Tides high and low
Published on Sep 15, 2006 at 12:40 pm.
6 Comments.
Filed under Earth, moon.
A few days ago, I posted about the Full Moon last week not being the Harvest Moon. I got a comment from Rob asking about tides. I thought about it, and I decided that the answer was a bit much for a follow-up comment. So, I decided to do a posting on tides.
In introductory textbooks tides are explained very simply. That is because these books tend to only look at the simplest cases. In real life, there are many factors that govern high and low tides. In practice, tides are really quite complicated to fully explain. Oh, the basic reason for high and low tides is easy, but it turns out that not all high tides are equally high, nor all low tides equally low. Explaining that fully takes some doing. I am not going to be able to do it here, but I can give somewhat of an explanation.
At their very simplest, tides were explained by Isaac Newton, in what we can call the equilibrium model of tides. Quite simply, the Moon pulls on the Earth. The side nearer the Moon gets pulled harder, and to it bulges outward. Since water is easier to move around than rock, the water tides are bigger than the land tides, so you see a high tide. But, there are two high tides per day. There’s another high tide opposite the Moon. Where does that come from? Well, as it turns out, the Earth isn’t really stationary. Both the Earth and the Moon are moving in orbits around the center of mass between the two bodies. That center of mass is below Earth’s surface, so it appears that the Moon orbits the Earth. But, the Earth is moving, too. You can think of centrifugal force pushing outward on the side of the Earth on the outside of that motion, and that is opposite the Moon. That isn’t an exact or complete answer to why you have to high tides, but it gets the idea across without a lot of math. The low tides will be between the high tides. If you are pushing water out from Earth in two directions, then it has to come from somewhere, right?
So, this gets the two high and low tides each day. The Moon moves a bit over the course of a day, so the high tides are a bit under 12 hours apart. But this doesn’t explain why sometimes the tides are higher than other times. One of the main reasons for that is that the Moon isn’t the only thing making tides. Even if there were no Moon, the Earth would still have high and low tides — two of each every day. So, what else is pulling on Earth to make tides? Why it’s the Sun! The Sun’s gravity pulling on Earth is what keeps Earth in orbit. And for the same reasons that the Moon pulling on Earth gives high and low tides, the Sun’s gravity gives high and low tides. The Sun is vastly larger than the Moon, so its gravity is far stronger, but it is also much farther away. The net result is that solar tides are about half as big as lunar tides. As with the Moon, high tide occurs on the side of the Earth in the direction of the Sun as well as on the opposite side.
But, if the Sun and Moon both are producing tides, how do these tidal effects work with or against each other?
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Well, it all depends upon the phase of the Moon. At New Moon and Full Moon, the lunar tides line up with the solar tides. In that case, the high tides will be extra high, and the low tides extra low. We call these extreme tides Spring Tides. Don’t let the name fool you. The do not only occur in the spring time. Spring tides occur every New Moon and Full Moon. At First and Last Quarter Moon, the solar high tide is located at the lunar low tide, and the solar low tide is at the lunar high tide. The lunar tides are bigger than the solar tides, so they dominate, but they are moderated. These smaller than normal tides are called neap tides. Like spring tides, neap tides occur twice per month.
But, there are even more things that affect tides! The Moon and Earth do not have circular orbits. Rather, the orbits are slightly elliptical. So, sometimes the Moon is closer to the Earth than other times. When the Moon is closest (astronomers call that perigee), the effect of the Moon’s gravity on the tides is going to be strongest. When the Moon is farthest (astronomers call that apogee), then the Moon’s effect is correspondingly weaker. But, the Earth’s orbit is also elliptical. When Earth is closest to the Sun (called perihelion, which occurs in early January), the solar tides are bigger than normal, and when Earth is farthest from the Sun (astronomers call that aphelion, which occurs in early July) the solar tides are going to be weaker than normal. Adding these these effects that I’ve mentioned, we find that if the Moon were New or Full and at perigee near the first of the year, then the tides will be exceptionally high and low.
