## Monday, August 1, 2022

### Moon Phases and Tides Overview

Physics Index

Where are we going with this? The information on this page introduces explores how the moon affects tides.

Moon Phases and Tides Overview

On earth, ocean tides "chase the moon" but are always around 50 minutes (about an hour) behind. So, as the moon passes overhead, the tides will be rising and will reach high tide about an hour after the moon has moved on.

Whereas it takes (approximately) 24 hours for the earth to rotate under the moon, the moon will appear to be directly overhead close to 6 hours after it rises, then will set about 12 hour after it rises. It will then be directly opposite 6 hours after it sets. (For the purposes of this discussion, we are going to just round off to 24 hours for the period of a day.)

Keep in mind that more than just the moon affects tides. Weather and the land masses change the magnitude of the effect the moon has the tidal levels. The timing, however, closely follows the moon's position as the earth rotates under it. (The moon also moves around the earth, which has an impact on timing, too.)

As a convenient way to describe the moons position, we can adopt the terms "overhead" to mean that it is on the same side of earth as and well into the sky above the observer. The "horizons" would be to the east and west of the observer. When the moon is above the opposite side of the planet as the observer, we can call that "underfoot."

Further, directly overhead would mean that it is at a right angle to a line that is tangent to the surface of… It is just directly up there! Directly "above" the viewer1(There's a footnote. Read it.)

Directly underfoot would be the opposite.

Suffice to say the EXACT timing is complicated. But the "hard and fast" rule is that the moon pulls the water toward it as it moves around the earth and as the earth rotates under it.

• Timing of high and low tide will be directly related to moon phase.

• There will be two high tides daily, when the moon is directly overhead and directly underfoot (opposite). They will occur roughly 12 hours apart.

• There will be two low tides daily, coming when the moon is on the horizons. Low tides will be very nearly 6 hours after high tides.
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1Full disclosure AKA, "It's not that simple.": the moon's path sort of wobbles around the equator due to it having an elliptical orbit that is around 5° tilted relative to the equator. Thus, depending on where you are on earth, the moon will be to the north or south of you by some amount, so "overhead" is actually not straight up. So, to say "directly overhead" means that it is perfectly on a north-south line on the same side of earth as the viewer. "Directly underfoot" would mean that it is exactly 180° away from the same line.

The tilt of the orbit also has an effect on how long the moon is visible. Most readers will know that days are longer in the summer; that is that the sun is visible for more than 12 hours in a day. In the same way, the moon's visibility will change based on where it is in its orbit.

### Key Points:

• Tides on earth are primarily caused by the gravitational pull of the moon. It draws the waters of earth to it (noticeable in oceans and other LARGE bodies of water).

• Whereas the moon is moving as the earth rotates under it, the waters are always "chasing" the moon. As a result, high tides occur about an hour (about 50 minutes) after the moon has passed directly overhead or directly underfoot.

• The magnitude of tidal fluctuation is connected to the physical geography of earth. As the waters of the ocean try to "chase" the moon, they will pile up on land masses.

• At high tide, the level of the ocean is higher. Thus, LESS of a beach will be visible. At low tide, the ocean is lower. Thus MORE of the beach will be visible.

## Phases and Tides

Full phase occurs when the Moon is on the opposite side of the Earth from the Sun and we see only the illuminated side. On that day it rises as the Sun is setting and sets and the Sun is rising. It is visible all night long. High tide is, therefore, near midnight and noon.

Waning Gibbous phase occurs when the Moon is mostly lit and the illuminated portion is egg-shaped (gibbous) with the western edge shaded. The amount of illuminated area visible is decreasing from one day to the next which is what is meant by "waning".

Third Quarter phase occurs when the eastern half of the Moon is illuminated. On that day it rises at midnight and sets at noon. It will be directly overhead six hours after it rises (sunrise)  High tide is therefore near sunrise and sunset.

Waning Crescent phase occurs when the eastern edge of the Moon is lit but most of the visible surface is dark. The amount of illumination is decreasing from day to day which is what is meant by "waning." During this time the illuminated portion of the Moon looks like the letter "C".

New phase occurs when the Sun and Moon are on the same side of the Earth and we see only the dark side. On that day the Sun and Moon rise and set approximately together. The new moon is visible all day long. High tide is, therefore extra high and near noon. A second high tide will be near midnight.

Waxing Crescent phase occurs when the western edge of the Moon is lit but most of the surface visible from Earth is dark. The amount of illumination visible is growing from day to day during this phase which is what is meant by "waxing."

First Quarter phase occurs when the western half of the Moon is illuminated so that it looks like the letter "D". On that day the Moon rises at noon and sets at midnight. High tide is, therefore, near sunset and sunrise.

Waxing Gibbous phase occurs when the Moon is mostly lit and the illuminated portion is egg-shaped (gibbous) with the eastern edge shaded. The amount of illuminated area visible is increasing from one day to the next which is what is meant by "waxing".