By Steve Humphrey
Since one of the themes of the June issue is Outdoor Living, I thought I would discuss what can be seen by looking up outside at night. In around 150 BCE, a Greek astronomer made an amazing discovery. Using around 200 years of records gained through naked-eye astronomy, he discovered something called the “Precession of the Equinox.”
The Solar System can be viewed as a disc of material revolving around the Sun, containing the eight planets, numerous asteroids and other debris leftover from the formation of the Sun. In ancient times, it was believed that all of that was revolving around the Earth. This disc, called the “Plane of the Ecliptic,” also includes the constellations that make up the signs of the Zodiac. The Moon revolves around the Earth at an angle relative to this plane, moving up and down through it. Eclipses, both Solar and Lunar, can only occur when the Moon is in that plane, in line with the Earth and the Sun.
If you draw an imaginary line from the Sun, through the Earth and onto those Zodiacal constellations, it will pick out a point among those “fixed” stars. If you do this on the day of the Vernal Equinox (that day in the Spring when the daytime hours exactly equal those of the nighttime), that point will be in some particular constellation (currently Pisces). What Hipparchus discovered was that this point moves, or “precesses” through the Zodiac until it makes a complete circuit, and this circuit takes almost 26,000 years. In 2160, it will move from Pisces into Aquarius, which was the inspiration for the song from the play “Hair” called “The Age of Aquarius.” (“Hair” was my favorite play during the 1970s. I must have seen it half a dozen times at the Aquarius Theater in Los Angeles.) What I find remarkable about this discovery was that Hipparchus had only limited data to work with, and only naked-eye astronomical data at that, to calculate the exceptionally long period of this circuit.
It is not clear what Hipparchus thought was moving. Was it the ring of constellations revolving around the Earth? Was it some feature of the rotation of the Earth? Today, we understand it better. The Earth rotates with the axis of rotation at an angle to the plane of the Solar System of 23.5 degrees. It almost certainly began with the axis perpendicular to that plane but was hit by some massive object in its early history which knocked it awry. Many think that a collision with a Mars-sized object is what created the Moon, blasting a large part of the Earth into orbit around it. It is the angle of the axis of rotation of the Earth that accounts for the seasons. In the summer, the Northern Hemisphere is closer to the Sun, while the Southern Hemisphere is farther away. Now, this axis of rotation wobbles like a top, whose spin rate is decreasing. That is, not only does the top rotate around its axis, but its axis also rotates. The precession of the Earth’s axis makes a complete circuit every 26,000 years.
So, even though Hipparchus got the details wrong, and held a geocentric view of the planets, it is still extraordinary that he was able to observe and calculate this phenomenon to the degree of precision that he did.
Steve Humphrey has a Ph.D. in the history and philosophy of science, with a specialty in the philosophy of physics. Questions? Comments? Suggestions? Email him at firstname.lastname@example.org.