What is the ecliptic?
As the Earth orbits the sun, the sun appears to pass in front of different constellations. Much like the moon appears in a slightly different place in the sky each night, the location of the sun relative to distant background stars drifts in an easterly direction from day to day. Itâ€™s not that the sun is actually moving. The motion is entirely an illusion caused by the Earthâ€™s own motion around our star.
Over the course of a year, the sun appears to be in front of, or â€śinâ€ť, different constellations. One month, the sun appears in Gemini; the next month, in Cancer. The dates listed in the newspaperâ€™s horoscope identify when the sun appears in a particular astrological sign. For example, March 21 through April 19 are set aside for the sign Aries. But your astrological sign doesnâ€™t necessarily tell you what constellation the sun was in on the day you were born.
If only it were that simple!
To understand why constellations no longer align with their corresponding signs, we need to know a little bit more about how the Earth moves. And something about how we measure time.
Time is a fiendishly difficult thing to define, especially if we insist on using the sun and stars as a reference. Our calendar is, for better or worse, tied to the seasons. June 21â€”the summer solstice above the equator and the winter solstice belowâ€”marks the day the sun appears at its most northerly point in the sky. At the June solstice, the North Pole is most tilted towards the sun.
What makes this complicated is that the North Pole is not always pointing in the same direction relative to the backdrop stars. Our planet spins like a top. And like a top, the Earth also wobbles! A wobbling Earth makes the North Pole trace out a circle on the celestial sphere. Now, the wobble is quite slowâ€”it takes 26,000 years to wobble around onceâ€”but as the years go by, the effect accumulates.
Over the course of one orbit around the sun, the direction of the Earthâ€™s axis drifts ever so slightly. This means that where along our orbit the solstice occurs also changes by a very small amount. The solstice actually occurs about 20 minutes earlier than one full trip in front of the backdrop stars!
Since we tie our calendar (and astrologers tie the signs) to the solstices and equinoxes, the Earth does not actually complete an entire orbit in one year. The seasonal or tropical year is actually a hair less time than one full orbit (sidereal year). This means that, each year, where the sun is relative to the stars on any given dayâ€”June 21, for exampleâ€”drifts a very tiny amount.
But wait about 2000 years, and the sun will be sitting in an entirely different constellation
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Article Added on Saturday, June 18, 2016
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