Discovery

From Moon to Sea

Can you tell the state of the tide from the position of the Moon?

Geoff Hitchcox

Is there any relationship between the Moon's position and the tide? Is it the same everywhere in New Zealand or is it just a random association?

If you are like me and enjoy natural history, the ocean, and walking on the beach, tidal behaviour becomes part of this fascination. We all know from school that tides are caused mainly by the Moon and, to a lesser extent, the Sun. If you reach for your nearest book about tides, there is usually a simple drawing showing the Moon causing a tidal bulge on the Earth, due its gravitational pull. In these simple diagrams, you can see that if the Moon is directly overhead, then it is high tide. Is this true for where you live?

A few years ago, whilst walking along the beach, I thought (being a software writer) that it would be a simple matter to produce a tide table. This launched me into a study of tidal prediction which is not as simple as I had thought. Indeed the number-crunching required to predict tides was (along with Large Gun Aiming Tables) one of the main motivations for building electronic computers.

In simple terms, the Moon and Sun are but triggers for the slip-slop of the water in the marine basin that makes up the local coastline. The main triggers are where the Moon and Sun are in their orbit, and how close they are (remember the orbits are elliptical not circular, so the distance between these objects and the Earth can vary). These and many other factors combine, adding or subtracting to produce the height of water at the beach.

To obtain these factors, the tide at any location must be measured accurately over a long period of time. The waveform of the tide is then mathematically analysed to work out the component amplitude and phase of the triggers. For every reference harbour there are therefore many constituents that, when fed into a computer, along with the position of the Moon and Sun, predict the tide. Not included in this prediction are significant factors such as wind, which causes waves, and atmospheric pressure.

Having produced or obtained a tide table for your local area, the next step is to plot on paper where the Moon happens to be in the sky at high or low tide. You could do this by careful observation of the position of the Moon, but this would require some unsociable hours and you'll have to have a clear sky! You could also sit down with a calculator and spend a lot of time determining the Moons position at the extreme tide times, but this is hard work and very slow. It's more comfortable and quicker to get your computer to do the hard work.

Plotting the Moon's position in relation to high or low tide produces what I call a Moon-Tide Sky Picture. The Moon rises in the east, passing through the two shaded rectangles to then set in the west. The vertical line in the diagram is true north (not magnetic); the zenith is the point directly above your head -- the centre of the sky. Hold the Moon-Tide Sky Picture up correctly and you can transfer the two shaded areas to the same coordinates in the sky. You can then read the tide by looking for where the Moon is.

There are two Moon positions that mark low and high tide -- the other high and low tide in a 24-hour period occur when the Moon is below the horizon.

Why is the Moon's position represented by a rectangle instead of a single point in the sky? If you plot the Moon's position on paper for a month of tides, you will find it produces interesting figure-eight type patterns. The rectangle represents the approximate boundaries of the plotted positions.

The length of the rectangle reflects the path of the Moon near the ecliptic (the path which the Sun and planets follow); this is a curve in the sky which changes on a seasonal basis. The Earth revolves daily about its axis which is tilted by 23.5^o to the plane of the annual orbit around the Sun. This 23.5^o tilt gives us our seasons and, for sky watchers, an ever-changing position of the Sun, Moon and planets, compared with the more fixed positions of the stars.

The width of the rectangle is due to the fact that it is not just the Moon's position in orbit that determines the tide. If the Moon orbited in the same plane as the Earth's equator in a circular orbit, and the Sun was much further away, then our plot would just be a single point not a rectangle.

The Moon-Tide Sky Picture shows that, if you know where true north is and you can see the Moon, you can accurately determine the local tide. Each location will have a different sky plot. In Auckland, the two main harbours of the Waitemata and Manukau are close geographically, but they have different pictures because the Waitemata has an east coast tide, and the Manukau follows the west coast tide.

The Moon's position in relation to the tide is different at each location. For the same position of the Moon in the sky as seen from Christchurch and Auckland, the tides at Lyttelton and Manukau are the complete opposite.

What is the use of this? Well for those of us who like to watch the clockwork motions in the sky, it is very rewarding to be able to see the Moon and read the tide. Its like being able to judge the time of day by the Sun's position.

If you spend time near the water, swimming, fishing or boating, you may need to know if the tide is ebbing or flowing. So if you can see the Moon and can translate the local skyplot to the real sky, you can tell the direction of the tide without having to wait for it to become obvious. If you can see the Moon, you can decide very quickly if it's wise to build a sandcastle at the water's edge! Most people won't know the Moon-tide relationship in their locality, so it is a chance of doing a little original research.

You only need plot the Moon for one month to give a good enough chart. However plotting over a longer period, such as a year, will generate a more accurate picture. Also you can test whether the old saying "Moon overhead, then it must be high tide" is true or not for your location. For Christchurch, if the Moon is directly overhead, the tide has just been low and is now flooding -- it certainly isn't full tide.

Of course the Moon is not always visible when we go to enjoy the beach or coastline, especially around New Moon. By using the Moon-tide diagram, if you can see where the tide is, you can then figure out where to look in the sky for the Moon (assuming it is above the horizon).

Those of us who dabble in astronomy know that a Full Moon rises just after sunset, so if you live in Christchurch and you take your paramour or partner for a midnight stroll on Brighton Beach at Full Moon, the tide will be out and you can walk more comfortably on the hard sand -- who said science wasn't useful?

For those who like complex 3D puzzles, if you know the current phase of the Moon, you can determine the Moon's position (and hence the tide) from where the Sun is. For example, at New Moon, the Sun and Moon are relatively close, so for one day a month you can use the Moon-tide diagram effectively as a Sun-tide chart. At First Quarter (when half the Moon's face can be seen), the Moon is 90^o to the east of the Sun, so that will tell you where the Moon can be found and, thus, what the tide will be doing.

The tides affect our trade, wash our beaches and give wonderful variety to our coastlines. The Moon is said to be one of the reasons why diversity and life developed on earth, due to the big wooden spoon effect of the Moon-induced tides mixing the primordial soup. We owe a lot to the Moon, so next time you observe it, see if you can determine the tide in your area, and think of the Moon as the little hand of the celestial clockwork that drives the tide.

Geoff Hitchcox can be found wandering Christchurch beaches.