SJAA Ephemeris August 2004 | SJAA Home | Contents | Previous | Next


Opposites Attract

Dave North


After July's two full Moons, we won't see another until near the end of August, after the Perseids have pretty well died down. If you stayed up to see them, you may have watched the late moonrise, often a nice reward for losing all that sleep.

This particular full Moon will have quite a strong libration, principally in the North. It's one of those nifty opportunities to see some of that stuff on the far side, and if I have the calcs right there will also be a small terminator up there throwing some shadows for relief.

When you get that close to the limb foreshortening takes a strong grip on what you see. Because of that, it's not easy to predict what might look good or not, but it only takes a few minutes to look and find out for yourself.

Full Moon is, of course, also the time of weak tides.

Of course?

Well, yeah. The thing is, at that point the Sun and Moon are opposition. To state the obvious, that means they are opposite each other in our sky. Which also means their gravitational influences are pulling against each other, to some extent canceling each other out.

The Moon has the weaker gravity of course, but because of proximity it has the stronger influence. So even the Sun can't completely overcome its tidal pull.

The result is the "neap" tide, or lowest high tide (and of course we'll also see the highest low tide about six hours later).

I think "neap tide" is a neat phrase.

Oh wait, did I say six hours? If you're visiting in Texas, that won't be true.

Over most of the earth, there are two tidal bulges and two tidal troughs -- each bulge will be opposite (and slightly trailing) the Moon. The other two are 90 degrees off. The near bulge is easy enough to get -- that water is closer to the Moon and therefore "tugged into the sky" just a little bit.

The other opposing bulge is for the opposite reason. It's further from the Moon so there's less attraction than at any other point on Earth, so it tends to just slough away. So in that case it's sort of pushed into the sky, though that's an awful way to say it.

Good thing I'm not a real scientist or I'd probably be in trouble.

And for another small digression, astrologers probably just love it that the Moon and Sun both actually do have an observable effect on our lives here on Earth. But I'm not an astrologer either, so I can say it's all just a bunch of hooey anyway.

Oh, I was talking about Texas.

Tides are basically a great big wave that rolls slowly around the earth each day. As you've already seen, they get bigger and smaller depending on the relationship of the Sun and Moon.

They also depend on the topography of the oceans and the land that interferes with their flow. Because the Gulf of Mexico is pretty constricted from the main Oceans surrounding it, the tides are somewhat crippled and there are only really two per day.

This is not uncommon. The same situation prevails also in the Mediterranean Sea, but it's so cut off from the major flow of The Wave that the tides can more or less simply be ignored for navigational purposes.

The opposite effect can be seen in the famous Bay of Fundy, where a massive tidal bore causes the water level to shift suddenly by several feet, and tidal differences typically run well over 20 feet in a day.

Nevertheless, most coastal zones will see a very significant tide because of the Moon. Those tides are a critical part of the dynamic that 'sucks' nutrients up from the bottom of the continental shelves (where they have simply fallen in the form of dead plants and animals and, well, fish poo).

Without that recirculation the coastal zones wouldn't have their heavy fish populations. Fishermen's Wharf would be a mere shadow of itself.

The implication is, of course, that the highest high tide (spring tides) happen at new Moon, and average tides at first and third quarter.

So if you live anywhere near the seashore, you can observe the Moon even when it's not up.

Just watch the water level rise or fall!

If you'd like to get a closer look on how this works locally (or anywhere else in the world) I highly recommend the wonderful (and free!) program xTide to run on your Linux (or other Unix) machine.

With it you can view tidal patterns, including graphs, for just about any time or place you can think of. It's surprisingly granular.

Those of you who are operating-system challenged can always take an afternoon to set up a dual-boot machine. Who knows, after a while you might even find a use for your original OS.


Previous | Contents | Next