The Ecliptic. Because of the orbital motion of the Earth, the Sun appears to us to move around the celestial sphere taking one year to complete a revolution. This apparent movement of the Sun is called the Ecliptic. A year is approximately 365.25 days in length. However; for the sake of convenience, the Gregorian calendar divides three years of the cycle into 365 days and the fourth (the leap year) into 366.
The Age Of Aquarius. In a popular song, the words the ‘dawning of the age of Aquarius’ refer to the period when the vernal equinox will lie inside the constellation Aquarius. The vernal equinox is the point where the Sun crosses the Equator on its northward movement along the ecliptic and heralds the first day of spring in the northern hemisphere on 20th./21st. March. This point is known as the ‘First Point of Aries’ because in 150 B.C. when Ptolemy first mapped the constellations, Aries lay in that position. However, although still named the ‘first point of Aries’, due to precession, the vernal equinox now lies in the constellation Pisces, so logically, it should be named the ‘first point of Pisces’ since we are now in the ‘Age of Pisces’. There are various predictions of when the next ‘age of Aquarius’ will begin but the most prominent of these is about 2600 A.D.
The Tropic of Cancer. These days, the Sun passes through the constellation Cancer in late July; however, in the time of Ptolemy, this occurred during the summer solstice when the Sun reached 23.4o N, the northern limit of the ecliptic. The latitude 23.4oN is still called the tropic of Cancer even though the Sun now resides in Taurus at the summer solstice.
The tropic of Capricorn. In a similar way, the Tropic of Capricorn is so named because the Sun once passed through the constellation Capricornus during the winter solstice on 21/22 December when the Sun’s declination reached its southernmost latitude of 23.4oS. However, due to precession, the Sun is now over the constellation Sagittarius at the Winter Solstice.
Note. The latitude of the tropic of Cancer is currently drifting south at approximately 0.5 arc seconds per year while the latitude of the tropic of Capricorn is drifting north at the same rate.
Astronomy for Astro Navigation
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