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Earth spins on an axis tilted about 23.4°, and the tilt gives us the seasons. Over a cycle of about 41,000 years it rocks only between about 22° and 24.5°. The Moon's pull is thought to keep it this steady; without a large moon, the tilt could swing far more, with much bigger shifts in climate.

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Even this small rocking matters: it is one of the slow orbital cycles, worked out by the Serbian scientist Milutin Milanković, that help pace the ice ages. The Moon's pull on Earth's slightly bulging equator makes the axis wobble at a steady pace that stays out of step with the tugs of the other planets, so those tugs cannot build up. In 1993 the French astronomer Jacques Laskar and colleagues calculated that without the Moon the tilt could swing chaotically, at times approaching 90°. Later simulations, in 2011, found smaller but still large swings of about 20 to 25°. Mars, which has only two tiny moons, has seen its tilt vary between about 0° and 60° over millions of years.

Türkçe oku
Dünya yaklaşık 23,4° eğik bir eksen çevresinde döner; mevsimleri bu eğiklik yaratır. Eğiklik yaklaşık 41.000 yıllık bir döngüde yalnızca 22° ile 24,5° arasında gidip gelir. Bu dengeyi Ay'ın çekimine borçlu olduğumuz düşünülür; büyük bir uydumuz olmasaydı eğiklik çok daha fazla değişir, iklim de çok daha sert dalgalanırdı.

Bu küçük salınımın bile etkisi büyüktür, çünkü buzul çağlarının zamanlamasında rol oynayan ve Sırp bilim insanı Milutin Milanković'in hesapladığı yavaş yörünge döngülerinden biridir. Ay, Dünya'nın hafifçe şişkin ekvatorunu çekerek eksenin düzenli bir hızla yalpalamasını sağlar. Bu hız öteki gezegenlerin çekimiyle aynı ritme girmediği için onların etkisi birikip büyüyemez. Fransız astronom Jacques Laskar ve ekibi 1993'te, Ay olmasaydı eksen eğikliğinin düzensiz biçimde değişip zaman zaman 90 dereceye yaklaşabileceğini hesapladı. 2011'deki simülasyonlar ise daha küçük ama yine de büyük, 20-25 derece dolayında salınımlar buldu. Yalnızca iki minik uydusu olan Mars'ın eğikliği ise milyonlarca yıl içinde yaklaşık 0 ile 60 derece arasında değişmiştir.

Sources and further reading

Background reading: Axial tilt
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