How Can the Moon Affect Tides?
DonÂ’t believe everything you hear, especially around marinas and bait stores where the “old salts” spin their yarns. When you hear boaters referring to ebb tide, or incoming tide or outgoing tide, it may be familiar language, but it is not accurate. Tide does not ebb, nor does it come in or go out.
- Tide is defined as the vertical movement of water and only goes up and down.
- Current is the horizontal or sideways flow of water. It floods in which makes the tide rise and ebbs out which makes the tide fall.
It is this up and down tidal movement that you should be concerned with and be able to estimate with some accuracy, especially when entering a potentially shallow port or harbor. LetÂ’s explore some definitions concerning tide.
First of all, tide is the rise and fall of water caused by gravitational forces of the moon and sun on the oceans of the earth. Generally speaking, tidal cycles contain two high tides and two low tides each day. During the time between high and low tide there will obviously be current flow. The time between high and low tides is a little over 6 hours and the entire tidal cycle repeats itself approximately fifty minutes later each day. So if you know that low tide is at 0800 today you can estimate that it will be at 0850 tomorrow.
The difference between the high tide and low tide is called the range of tide. For instance, if the water depth at high tide is 20 feet and at low tide is 18 feet, the range of tide is two feet.
There are two types of currents that you can expect in regard to tides.
- Flooding current is experienced when the tide is rising.
- Ebbing current is experienced when the tide is falling.
When the tide has reached it highest and lowest points there is a brief period where there is no current ebbing or flooding, this is referred to as slack water .
Although there are irregularities in some parts of the world, as a general rule the following can be predicted using publications such as the Tide and Current Tables, using software specifically designed to estimate tides or by accessing the information about tides online at BoatSafe Safety Links.
Times of high and low tide and depth of water at each of these times
Set and drift of strong tidal currents, including times of maximum current
Times of slack water
This link above provides a tutorial on how to use the official tide website. Once you are familiar with how it works, you can go directly to the tide website by clicking on the tide link at the top of the page.
How do the effects of the sun and the moon affect tide? The gravitational pull of the moon tugs on the surface of the ocean until its surface mounds up and outward in the direction of the moon. When the mound of water has reached its highest point it is called high tide. On the side of the earth opposite the moon, the centrifugal force caused by the earthÂ’s rotation produces another mound of water and high tide on the opposite side of the earth. Somewhere in between these two high tides are two flat areas on the surface of the ocean, these are low tides.
The moon appears to rotate around the earth each day, however it is the earthÂ’s rotation that gives this appearance. The moon actually orbits the earth in an elliptical pattern, taking 27.3 days to complete one orbit. The length of time that it takes for the earth to rotate around so that the moon is in the same position is actually a little over a normal 24 hour day. It is 24 hours and 50 minutes or a tidal day. That is why the tidal cycle starts approximately 50 minutes later each day. As the earth rotates, the moonÂ’s gravitational force continually mounds the water and that fluid mound moves around the earth. The actual height of the tide is influenced by the shape of the coastline and depth of the water.
Two other areas that need to be covered are “spring tides” and “neap tides.” It was mentioned previously that the tides are affected by both the sun and the moon. Actually, the sunÂ’s effect is less than half that of the moon but when these two bodies are in alignment and pulling in the same direction they cause higher high tides and lower low tides called spring tides.
On the other hand when the Sun and Moon are at right angles to one another with the moon pulling one direction and the sun pulling another there is somewhat of a canceling affect and you get lower high tides and higher low tides called neap tides.