File talk:Earth precession.svg

CW, not CCW

I am reverting to the original image because Earth's axial precession is opposite its direction of revolution around the Sun (also opposite the direction of rotation around its own axis), which is why it is called precession rather than an advance. Everyone agrees that the precession of Earth's north celestial pole is CCW around the north ecliptic pole, that is, in the direction from Thuban (the north pole star 5000 years ago) to Polaris (the modern north pole star) as shown in the image at the right (click on it for a large image which will properly show the signs of the years). Rotation directions, CW vs CCW, can be easily confused because they depend on one's point of view. If a car is traveling forward, its wheels rotate CCW if viewed from the left side of the car (driver's left), but CW if viewed from the other end of the axle (from the right side of the car). Similarly, even though the north celestial pole rotates CCW around the north ecliptic pole when viewed from Earth, if viewed in the other direction, that is from the north celestial pole looking toward Earth, Earth's north geographic pole rotates CW around a line perpendicular to the ecliptic.

A good "illustration from H. A. Rey's book" shows Earth's geographic north pole precessing CW (when viewed from above Earth's north pole) connected via several lines around a cone to a view of the north celestial pole precessing CCW around the north ecliptic pole (when viewed from Earth). PRECESIÓN Planetmad by Elplanetmad is an excellent animation also showing these two views, a CW precession on Earth smoothly shifting into a CCW precession among the stars.

Ironically, the NASA Earth Observatory pages from which both images were obtained link to a single animation showing a CW precession, both Earth's Orbital Precession (misnamed) with a CCW still image and Milutin Milankovitch Precession with a CW still image. — Joe Kress (talk) 00:45, 15 January 2010 (UTC)

My reversion wasn't successful as the image is still CCW! — Joe Kress (talk) 00:51, 15 January 2010 (UTC)

Nevermind. It was a caching problem. The image is now reverted to the correct CW precession. — Joe Kress (talk) 02:29, 15 January 2010 (UTC)