Chris Peterson wrote:
JimShea wrote:I have a bone to pick with today's discussion of the APOD.
Actually, Earth's axis of rotation is NOT pointed directly at the sun today (or on any day for that matter). A better description would be to say Earth's axis of rotation on the summer solstice is perpendicular to the plane of its revolution around the sun.
On either solstice, the Earth's axis of rotation "tilts directly toward the Sun" in the sense that it lies on the same plane as a line between the Earth and the Sun. The Earth's axis of rotation is never perpendicular to the plane of its revolution around the Sun. The angle between the two is called obliquity, and is about 23°, which doesn't change with the time of year.
The axis "tilts directly toward the Sun" at a solstice. This does not mean that the axis "points directly at the sun". (I might lean to the left, but not so far that I'm lying down.) Perhaps the confusion has arisen because the "tilt" was not expressly stated in the explanation as the obliquity, which as Chris stated, is the angle between the spin axis and the orbital plane axis (or the angle between the equatorial and orbital planes)
. Not only does the obliquity stay constant, but the direction of tilt, relative to the "fixed" stars, hardly moves over a human lifetime -- it must therefore tilt directly towards the Sun, exactly twice per year.
Edit: IMHO, this bit could have been improved with "tilts" instead of "points":
APOD Robot wrote:... Since Saturn's grand rings orbit along the planet's equator, these rings appear most prominent -- from the direction of the Sun -- when the Saturn's spin axis points toward the Sun. Conversely, when Saturn's spin axis points to the side, an equinox occurs and the edge-on rings are hard to see. ...
(Super impressive APOD by the way. Doubleplus drool.)