The Ecliptic and the Zodiac

The celestial equator goes all the way around the celestial sphere directly above the Earth’s equator.  There’s another circle that goes all the way around the sky.  It’s called the ecliptic, and it’s tilted with respect to the equator by 23.5º.  The ecliptic is the imaginary circle on the sky that marks the annual path of the Sun.  It’s tilted because the Earth itself is tilted relative to its orbit around the Sun by 23.5 degrees (see above).  If the Earth was not tilted in its orbit around the Sun, the celestial equator and ecliptic would be the same circle.

The tilt of the Earth’s axis, showing the plane of the ecliptic inclined to the celestial equa-tor and the position of the equinoxes and solstices.

The tilt of the Earth’s axis, showing the plane of the ecliptic inclined to the celestial equa-tor and the position of the equinoxes and solstices.

Because of Earth’s tilt, the Sun appears highest in the sky relative to the celestial equator when the Earth is at one position in its orbit.  This happens on or about June 21, and we call this the summer solstice (in the northern hemisphere).  When the Earth is at the opposition side of its orbit in December the Sun is at its lowest point in the sky relative to the celestial equator.  This is the winter solstice.  Between the two, the Sun is right on the celestial equator.  These are spring and autumnal equinoxes when spring and autumn begin.  The equinoxes and solstices are four points on the ecliptic.

What’s more, since all the planets lie near the same flat plane around the Sun, the ecliptic also marks the path of the planets around the sky as they revolve around the Sun.  So every planet, the Sun, and even the Moon, are always found on or very close to the ecliptic during the year.

As it turns out, the great circle of the ecliptic passes through 12 formal groups of stars called constellations.  This group of constellations is called the zodiac, and it includes Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Sagittarius, Capricornus, Aquarius, and Pisces.  (The ecliptic actually passes through a 13th constellation, Ophiuchus, but it is not included in the zodiac because ancient astrologers regarded the number 13 as unlucky).

You’ve covered a lot here.  Who would have thought the sky has so many points and great circles?  It may all seem a little confusing at first.  But don’t worry.  You just need to review this material, think about it on your own, and find these points and circles in the heavens as you examine the sky in the coming weeks and months.