There’s no real equivalent in the Southern Hemisphere. If you can see Polaris, you can orient yourself to Earth’s North Pole, and this fact has been used for celestial navigation for centuries. Credit: NASA/JPL-CaltechĪt this moment in history, the north celestial pole is very near the somewhat bright star Polaris-note the name-which marks the tip of the tail of the constellation Ursa Minor (the Little Bear, commonly called the Little Dipper). This visualization shows Earth’s axial precession, a wobble in the planet’s rotational axis with a period of about 26,000 years that is induced by tidal forces from the sun and moon. That circle is 47 degrees across (twice the 23.5 degree angle of Earth’s axial tilt), centered on the point in the sky 90 degrees from the plane of its orbit. As a consequence, the direction of Earth’s axis slowly wobbles in space, making a complete circle once every 26,000 years or so. The moon orbits Earth in a plane tipped toward the bulge, so the moon’s gravity yanks on the bulge as well. The sun’s gravity pulls on that misaligned bulge, creating a torque by trying to get the bulge to align with the plane of Earth’s orbit. Also, Earth’s axis isn’t vertical relative to its orbit around the sun but is instead tipped by about 23.5 degrees. If only things were so simple!Īs Earth spins, centrifugal force makes Earth’s equator bulge-so it’s not a perfect sphere but instead a flattened one, like a beach ball that someone is sitting on. If it were left to its own devices, Earth’s axis would always point in the same direction in space. If you were standing exactly on Earth’s North Pole, the north celestial pole would be exactly overhead. In fact, if you extend this imaginary axis into space, it points to two spots on the sky called the celestial poles. Draw a line between them, and you get Earth’s rotational axis. That spin defines its geographic poles, both North and South. Like a top, Earth spins as well, once per day. The key factor here is torque, a force on the spinning object that changes the direction of spin, which in this case is caused by gravity. The detailed physics is complicated (I wish I had a dollar for every time I had to say that), but the overall action pushes the top sideways, creating the slow precessional motion. The top can’t fall over, though, because it’s spinning rapidly, creating a gyroscopic effect. The rotation axis tilts just a little bit, and gravity tries to pull the spinning top down. The top does this because its spin is not perfectly vertical. But after a moment, the top will start to slowly wobble, and the axis itself will make a slow circle around the vertical, drawing out a virtual cone in space. If you spin a toy top on a flat surface, its axis of rotation will be very nearly vertical.

And you’ve likely seen it before but on a far smaller scale. But there is a third motion that Earth makes that is called precession. These have obvious effects on our daily life-it’s right there in the adjective-as well as during our lifetime. The other motion is Earth’s annual revolution around the sun, the time it takes to orbit once: our year. One is its diurnal rotation, the daily spin it makes around its axis, like a basketball twirling on top of a grandstanding player’s finger. You’re already familiar with the two major motions Earth makes in and through space. Earth’s wobble has affected climate, navigation and our calendar. Over a human lifetime, the effect is so small as to be unnoticeable.īut over humanity’s lifetime, the effect has been not only noticeable but profound. While the wobble is pretty big, it takes a long time to play out. You wouldn’t personally notice it, though.

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