Hipparchus and the precession of the equinoxes
The axial wobble was another effect Milankovitch studied, but he was not the first person to notice it. That person was the Greek astronomer and mathematician Hipparchus (c. 190 b.c.e.-c. 120 b.c.e.). Hipparchus was the greatest of all Greek astronomers, and some of the discoveries he made and the deductions he made from them are still important today—the axial wobble is one of them. He also calculated the length of the year as 365.25 days, diminishing by 0.003 day each year, and the lunar period as 29 days, 12 hours, 44 minutes, and 2.5 seconds—which is one second too short.
Hipparchus was born in Nicaea, now called Iznik, in northwestern Turkey, and he established his observatory on the island of Rhodes, in the southeastern Aegean Sea. His research centered on measuring the size and distance of the Sun and Moon. In order to achieve this he was obliged to study the rotation and orbit of the Earth.
At the equinoxes the Sun rises exactly in the east and sets exactly in the west. There were no reliable clocks in the time of Hipparchus, so astronomers could not determine when day and night were of equal length, but they could identify the points of the compass and so they could recognize the equinoxes and use them as a basis for the calendar.
As the Sun crosses the horizon at dawn on the equinox its position marks the point where the ecliptic—the path of the Earth's orbit—intersects the celestial equator. Imagine a line drawn from the center of the Earth to the Earth's geographic equator and then extended to the very edge of the visible universe. Then turn the Earth so that the end of this line describes a circle around the "inside edge" of the universe. That line is the celestial equator and the sphere it encloses is the celestial sphere.
Astronomers measure the position of the Sun when it intersects the celestial equator by reference to the stars. Their positions remain con-stant—they are sometimes known as the "fixed stars"—and the Sun's position can be referred to the patterns of stars known as constellations. Unfortunately, this presents a difficulty: in daytime the stars are invisible. Hipparchus solved the problem by waiting for a lunar eclipse, when the stars are visible and the position of the Sun is known. A lunar eclipse happens when the Moon, Earth, and Sun are in a straight line, with Earth in the middle. Earth's shadow then falls on the Moon, and therefore the center of the shadow—in fact the center of the Moon's disk, which remains visible because some sunlight is refracted by the Earth's atmosphere—is at a point on the celestial sphere directly opposite the position of the Sun.
In about 130 b.c.e. Hipparchus measured the intersection at the equinox and compared his results with old records made by earlier astronomers. He concluded that over a period of 169 years the intersection had moved by two degrees. He called this the precession of the equinoxes.
In Hipparchus's day, at the spring equinox the Sun intersected the celestial equator in the constellation of Aries. By the time of Christ intersection occurred in Pisces. Today it is moving into the constellation of Aquarius. In other words, Earth's orbital position at the equinoxes is changing. Each year Earth is a little to the west of the position it occupied the previous year. This is the effect of precession. Hipparchus calculated the rate of precession as 45 or 46 seconds of arc a year. In fact, the rate is 50.26 seconds per year, so Hipparchus was very nearly correct.
Precession has another effect. Nowadays, we can find north on a clear night in the Northern Hemisphere by locating Polaris, the North Star or Pole Star, which stands vertically above the North Pole. Changes in obliquity mean that this has not always worked in the past
When the cycles coincide
and will not always work in the future, because the Earth's axis does not always point to Polaris. Around 3000 b.c.e., for example, the axis pointed to the star Thuban in the constellation of Draco, so that was the Pole Star. Hipparchus was unable to use this method of finding north, however, because in his lifetime there was no star directly above the North Pole.
Continue reading here: The significance of precession
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