Cruising GP, Jonty Pearce, suggests giving the tide tables a rest and keeping an eye on the moon instead...


The sun was shining on the sea,
Shining with all his might:
He did his very best to make
The billows smooth and bright–
And this was odd, because it was
The middle of the night.

The moon was shining sulkily,
Because she thought the sun
Had got no business to be there
After the day was done–
“It’s very rude of him,” she said,
“To come and spoil the fun!

– Lewis Carroll

When gazing up at a full moon, It still amazes to me that man has actually stood there, thereby totally undermining the notion of the man in the moon living off its cheesy structure. The sun itself may rise and set in a daily predictable sort of way, but the moon has its own complicated life of waxing, waning, and being full or new which, I have to admit, has always slightly confused me. I’ve always forgotten which way round a crescent moon starts and finishes, and my understanding of the term gibbous is vague. In my line of work, PUN’s and DEN’s rule – Patient’s Unmet Needs and Doctor’s Educational Needs. With my annual appraisal looming this afternoon, I decided that I should apply the principles of SEN’s (Sailors Educational Needs) and update myself.

That great learning resource, Wikipedia, taught me that the life of the lunar cycle is best considered from the starting point of the ‘new moon’ – a situation where both the sun and the moon are aligned on the same side of the Earth, resulting in the side of the moon facing the Earth being in shadow apart from the reflected dim ‘earthlight’. In the Northern Hemisphere, as the moon waxes (the amount of illuminated surface as seen from Earth is increasing), the lunar phases progress through new moon, crescent moon, first-quarter moon, gibbous moon, and full moon. During this waxing process, we see the rim of light starting on the right hand side of the moon, spreading leftwards till the moon is full. The moon is then said to wane as it passes back through the gibbous phase, third-quarter moon, crescent moon (with the rim of light now on the left) and back to new moon.

I am now happy in my lunarcy educational status and can tell that a crescent moon concave to the left is waxing while conversely a crescent moon concave to the right is on the way out. And gibbous simply means that the moon phase is between half and full! Watch out for future references to ‘sailing under a gibbous moon’ now I know what I’m talking about.

It is very nice to know all about the phases of the moon, but why worry? The main practical sailing implication of the moon phase is, of course, the state of the tide. Spring tides, with a larger range, fill our shores just after a full or new moon. The lesser highs and lows of neap tides are seen around a half moon. In our own Welsh cruising ground around Milford Haven, an adverse spring tide current can take our speed over the ground down to a knot or two. Conversely, time it right with a fair current and passage times can be slashed. Spring and neap tides also maintain their same times of day – we find it useful to know that at Neyland morning neap high tides occur at around 11am, while spring high tides keep to 6am or thereabouts. With a cill limiting access to our berth to 3hrs either side of high tide, we need an earlier marina departure on a spring tide to be away by 9am, whilst our Sunday evening return can be late in the afternoon. During neap tides, we can have a Saturday lie-in but are unable to return on Sunday until 8pm – rather late with a 3hr drive at the end, so we generally choose to cross the cill by 2pm. The moon makes us choose between an early start with a longer weekend sail, or a lie-in and an early return. In between full and half moon, of course, we have to check the tide tables!

So, watch the moon as it sails gaily through the night (and sometimes day) skies. A little thought and application can make the tide tables a less thumbed publication, and you will feel more attuned to nature.

Jonty Pearce