One of the main things we know about the moon is that it changes phases throughout a month. This changes the way the moon looks in the sky and it also affects tides.
Tides are the fall and rise of ocean water at a given place. This happens because gravity from the sun and the moon pulls on the earth’s water and Earth itself.
Tidal range
The moon phases and tides are influenced by the Earth’s rotation, as well as other factors. Tidal ranges, or the difference between high and low tides, vary from place to place. In some places, tidal ranges are very small. In other places, they are much larger. The shape of a beach, ocean floor, and the weather also influence the ranges.
In many parts of the world, high and low tides are a daily occurrence. However, there are a few special times that bring these tides to a peak level: full and new moons, and when the moon is at perigee.
Spring tides are a result of the Sun and Moon acting together twice a month when they are in a straight line (full or new moon). They produce high and low tides that are significantly greater than average.
These high and low tides happen a few days before and after the new or full moon. These tidal ranges are called perigean spring tides.
When the new or full moon is close to perigee, the tidal range can be so great that coastal areas are at risk of flooding. Scientists can predict these high and low tides so that warnings can be issued.
This activity shows students how the phases of the Moon affect the tidal ranges in different areas. They will investigate how the moon’s position changes with respect to the sun and Earth to explain why there are two high tides and two low tides each day in most places.
They will also learn how the tidal bulges created by the Sun and Moon line up at different times to increase tidal range. They will discover how a new moon and a full moon can both cause a very large spring tide, and how a neap tide can make the difference between high and low tides even smaller.
This activity is a simple and fun way to help students understand how the moon affects the tides in our planet’s oceans. It is especially effective with older students who have had a chance to study tides.
Spring tides
The moon phases and tides affect the oceans in many ways. It’s not surprising that coastal Aboriginal and Torres Strait Islander people have observed these cycles and used them to determine when and where the best times are to hunt, fish, and grow crops.
Tides are caused by the combined gravitational pulls of the Sun and Moon on the water below Earth’s surface. When the two are aligned, their gravitational forces reinforce each other, creating larger-than-normal tides, called spring tides.
Spring tides occur twice a month, during both the new and full moon phases. In these times the tidal bulges are lined up in the same direction, creating a “+” shape and making the tides much higher than normal.
However, if the Sun and Moon are at right angles to each other during quarter moons, their gravitational pulls are at odds and cause smaller-than-normal tides, called neap tides. This occurs because the Sun’s gravity is slightly stronger than the Moon’s, but it is much farther away from Earth and thus has a smaller difference between it and our planet.
These tides may not seem like much of a difference, but they do cause the sea level to rise and fall. They also push the water up on the shore. This can make the beach seem higher than usual, depending on how close to the ocean the beach is located.
The same phenomenon can happen if the moon is around its closest point to Earth, known as perigee. This can happen several times a year and can produce even larger variations between high and low tides, called perigean spring tides.
In a few weeks, the Earth and Moon will again line up, at which time the gravitational forces will once again strengthen, causing yet more high tides. This happens around the March equinox and September equinox, which are often called the “spring tides of the year” because they have the largest tidal range.
Neap tides
The moon, Earth and Sun all play a role in determining the tides of our oceans. This is primarily due to the gravitational pull of these bodies as they orbit the Earth.
In the same way, the rotation of the Earth also plays a role in determining the rise and fall of our sea levels. As the Earth spins, it creates a series of high and low tidal bulges, which result in two high tides and two low tides on any given day.
But the two bulges are not always aligned with each other. Instead, they tend to be a little ahead of each other in the sky. This is called a “neap tide”.
Neap tides occur when the Sun, Moon and Earth are all at right angles to each other (first and last quarters). The gravitational forces between these three bodies are weaker than they would be at their maximum. This causes the tidal ranges to be less than normal.
As a result, low and high tides occur at these times of the month, but the difference between them is very small. This is why we call them neap tides!
When the sun, moon and Earth are lined up with each other–at full or new moons–the lunar tide and solar tide line up and reinforce each other, creating a large total tide. This is what creates spring tides.
The interaction of the lunar and solar tides also produces a tidal bulge that is not as big as the one produced by the Earth. The tidal bulges cause the high tide and low tide to be higher and lower than normal, making them appear to rise and fall more rapidly than usual.
Because the Earth’s surface rotates in the same direction as it revolves, these high and low tidal bulges can take a little extra time for the Earth to spin through each of them. That means that in some parts of the world, only one tide a day is experienced.
However, in other areas of the world, the combination of the effects of the lunar and solar tides produce much larger tides. This is what creates spring and neap tides.
Moon at perigee
A supermoon happens when the moon closely aligns with perigee – the closest point in the moon’s orbit to Earth. It is called a supermoon because it is usually larger and brighter than normal.
The elongated orbit of the moon causes it to reach perigee, its closes point to Earth, about every 28 days and apogee, its farthest point from Earth, about 14 days later. This is important to know because it explains why high and low tides vary in intensity during the course of a full moon cycle.
When the moon is at its apogee, its farthest from Earth, the effect of gravity is lessened, contributing to lower tides. Conversely, at its perigee, the moon’s closeness to the Earth increases its gravity and contributes to higher tides.
For instance, in 2018, the new and full moons closely aligned with perigee on January 1-2, bringing especially high tides that caused major flooding in Europe. The following day, Storm Eleanor hit the UK with winds of up to 100 mph (160 km/h) and left thousands without power.
In this case, the moon was about 31,000 miles closer to Earth at perigee than it was at apogee, giving it a much bigger appearance. This effect was visible to the naked eye and can be easily seen when taking pictures with a camera or smartphone, but it might not be apparent when using conventional film.
You can find out exactly how close a full moon is to perigee on the Lunar Perigee and Apogee Calculator. It also shows you the dates and times of perigees and apogees throughout the year as well as New and Full Moon phases.
This calculator also gives you the brightness of a full moon at each phase, and shows how bright a full moon is during a full moon cycle. As a rule, a full moon is 1.3 times (or 30%) brighter at apogee than it is at its mean distance.
The closest full moon to perigee in recent memory was November 2016 and the closest one since 1912 occurred on January 26. The next one is scheduled for November 2034.