Expert: Philip Stahl - 3/10/2008

Question
QUESTION: Hi Philip,

I'm not sure you're the right person to ask, but here goes:

Why are tides so low at the soltices?

I don't understand why at both summer AND winter solstice, the tides are super low. In the summer, at New Moon, shouldn't the lowest tide be at night? What's going on here?

What makes the highest and lowest tides happen?

Thanks in advance!

With Blessings,

erin

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ANSWER: Hello,

Actually it isn't plausible that the tides would be highest or lowest at the solstices (unless we are talking about ONE specific gregraphic location - in which case the seabed topography will also come into play - so it isn't a simple matter of the solstices being responsible)

The reason for this is that the Moon's orbital plane is inclined at 5 degrees and 9 minutes to the plane of the Ecliptic. Thus, for extra high tides (or their opposites) one would have to have the Sun in alignment *at the solstice* with the Moon *on the ecliptic*. But since the Moon will only cross the ecliptic on two occasions per year - and this need not be the same times as the solstices - we can see this is purely coincidental.

Now in terms of your more general question, of what produces the high and low tides, bear in mind thje Earth revolves once every 24 hours on its axis, and the Moon's own orbit superimposes another 50 minutes each night. This means that two high tides will be produced about every 24 hrs. 50 minutes, or once every 12 hrs. 25 minutes. Bear in mind these are only approximations, but if you get a daily paper and it features tides (assuming you live near the sea) you can actually check this by taking the time difference.

Now, just as there are two high tides, so also there are two low tides every 24 hours 50 minutes. These will occur where the gravitational force produces maximal tidal bulges  - that is, at the point on Earth nearest the Moon (new or Full) and at the exact opposite point. In between these, there will occur the two low tides.

This explanation of course, leaves out a lot of details - such as the fact that Earth's axis is not at right angles to the Moon's orbital plane. However, it does provide the best *simple* explanation for why the Moon has two high tides, and also why they occur at the time intervals they do.

Don't forget also that one needs to factor in the 50 mins. differential 'delay' time of the Moon, i.e.  in rising on subsequent nights.

Hence, a high tide at 6.45 a.m. one day will not occur at the same time the next - but later.

Another point, high tides don't only occur at New Moon or Full! Rather, a higher than normal tide called "spring tide" (nothing to do with the season, but rather originated because people spoke of the tides "springing forth" then) occurs at those particular times - when Earth is in alignment with Moon AND Sun, so the bulge effect is magnified.

But one can also get high and low tides at first and last quarter Moon, they just aren't as pronounced because the Moon is now at right angles to the Sun. We call these "neap tides".

You can obtain much more info, including graphics, here:

http://home.hiwaay.net/~krcool/Astro/moon/moontides/

Pay particular attention to the question (which is among many at the end): Why are ocean tides so different everywhere?

This may well be the key to explaining your own observations of tides. That is, the topography of the coastline combined with the seabed surface where you are may have engendered the observations you noticed (and referenced in your question) at THAT place.

BUT - that doesn't mean they can be generalized to ALL locations!

Having seen (over decades) the behavior of the tides myself, from places as different as Miami, FL; Anchorage, AK;  Barbados, Trinidad and Martinique - I can assure you that significant differences are the rule rather than the exception.




---------- FOLLOW-UP ----------

QUESTION: Thanks for all that info, Philip!  

It was very interesting to learn that the Full Moon doesn't affect the tide... I've been mistaken all these years!  Now that you say it, though, it makes sense: the phases of the moon are just a trick of the light, really, and wouldn't affect the gravitational forces... : )

About the Solstices: Let me explain why I'm asking.  I live on an island, and twice a year, on the summer solstice - give or take a day - the tides are low enough to enable us to walk out to a little lighthouse island.  Only that one time of the year, every year.

We think that it's the same with winter solstice, but to be honest it's too cold to attempt the crossing!  So maybe we're wrong there.

So, are you saying that we have a lowest tide here every year because of topography, and not because of the Solstice?  I'm confused about this.

Can you clarify?  

Thanks again for all your time.  It's a real gift to be able to get answers to these questions that have plagued me for years!

With Blessings,

Erin

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ANSWER: Hello, Erin

Actually, I didn't say that the Full Moon "doesn't affect the tides", I said that high tides *don't only occur at New Moon or Full* - which is a wholly different thing. In other words, high tides can also occur at first and last quarter, they are just lower relatively ("neap tides") because at those phases the Moon is at right angles to the Sun-Earth and not aligned with them.

SO, it really IS true that the Moon does affect the tides (by virtue of its gravitational pull on Earth) and isn't merely a "trick of light".

Now that you mention you live on an island, it makes sense that what you have observed with regard to the low tides - at particular phases -amounts to coincidence, nothing more.

I also lived on an island (Barbados) for 20 years, and had many occasions to study tidal behavior. One thing currently affecting many beaches around the island (seen when I visited last June, for example) is the EROSION of the shoreline - leading to much higher tides than used to be experienced. Other areas, shores of Barbados - such as on the south coast have much sand deposit and the topography has altered so there are lower tides there now - than had been observed before (say in the late 1970s)

What I am saying again, is that the occurrence of the solstices (winter or summer) is purely coincidental to your observations of the low tides you report - especially now that I know you live on an island. The point here is that the occurrence of a solstice will not determine in general when a lowest (or highest) tide is produced at a location.

The topography of the sea floor, not to mention the shore line, is equally important to what kind of tides - how high or low - occur at a place. Changes in the sea floor (which in general won't be accessible to you unless you go diving or snorkel a lot) can also contribute to the height of tides.

At the Bay of Fundy in Newfoundland, for example, exceptionally high tides can occur. These are a direct function of the sea floor topography and the nature, shape of the coastline. These are important aspect to bear in mind when considering tides.

Hope this sheds some more light.

---------- FOLLOW-UP ----------

QUESTION: Hi Philip,

Thanks so much for your answers!!  I really appreciate it, and i want to rate the answer, but there was nowhere to click...

But i wanted to let you know that I thought it was great!

With Blessings,

erin

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Answer
Thanks again for your feedback, Erin.

I removed my profile from being rated about a year ago because I found too many did not know how to fairly assign ratings. For example, I'd receive a '7' for timeliness when the answer was given well within the time limit - often the same day.

Anyway, your verbal acknowledgement more than makes up for a numerical rating!