Wednesday, June 1, 2011

How earth's path around the sun is connected to surfing

The previous post discussed the connection between the motion of the earth around the sun and the ancient double spiral symbol.

We saw that the tilt of the earth's axis (the obliquity of the ecliptic) causes the sun's path to cross back and forth above and below the celestial equator throughout the year, and also that the eccentricity of earth's orbit around the sun causes the earth to move faster as it approaches perihelion each year, a fact largely responsible for the analemma or figure-eight pattern the sun will trace out in the sky if viewed at the same time on successive days for an entire year.

The perihelion is the point on earth's elliptical orbit where the earth comes closest to the sun, so-called because it is derived from the Greek peri- meaning "around" and Helios, the sun or the sun god (a "perimeter" is the meter or "measurement" around something, and the "pericardium" is the sac that is around the kardia or heart). The point of perihelion is opposed by the aphelion, or furthest point in earth's orbit from the sun.

This brilliant animated video from the excellent analemma explanation site of Bob Urschel of Indiana illustrates visually the reason that the earth travels faster as it approaches perihelion and slows down as it approaches aphelion, as opposed to the steady rate of orbit that would occur if the earth followed a perfectly circular path.

In the video, a green earth on a circular orbit is contrasted with a blue earth on an elliptical orbit, and the point of perihelion is placed at the bottom of the screen, so that the earth on the elliptical orbit speeds up as it falls "down" towards perihelion and then slows as it "rises" towards aphelion, only to begin to accelerate again as it passes aphelion and begins to fall again towards the sun and the bottom of the diagram. In reality, this is very much what is actually taking place. We think of the bottom of the diagram as being "down," because the earth's gravity pulls us downward towards it at all times, but in earth's orbit "down" is towards the sun, whose gravity pulls earth towards it, so that what is "down" in the diagram is actually "down" towards the sun and the point of perihelion, where earth travels fastest before it slingshots around the sun and begins to rise up again.

For the earth, perihelion occurs roughly fourteen days after winter solstice in the northern hemisphere, illustrating that winter is not caused by the earth being further from the sun (since perihelion is where earth is actually closest to the sun) but rather by the fact that the axis of that hemisphere is pointed away from the sun, creating shorter days with less direct sunlight and a lower path of the sun across the sky than during summer.

When I watched this video of earth's motion around the sun each year, I was struck by the parallel to the motion of surfing. The video at top shows the great Shaun Tomson surfing at Jeffreys Bay, South Africa. Notice that he initially plunges towards the bottom of the wave (perihelion) where he executes a bottom turn and then carves back up the face of the wave towards the lip (aphelion), where he then turns again downwards to pick up speed again.

Now watch this animated video from the same analemma site that shows the sun's motion across the celestial equator as seen from earth, while keeping in mind the motion of Shaun Tomson going up and down along the wave at J-Bay. Watching it, you can understand why the ancients came up with metaphors to encode in their myths such as the serpent that winds all the way around the earth (Midgard's Serpent), but you can also see that they could have equally well selected the metaphor of surfing along a very long right such as the kind found at Jeffreys.

The mysterious connections between the rhythms of the celestial motion and the act of surfing have not been sufficiently explored yet, but there is definitely something going on here that is worth investigating further. For other musings on this topic, check out this previous post as well.

For a more detailed explanation of the concepts of the ecliptic, the celestial equator, and the passage of the ecliptic back and forth across the celestial equator throughout the year, check out the lengthy third chapter of the Mathisen Corollary, which is designed to make sure you never feel intimidated by those terms ever again but instead find them completely comfortable and understandable.