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The ecliptic, zodiac and precession

Mapping the sky

Astronomers use many imaginary lines for mapping the sky, one of these is the ecliptic. This is the apparent path the Sun appears to take through the sky as the Earth orbits around it. The Earth's annual motion around the Sun causes the Sun to appear to shift eastwards through the zodiacal constellations eg, Aries, Taurus, Gemini, Cancer and so on.

The moon and the planets are mostly seen within a zone of about 8° either side of this imaginary line, because the solar system looks like a flattened disc, the apparent paths of the sun, the moon, and the major planets all fall within this zone, as most of the planets orbit in the same plane. 

The ecliptic is inclined at a 7° angle from the plane of the Sun's equator. The Earth's axis is tilted at a 23.5° angle from the ecliptic (this produces the seasons.) The planet Pluto's orbit deviates more than any other planet from the plane of the ecliptic, with an inclination of 17°.

The zodiac is divided into 12 equal parts of 30° each, forming the twelve constellations of the zodiac, (zodiac means - circle of animals), as most of the zodiacal constellations are animals - Aries the ram, Taurus the bull, Leo the lion, Cancer the crab and so on. The Sun slowly passes through each of these constellations over the course of a year. Actually the sun passes through thirteen constellations, Ophiuchus is not included in the zodiac, yet the Sun spends more time in this large constellation than nearby Scorpius. In 2003 the sun spent less than a week in Scorpius and just over two weeks in Ophiuchus (the serpent bearer). This anomaly is due to the redrawing of the constellation boundaries in 1928.

When the constellations of the zodiac were named over 2,000 years ago, the vernal equinox coincided with the beginning of the constellation Aries. For this reason, the first 30° section of the zodiac is called Aries and it extends eastward 30° from the vernal equinox, which is therefore called the first point of Aries. However, because of the precession of the equinoxes (see below), the vernal equinox has moved westward about 36° and now lies in the constellation Pisces; the zodiacal constellations thus no longer correspond to the segments of the zodiac represented by their signs. The constellations will again coincide with the original sections of the zodiac in about 23,000 years. The zodiac probably had its origins among the Assyrians or Chaldeans, although it may have originated among the Babylonians as early as 2000 B.C.

(Precession - In Physics = The motion of the axis of a spinning body, such as the wobble of a spinning top, when there is an external force acting on the axis. In Astronomy = A slow gyration of the earth's axis around the pole of the ecliptic, caused mainly by the gravitational pull of the sun, moon, and other planets on the earth's equatorial bulge with respect to the celestial sphere.)

The Earth's equatorial bulge, (the equatorial diameter is 21 kilometres greater than the polar diameter) in combination with the obliquity of the ecliptic, causes the moon, the sun and the other planets to exert forces on the Earth which produces a torque about its centre of mass. This torque causes the Earth's axis to precess. The radius of this circle subtends an arc equal to 23.45 degrees (the obliquity of the ecliptic), the precessional period is approximately 25,800 years. The corresponding oscillation of the celestial equator causes the Vernal Point (0 Aries) to move slowly westward at roughly 50 arc seconds per year. In other words the Earth's rotational axis gyrates around the vertical, tracing a cone shape in space over a period of 25,800 years

Precession was discovered in the second century BC by Hipparchus, a Greek astronomer. Hipparchus made measurements of stars brightness and positions, compiling a star catalogue that was used for centuries. While doing so he compared stars he observed with observations made by astronomers over a century earlier. He found that there was a systematic shift in the ecliptic longitudes of the stars. Since the vernal equinox marks the zero point of the ecliptic co-ordinate system, he called the motion "precession of the equinoxes.”

Earth' axis meets the sky at the north and south celestial poles (NCP/SCP), as the axis precesses, the NCP and SCP change.

At present the NCP is about 1degree from Polaris the north star, in a century Polaris will come even closer to the NCP before passing on. 2,000 years ago Polaris was 12° from the NCP. Currently there is no bright star near the SCP, Sigma Octanis is close by but at Mag 5.4 is fairly faint. Two bright stars will become the SCP star in the future, Aspidiske (Iota Carina) and Delta Velorum, both about Mag 2 stars.

In about 12,000 years the NCP will pass about 6° from the Star Vega (Mag 0.) Because of precession the position in space using the Declination and Right Ascension of a particular star changes as well, RA and Dec of a star given today will change with time so these are updated every 50 years.

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