Where The Planets Are

Some 5 billion years ago, a swirling light-year-sized pocket of gas and dust collapsed into a disk to form our solar system.  This flat disk continues to swirl as planets and asteroids experience their perpetual Newtonian fall around the Sun.  The flat disk of the solar system has a practical implication for stargazers: it means most solar system objects are confined to a narrow band of the sky as seen from Earth. The imaginary line through this band is called the ecliptic, and the group of constellations that fall along the ecliptic is called the zodiac.

The concept of the ecliptic and the zodiac makes it easier for stargazers to find the planets.  You will nearly always find major solar system objects within a few degrees of the ecliptic (because the solar system’s disk is not completely flat) and in one of the constellations that lie along the zodiac.  In 1930, the International Astronomical Union sliced up the zodiac into 12 equal spans of 30º along the ecliptic.  With this definition, 13 constellations fall along ecliptic: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Ophiuchus, Sagittarius, Capricornus, Aquarius, and Pisces.  Though for historical reasons, Ophiuchus is not officially included in the twelve zodiacal constellations.

This is why you will never find a major planet, the Sun, or the Moon in constellations like Perseus or Ursa Major or Centaurus.. they are nowhere near the ecliptic.  Though very rarely, a planet will be found far enough from the ecliptic to just barely skirt into constellations like Orion that are close to the ecliptic.

And some objects like comets and asteroids have orbits that are highly inclined to the plane of the solar system, so they can appear in non-zodiacal constellations.

The ecliptic itself is a great circle around the sky, much like the celestial equator.  But it’s tilted by 23.5º with respect to the celestial equator because the Earth itself is tilted 23.5º with respect to the plane of the solar system.  Throughout the year, the Sun follows the path of the ecliptic.  When the Sun arrives t the southernmost and northernmost points on the ecliptic– the solstices– summer and winter begin.  When the Sun arrives at the points where the ecliptic crosses the equator– the equinoxes– spring and autumn begin.