in Figure 2. The high tides happen close to where each long end of this tidal bulge is located.
Tide levels are also affected by weather patterns, particularly strong offshore winds. They can move water away from coastlines and exaggerate the low tide.