The Moon does not reach down and lift the sea
the way a hand might lift a sheet of cloth;
it pulls on everything, on you and me,
on rock and wave and every drifting moth.
But pull grows weaker as the distance grows:
the nearest sea is tugged the most of all,
the solid Earth a little less, and those
far waters on the other side, the faintest call.
The near sea swells towards it, and the far,
pulled least of all, is left to swell behind;
two bulges, then, wherever oceans are,
and Earth rolls under both, the two combined.
That’s why the beach is swallowed twice a day,
and why it comes some fifty minutes late:
the Moon has moved along its own slow way,
and we must turn a little more, and wait.
The Sun pulls harder on us, yes:
it keeps the whole Earth on its track.
But tides come from the more and less,
the near side’s pull against the back,
and that fades out so fast with space
that, being so far away, the Sun
can raise, in any time or place,
not half the tide the Moon has done.
When both line up, at new and full,
their bulges stack, and spring tides rise;
at quarter moons they crosswise pull
and leave the neaps, of modest size.
So when the harbour empties, and the boats
lie tilted in the mud, and gulls walk where
the fleet, a few hours earlier, was afloat,
it’s just the Moon, being stronger here than there.
A small experimentWatch the tide run late
- Find the high tide times for the nearest coastal city for today and the next six days.
- Write down the time of the first high tide each day.
- Work out how much later it is each day than the day before.
- Look up the Moon's phase for the same week and note which days have the biggest high tides.
What's going on?
High tides arrive roughly 50 minutes later each day because the Moon moves along its orbit, and the Earth has to turn a little further to catch up with it. The biggest tides, the spring tides, come around new and full moon, when the Sun and Moon line up.