But, that isn’t all! (Remember, I said that tides can be complicated?) It turns out that the angle between the tidal bulges and the Earth’s rotation makes a difference. The Moon does not orbit Earth right above its equator. Rather, the orbit of the Moon is inclined, and so sometimes it is over the northern hemisphere and sometimes over the southern hemisphere. When that happens, the tidal bulge is skewed, so as the Earth rotates, a person on Earth does not pass through the high tide bulges symmetrically. NOAA has a nice diagram of this here. That means that the two high tides during the day won’t be the same. One will be bigger than the other. The same goes for the Sun. The Earth is tilted on its axis, to the Sun appears more over one hemisphere than the other most of the year. But, near the equinoxes, the Sun appears to be right over the equator. A Full Moon or New Moon will also be over the equator at the equinox. And, it is easier to pull the tides when you are pulling with the rotation of the Earth, so that makes the tides slightly more extreme when you get a Full Moon or a New Moon near the equinox that an ordinary spring tide, which is already rather extreme!Â
Even bigger tides can occur if the Full Moon or New Moon occurs near the equinox and the Moon is also at perigee. This is actually what is going on this month and the first part of next month! The Moon was full September 7, and perigee was just hours later. The Moon will again be full October 7 (October 6, here in Texas), and the Moon will be at perigee earlier in the day. Again there will be very high tides. In fact, that is even closer to the equinox than this last Full Moon. New Moon occurs less than a day before the actual equinox, which is on September 23 this year (or September 22 where I am). So, that would give a spring tide at the equinox, but the Moon will only be hours after apogee, so the tides won’t be nearly as big as they will be on the Full Moons before and after.
So, there you have some of the things that influence tides. On top of all that, local topgraphy and coastal features also have a major effect, since water has to move around these features. They can either magnify or mitigate the tidal effects. If you are near the coast, go take note of the tides over the next month, and look for the extra high high tides of October 6 and 7.
-Astroprof
Rob on September 16, 2006 at 4:49 pm: 1
Thank you very much for sorting that one out!
Astroprof’s Page » The Future of the Moon on January 24, 2007 at 5:06 pm: 2
[…] The answer comes from understanding a little about orbits and tides. You can look at my earlier post on tides for more about the tides themselves. This time I’ll talk more about orbits. Basically, the gravitational pull between the Earth and the Moon is what keeps the Moon in orbit. Newton’s First Law tells us that an object will move in a straight line unless a force acts on it. The gravitational force pulls on the Moon. The Moon is falling towards Earth. But, it is also moving to the side so fast that it keeps missing. The Earth’s surface just curves away from it, and the gravitational force is directed towards the center of the Earth (in an idealized system). For a circular orbit, the gravitational force provides just the right centripetal force to keep the Moon in orbit. But, the Moon also pulls on the Earth. The Earth also moves. You get tides. The high tides are going to be under the Moon and opposite the Moon. Again, read my earlier post to understand more about how this works. […]
Joan on February 4, 2007 at 10:43 pm: 3
Extra high tides - this evening, I was “stuck” on a ferry that could not dock due to extra LOW tides in the Sound (ocean would have HIGH tide at that time). After reading your post, I believe that the nearly full moon must be at perigee and the tides are abnormal. The large boat had to wait almost two hours before cars could disembark.
What do you think?
Astroprof on February 5, 2007 at 2:18 pm: 4
Any Full Moon will give extra large tides, and the Earth is near perihelion now, so that makes spring tides even more extreme.
Astroprof’s Page » Big tides on November 24, 2007 at 5:53 pm: 5
[…] Today is Full Moon. This Full Moon also falls on the same day that the Moon reaches perigee (the closest point in its elliptical orbit to the Earth). That means high tides. I’ve blogged about tides twice before: here and here. You can read those postings to find more about tides. […]
Christinuhh on February 2, 2008 at 9:00 pm: 6
Thank You So VERY Much!! You helped me prove to my Mother that the tides are affected my full moons!! And I highly appriciate that!!