Time and Tide...

[khiemtran]

Driving home earlier this week, I noticed a near-full moon low on the horizon to my left, and the sun setting over to my right. The sun, earth and moon are in a line, I thought. It will be a Spring Tide tonight. As it happened, I already knew it would be high tide in about an hour, but it set me wondering just how I could predict the tides just from the sun and moon. And it turns out in Sydney, it's much simpler than I thought....

First, some basics from Hunter Gatherer 101. These are all things I knew or could have deduced intellectually, but which I'd never really thought about before.

1) The full moon is always on the opposite side of the sky from the sun. The only time that anyone on earth can see a full moon is when it is on the far side of the planet from the sun. Likewise, a new moon is going to be when the moon is on the sunward side of the earth. A quarter moon (half-lit, half-dark) is when the moon and the earth are alongside each other, roughly the same distance from the sun.

2) The moon orbits the earth in roughly the same plane that the earth orbits the sun. Not exactly the same plane - it sweeps through higher and lower latitudes, but in general it means the moon follows the same general arc across the sky. It will start in the east, somewhere near where the sun rose, track across the sky following a vaguely similar road to the sun, then finish in the west, somewhere near where the sun sets.

3) The moon orbits the earth much slower than the earth spins. It takes the moon (roughly) twenty-eight days to complete a full cycle, while the earth rotates every twenty-four hours. That means that the moon's flightpath and timing is only going to change a little each night.

4) Tides are amplified when the sun, earth and moon are aligned (known as "Spring tides") and dampened when they form right angles ("Neap tides"). From 1) above, it should be clear that Spring tides happen when the moon is either full or new; and neap tides happen during a quarter moon.

Now, in Sydney, we get two high tides a day: a lesser one and a greater one. The pattern is little high tide, little low tide, big high tide, big low tide. I looked up some tide charts and then used the moon calculator on timeanddate.com to work out where the moon would be for each of the high tides at different times of year.

As far as I can see, it looks like the greater high tide happens when the moon is over the middle of the Pacific (typically in the longitudes south of Alaska). Here's what the moon looks like then when viewed from Sydney. (Note that if you were in the middle of the Pacific, it would be the middle of the night, the moon would be directly above you, and it would still be a full moon.)



From this distance, a moon over the middle of the Pacific will always be low in the east. The lesser high tide occurs when the moon is over Africa. In this case, it isn't visible from Sydney anymore, having just passed now over the horizon.

Now, putting all this information together, it is actually quite easy to predict what the tide will be. If it's a full moon, as in the photo, then the sun must be on the opposite side of the sky; so if the moon is low over the east, the sun must be setting. Hence, if it's a full moon, there will always be a spring tide around early evening. If it's a quarter moon, the sun must always be at ninety-degrees to the moon, so when the moon is in the right place over the Pacific for the greater high tide (which will be a neap tide); then the sun must be nearly directly overhead. Hence, a neap tide will always occur on a quarter moon some time after midday.

Compressing this down into a simple table, we get:

Phase:	Greater High Tide:	Lesser High Tide:
Full	Sunset	Sunrise
3rd quarter	After Midnight	After Midday
New	Sunrise	Sunset
1st quarter	After Midday	After Midnight

And from there, it is quite easily to interpolate for all the intermediate phases. There is a bit of a catch during daylight savings though, as the sun and moon remain strangely oblivious to the laws of man. Each of the tides when appear an hour later during daylight savings time.

So, if you ever come to Sydney and happen to see a full moon, you can cock your one good eye at the sky and say with confidence: Aarrghhh, it be a fine Spring Tide tonight!"; or if you see a quarter moon: Shiver me timbers, if it won't be a low neap at the beach tomorrow morning!".

Comments

[mnfaure.livejournal.com]

I really loved observing the moon and tides when we lived on Mayotte. I love watching the moon anytime, but there is something extra special about it when you live near a body of water and can watch the moon's effects on it.

[khiemtran.livejournal.com]

Yes! It still feels magical (and awe-inspiring) that such an amount of water can be moved by a distant (but huge) object in space). Or, for that matter, when I'm in a canoe and paddling against the tide, that it's actually the moon that I'm up against.

[mnfaure.livejournal.com]

Or, for that matter, when I'm in a canoe and paddling against the tide, that it's actually the moon that I'm up against.

I love this thought! Thank you for making me think in new ways. :D

[asakiyume.livejournal.com]

Ditto what mnfaure said! What a great thought--the moon hampering your paddling!

[asakiyume.livejournal.com]

Like you, I knew/know these things intellectually, but it's neat that they hit you afresh, or started to, when you saw the moon to your one side and the sun to your other--the very sight of them becoming body knowledge, in a sense.

I saw this same moon last night, here on the other end of the earth. It was very, very golden, early in the evening, as if it was candlelit from within.

Remind me to try out my Dorset pirate accent next time I'm in Sydney!

[khiemtran.livejournal.com]

The moon was just waning past full as I walked to the pool this morning. The sun hadn't risen yet, so the moon was low in the west (a full moon is always going to be opposite the sun), probably somewhere over Madagascar. If a Madagascan sailor was looking up at that moment, they might have thought a full moon right above me, it must be the middle of the night. By the time I had finished my swim, the moon had disappeared over Africa and the sun had risen. And, though I couldn't see it, I knew that, at the beach, the tide had peaked.

[asakiyume.livejournal.com]

The world--the sea--was doing its deep-breathing exercises, and you were in tune with its exhaling.