The Moon is Earth's only natural satellite. It is the largest natural satellite in the Solar System relative to the size of its planet, the fifth largest satellite in the Solar System overall, and is larger than any known dwarf planet. The Moon's orbit around Earth has a sidereal period of 27.3 days. During each synodic period of 29.5 days, the amount of visible surface illuminated by the Sun varies from none up to 100%, resulting in lunar phases that form the basis for the months of a lunar calendar. Though we can see the moon’s size change throughout the month, it is really always the same size. Yet we see these different sizes or moon phases at regular intervals every month. How does this work?

It is very important to first understand the following facts before you proceed

 The moon revolves around the Earth every 27 days, but from Earth it appears to be 29.5 days, because the Earth’s motion extends its full revolution. The moon takes the same amount of time to rotate once around on its axis as does to revolve around the Earth - so the same side of the moon always faces the Earth.

Figure 2 The orbital period (also revolution period)
moon phases
Figure3 moon phases https://www.abrighterwild.com/blog/moon-phase-wisdom

The moon looks different during its revolution around the Earth, because at each position it is getting a

different amount of sunlight on its surface and reflecting it. When the moon is positioned between the Earth and the sun, we face the dark side, so we cannot see the moon at all. This is called a new moon. As each day passes and the moon moves at an angle out from between the Earth and the sun, we begin the see a sliver of the moon getting sunlight. By day 4 this is called a waxing crescent,When the moon has revolved to a 90 degree angle from the Earth and sun, on about day 7, it has reached its first quarter. We can now see half the moon, while the other half sits in invisible shadows. The next phase, at about day 10, we can see roughly three quarters of the moon. This is called the waxing gibbous phase.

After roughly 2 weeks, the moon is now in position with the Earth sitting between it and the sun, so we see its fully lit side as a full moon. It is not at an exact alignment though, or the Earth would block the sun from the moon causing a lunar eclipse.

As it continues on in its revolution around the Earth, the moon begins to move into shadow as the waning gibbous moon by day 18, then the third quarter half-moon at day 22, then waning crescent at day 26 and finally the invisible new moon again on day 29.

Like the full moon, the new moon does not block the sun from reaching the Earth, because it is not an exact alignment. On the months where the alignment is exact, we experience a solar eclipse.


The moon's gravitational field is so strong that it affects the Earth, most notably the water in the oceans. The side of the Earth that is closest to the moon will have a distinct bulge. The rise and fall of the ocean level results from the moon's gravitational field pulling as it moves in orbit around the Earth.

Moon gravitation field
Figure 3 Moon gravitation force on earth

The Moon acts like a strong magnet to the ocean. This means that it pulls and tugs at the surface of the ocean until it swells and rises up towards the moon. This causes a high tide. The ocean also swells out on the other side of the Earth from the Moon. This is because the pull of the moon is less on that side of the Earth.


Tide Levels

Tides are the rise and fall of the ocean water level at any given place. For six hours, a tide will rise on the beach. Then for six hours, the water level will recede into the ocean. Because oceans are liquid, their bulge is more obvious than the land bulge.


High Tides

The side of the Earth facing the moon will have a tidal bulge called the direct tide. Similarly, on the opposite side of the planet, the ocean will also be bulging. This is called the opposite tide, and it happens because the inertial force of the Earth exceeds the gravitational force of the moon at this location. Therefore, high tides occur simultaneously on the opposite sides of the Earth.



Low Tides

Low tides are the receding waters between the high tides. In some places, low tide can be only a few feet, while in others the ocean can recede much farther. High and low tides both appear two times each in a 24-hour day, but since the moon rises 50 minutes later each day, the tide cycles will differ by the same 50 minutes daily.

Spring Tides

The phases of the moon also affect tides. When the moon is at its full or new moon phase, high tides are at their highest, while low tides are lower than usual. Called spring tides, these tides occur when the sun, moon and the Earth all line up. The added gravity of the sun can make the oceans bulge more than at other times.

National Ocean and atmospheric administration
Figure 4 image from NOAA's National Ocean and atmospheric administration
Spring tide
Spring tide

When the sun and the moon are in line with each other, they pull the ocean’s surface in the same direction. This causes higher high tides and lower low tides. These tides are known as ‘spring tides’, although they have nothing to do with the spring season! Instead, there are spring tides during a new moon and a full moon.

Neap Tides

During the moon's quarter phases, the sun pulls against the moon's gravitational pull instead of with it. During these tides, the result is the lowest high tide and the highest low tide -- in other words, the least extreme difference between high and low tides. This is called a neap tide.

Neap tide
Neap tide

At other times, when the sun and the moon are at right angles to each other they pull in opposite directions. When this happens, there are lower high tides and higher low tides than usual. These are called ‘neap tides’ and happen when there is quarter moon.

Too Close for Comfort

If the moon is at perigee, or the closest point in its orbit around the Earth, the tides can also be affected. Combined with a full or new phase, a moon at perigee can produce the highest and lowest tides of all. Scientists can easily predict these highest of tides so that warnings can be issued for possible coastal flooding.

What Are Diurnal Tides?

Ocean tides are caused by the pull of gravity of the moon and the sun on the ocean's surface. As the moon is much closer than the sun to earth, its influence is far greater. The moon's gravitational force causes a bulge in the ocean’s surface on the side of the earth facing the current position of the moon.

oceas/sea waves

Due to the law of inertia, a bulge also forms on the opposite side of the earth. At the peaks of each of these bulges is high tide at the troughs and low tide. We experience high and low tides at the beach when these peaks and troughs reach our shores.

What Tide Corresponds With a Solar Eclipse?

The gravitational force exerted by both the sun and the moon causes the tides in the Earth's bodies of water. Proximity to Earth means the moon is the predominant factor in determining the Earth's tides because the moon exerts greater immediate gravitational shifts. The most drastic high tides, called spring tides, occur when the Earth, moon, and sun align. Therefore, during a solar eclipse, spring tides occur.

references and resources
  • https://spaceplace.nasa.gov/eclipses/en/
  • https://www.enchantedlearning.com/subjects/astronomy/moon/Tides.shtml
  • https://oceanservice.noaa.gov/facts/springtide.html
  • https://oceanservice.noaa.gov/education/tutorial_tides/tides03_gravity.html