Probably the most accepted current theory of Earth's tilt (currently 23.4°) is an ancient collision with a large body, which sent a large part of our crust into space to become our Moon.
Axial tilt is measured relative to the ecliptic plane (the plane in which the planets orbit), which is a sensible zero point that indicates the rotation direction of the accretion disk of collecting dust and gas that formed the planets (and Sun) originally.
Now consider that all planets have some axial tilt (though Mercury's is negligible, probably due to the overwhelming influence of solar effects), and some are quite extreme. Uranus is tilted 97.77° -- not just tilted onto it side, but slightly beyond horizontal. Venus, tilting 177.4°, is almost exactly upside down.
Consider the immense forces that knocked these planets over. What remaining evidence can we see of those cataclysms? The sun itself is tilted -- a modest 7.25° -- but the Sun has 1000 times the mass of Jupiter and 500x the rest of the solar system put together. What could have tipped it? An encounter with another star would likely have affected the planetary orbits.
My point isn't to advance theories (though you should feel free to). It's just to inspire a little "thought and awe" about the stodgy boring crawl of the planets around the sun.
Planetary axial tilt - something to think about
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