What is the ecliptic?
If you live in the high Arctic or Antarctic, youll see the sun or moon track along your horizon. But most of us  on most parts of Earth  see the sun arc across the sky from east to west, as Earth spins each day. The moon follows the suns path. And so do the major planets in our solar system. This imaginary track across our sky  the suns path during the day  is called the ecliptic. Technically speaking, Earths orbit defines the ecliptic. As viewed from space, the ecliptic is the Earth-sun plane. As viewed from Earth, the ecliptic is a great circle around our sky, formed by the intersection of Earths orbital plane with the imaginary celestial sphere surrounding us. The sun travels around our sky on the great circle of the ecliptic. The moon and planets do, too, more or less. Its mainly because, long ago  before there was a solar system as we know it today  there was a vast cloud of gas and dust in space. This cloud was spinning, and, as it spun, it flattened out. Our sun formed in the center of this cloud. The major planets, including Earth, and most other solar system objects formed in the flat disk surrounding the sun. The ecliptic is this flat disk of planets in our suns family  our solar system  translated onto our sky. Planets follow the ecliptic  So the major planets  and many of the minor planets, aka asteroids  orbit the sun in more or less the same plane. We can speak of this plane as defined by Earths orbit around the sun: the ecliptic. If we could watch the solar system from far above the Earths north pole, wed see the planets, moons, asteroids, and some of the comets (but not all of them) rushing around the sun counterclockwise in this plane, like marbles rolling around a flat dish. Actually, the major planets are more within the dish than on it. Theyre within the plane of the ecliptic, more or less. They retain the outline of the original cloud in space from which they were born, and their movement around the sun is an echo of the original spin of the cloud. Ecliptic defines the zodiac  Far beyond the cold edges of our solar system, we see the stars of our Milky Way galaxy. The stars are moving, too, but theyre so far away that they dont seem to move over the course of a human lifespan. And so we speak of the fixed stars. Fixed stars on the ecliptic  or suns path  seemed special to the early stargazers. They identified constellations made of these stars, and used the word zodiac for the wider pathway traveled by these constellations. And so we find the sun, moon and our major planets within the constellations of the zodiac. Now about that phrase we keep using, the phrase more or less   The other planets dont orbit exactly in the Earth-sun plane. Each major planets orbit is inclined a little bit to this plane. Some of the asteroids have orbits that are more inclined. And comets tend to have the most inclined orbits of all. See a chart at Wikipedia with the inclinations of the major planets orbits. The moon and the ecliptic  Interestingly, Earths moon isnt exactly on the ecliptic, either. Its orbit around Earth is tilted by about 5.15 degrees relative to the ecliptic. This means the moon spends most of its time above or below the ecliptic. It crosses it twice each orbit: once going upward and once downward from our point of view. Therefore, we usually see the moon close to, but not exactly alongside the other solar system objects. On the other hand, the moon sometimes passes right in front of other solar system objects, in an event called an occultation. So there are little variations. But  for all practical purposes of skywatching  you can think of the ecliptic as a line across our sky. You can think of the sun, moon and major planets of the solar system as moving along that line. One thing to remember, though. The suns path is high in summer and low in winter. So the location of the ecliptic in your sky shifts a bit, seasonally. Ecliptic, eclipse  If the word ecliptic sounds familiar, youre right. Its from the same root as the word eclipse, from the Latin and Greek meaning to fail to appear or to be hidden because the moon hides the sun during an eclipse. The ecliptic got its name because the ancients saw that solar eclipses happen when the moon crosses the ecliptic during the new moon phase. Later, astronomers gave the name node to the places where the moon crosses the ecliptic. If the moon traveled exactly on the ecliptic, and the other planets did, too, the moon would occult, or block out, all the planets and the sun every orbit. Wed have lunar and solar eclipses every month. Ho hum. Watch the sky  If youre able, keep an eye on the sun, the moon and the planets for a while. Watch for a few days, a few weeks, months, years, even. Youll begin to get a feel for the ecliptic in your sky. Youll notice the planets, sun and moon are always on or near the ecliptic, and you can use this line across your sky to help you find your way around, making your way between the constellations and stars. Youll notice the suns path  the ecliptic  higher in the sky during the summer months and lower during the winter. Eventually, youll be able to imagine the suns path in your sky, long after the sun has set. When that happens, youll be able to pick out a planet from a star very quickly and easily, which is a great party trick. Mars is the red one; Saturn the yellow one; Venus the bright white one that never gets too far from the sun; Mercury the seldom-seen one; and Jupiter the very bright one (but never as bright as Venus) that often gets far from the sun. Welcome to stargazing, friend! Bottom Line: The ecliptic is the path the sun takes across our sky. Its the Earth-sun plane. And, more or less, its the plane of the orbits of the major planets and their moons, and some asteroids, our solar system. Stargazing tip: Learn the whereabouts of the ecliptic in your sky. Youll always find the sun, moon and planets on or near it.