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Ocean tides are caused mainly by the Moon: it pulls more strongly on the side of Earth facing it than on the far side, and that difference stretches the oceans. The Sun adds an effect a little under half as strong.

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The story behind the card

The stretching raises two bulges of water, one facing the Moon and one on the opposite side of Earth. As the planet turns, coasts pass through both, which is why many places have two high and two low tides a day. At new and full moon the Sun and Moon line up, their effects add together and the difference between high and low water is greatest, called spring tides; at half moon they pull at right angles and the range is smallest, called neap tides. The shape of the coast then changes everything: in Canada's funnel-shaped Bay of Fundy the water can rise and fall by about 16 metres, the largest tidal range in the world.

Türkçe oku
Gelgitlerin asıl nedeni Ay'dır. Ay, Dünya'nın kendisine bakan yüzünü uzak yüzünden daha güçlü çeker ve bu fark okyanus sularını gerer. Güneş'in etkisi bunun yarısından biraz azdır.

Bu gerilme iki su kabarıklığı yaratır: Biri Ay'a bakan tarafta, öteki Dünya'nın tam karşı tarafında. Dünya döndükçe kıyılar ikisinin içinden de geçer; bu yüzden birçok yerde günde iki kez su kabarır, iki kez çekilir. Yeni ay ve dolunayda Güneş ile Ay aynı hizaya gelir, etkileri toplanır ve kabarmayla çekilme arasındaki fark en büyük hâlini alır. Yarım ay evrelerinde ise ikisi birbirine dik açıyla çeker ve fark en aza iner. Sonrasını kıyının biçimi belirler: Kanada'daki huni biçimli Fundy Körfezi'nde su 16 metre kadar yükselip alçalabilir; bu, dünyadaki en büyük gelgit farkıdır.

Sources and further reading

Background reading: Tide
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