# Rotational axis derived via tides.

Did any known people derive the rotational axis of a planet in our solar system via observations of tidal force, and not the sun’s path of travel through the sky?

I don’t understand your question. People figured out the time of rotation of the Earth from watching the sky rotate, although until Copernicus they thought it was the sky rotating and not the Earth. People figure out the time of rotation of other bodies from watching their surface markings move around. (Before the invention of the telescope, this was possible only for the Moon.) Tides have nothing to do with it, and the sun’s path of travel has no relevance for any body except the Earth itself.

The moon orbits the Earth pretty much on the axis of the Earths rotation. Correct?
The tides follow the moon around the earth, so they rotate around the earth on the same path as the moon’s orbit.
On an island surrounded by ocean the tide will rise starting at one end, and sweep by until the tide can be seen to rise on the other end. In some places this is very apparent, in others not so much.
A person could mark where the tide first hits the island, and another the last place the tide rises on the island.
The two points could be connected by a line and you have east and west roughly alligned for the Earths axis.
Has a group of people been known to do this at one time.

Somebody could do this more acuratly with floats and real time data collecting today. A good enough result should be possible in more primative situations.

The sequence of the tides is a resultant for both lunar, the larger effect, and solar tides. Deriving the earth’s axis of rotation requires watching and noticing that Polaris doesn’t move. The guy smart enough to resolve the math of the first case is probably going to notice the second case.

Observing the increase in river flow as temperature rises in the spring could be use to determine that snow turns into water when it melts. Having a snowball melt down your back will also give you this information. Which one seems more likely?

Tris

Problem I see with your island example is that you are assume that the tides will move around the island from east to west. But what if the point on the island that shows the “last place the tide rises” is on the southwestern tip?

Drawing a plum line from the (SW) cape to the eastern shore (where you took your “first tide rising” measurement) will not give you a true west to east line… it will be skewed a little SW to NE…

It’s a lot easier to use sky observations.

I can’t answer for sure, but I think the answer to the OP’s question is “no”.

Now, I know that some polynesian cultures made “maps” of the surrounding seas that included tide info, but that is not the same as saying “Forget the Sun. Based on the tides, that way is true East”.

Is it apratical way to do it though as I’d like to know if that method could be used. I’m not talking 100% accurate that people want today. I mean if an explorer 300 300 years ago could have done this experiment, and gotten a usable result.

The tides, unfortunately, are incredibly complex. You’d think that water being fluid and finding its own level and all would make it easy, but it doesn’t. The effects of currents, different temperatures, different salinities, Coriolis force, continents being in the way, and various kinds of drag mess up the way the sea levels vary. I recall years ago reading a catalog that had a tide “clock” that would tell you when High and ow Tide were if you told them where you lived. They wouldn’t sell to some locations, because the tide calculations were so complex and unreliable.
Trying to deduce features of the earth from sea level might be responsible for a major blunder in the ancient world. As L. Sprague de Camp has pointed out, Claudius Ptolemy thought that the Indian Ocean was a land-locked sea, having no direct connection with the Atlantic or Mediterranean. Maps drawn based on his work show a “bridge” of land going from Africa, around the Indian Ocean to Asia, cutting it off as its own lake. Why?

De Camp speculates that Ptolemy knew that the Red Sea and the Mediterranean were of different heights. Since water “seeks its own level”, if the Red Sea connected with the Mediterranean around Africa, then there is no way the levels could be different. Therefore there was a barrier between the two.

Only there isn’t. Sea level isn’t of uniform height around the world, for the reasons above. Ptolemy was incredibly clever (if de Camp’s interpretation is true, as I suspect), but there were still things he didn’t know, and his impeccable reasoning gave him the wrong result.
I think anyone trying to determine the axis of the Earth’s rotation by looking at tides (and how do you compare tides at different places? at least at the isthmus near Suez they’re close together) will end up with an incoherent mess.

The two points and an intersecting line can vary up to 45% from the true east to west vector in either direction. Add in observations at a line tangent to the intersecting line and you can see if both points of intersection at the shore have the tide pass sooner or later than the other opposite point. You can rotate the line over time until the tide reaches the two points of shore intersect, at the same time. doing a physical indicator of how many degrees off east to west the original line was. - or - You could measure the time at which the tide reaches the two shore intersect points made by the perpendicular line, and use time to calculate a angle by which the first line was scewed from the true east to west vector. I’m sure I made that clear as mud. Thanks for the help people. I see there is more problems then I considered, but I think a determined person could accomplish this, with help and equipment. The reason I stated a island i the ocean was to elliminate stuff like shallow water. Maybe a volcanic island in the pacific with no neighbors for better results.

You’d run the data averaged out over time. Maybe a statistical analysis of the collected data, would enhance the results. Throw out any data the doesn’t fit the limits set by the statistical analysis. So in other words a trend analysis like any process you want to garantee will stayed in control during the data collection period, will refine the results, or more of a capability study really. I hadn’t thought of process control statistics, being used on natural processes you can’t adjust, instead of manufacturing you can, before now. I don’t know why. Duh!

A stick and a string, and two days of observations will give you true north to within a degree. Tides, two months as an absolute minimum, assuming a very conical shape, and no other islands within many miles.

Might I ask exactly why you think this might be a good plan?

Tris

What’s the stick, string, and 2 day method? Sounds like something I missed by not being a Boy Scout.

It’s an alternative method I was trying to figure out if it worked. In other words can I do this, not is it the easiest way. I like to think of alternatives to doing things. It broadens your thinking. Now if I’m on a planet without visability of outerspace, because of clouds, I have a possible alternative to determine direction. The challenge, it’s all about the challenge.

Isaac Asimov wrote a neat essay(Constant As The Northern Star) about Polaris and its motion. He was chastising himself for writing something about the ancient Phoenicians being the first to recognize that Polaris marked true north and use it for navigating at sea. However at the time the Phoenicians were doing their navigation there wasn’t any north star. Polaris was nearly 17[sup]o[/sup] from the place where the axis of the earth’s rotation points.

Because of the unbalnced gravitation of sun and moon on the earth’s equatorial bulge, the earth’s axis precesses in a circle that takes about 25000 years to complete. Right now the axis is close to Polaris, just under 1[sup]o[/sup] away, and it will be at its closest in roughly 2100. Then the axis will start to move away from Polaris and in 12500 years will be near the star Vega in the constellation Lyra.