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Written by Stephen Langfur
We cannot look into the sun, but we can deduce its position in relation to the zodiac as follows. If we chart the sun's course during the day and then consult the night sky, we can find a point where the charted course goes through a zodiacal constellation. The constellation that is opposite that constellation on the zodiacal belt is the one that the sun would have passed through during the day.  

The earth is tilted on its axis, and the tilt remains the same as it revolves around the sun. In the winter of the northern hemisphere, the tilt exposes less of that hemisphere to the sun. The sun appears low in the sky and the days are short. On March 21 (the spring equinox) the tilt doesn't make any difference: both hemispheres are equally exposed to the sun. In the northern summer, the tilt exposes more of the northern and less of the southern. Around September 22 (the vagueness is because of the extra quarter day in a year), the tilt again doesn't make any difference, and we have the autumnal equinox. At an equinox, the sun rises directly in the east and sets directly in the west. In the northern hemisphere, before reaching the autumnal equinox, the sun rises and sets more and more to the north;  afterwards it rises and sets more and more to the south. Since the earth is roughly a sphere of 360 degrees and the year has 365 days, the sun's course seems to change position by about one degree each day.  

The earth's axis does not spin in a perfect straight line, however. The moon and the sun exert gravitational attraction on the bulge at the equator, so that the spinning of the axis, if pictured from the center of the earth to one of the poles, resembles a cone (think of a spinning top). Now if we project the earth's equator onto the (fictional but useful) celestial sphere, calling the result the "celestial equator," we see that as a result of the imperfect spinning, the celestial equator moves a bit on the celestial sphere. On the other hand, the plane of the earth's orbit around the sun remains unaffected by the warp in the axial spin. Since the equinoxes lie at the points where the celestial equator intersects the plane of the earth's orbit, they move on the celestial sphere. This movement is called the precession of the equinoxes. (They make a complete round, returning to the same point, about every 26,000 years.) Because of this precession, there is discrepancy between the astrological signs, established thousands of years ago, and the actual constellations that the sun "passes through." For example, the sun enters the astrological sign of Aries (the ram) at the spring (vernal) equinox, but it will take nearly a month before it reaches the actual constellation of Aries.