The Sun and the planets formed out of one rotating cloud, and a rotating cloud collapses into a disc. Everything that followed inherited that plane, which is why the planets are always found along one line in the sky.
The result is a solar system some 60 AU across and about 2 AU thick. Drawn honestly on a screen that is a horizontal line and nothing else, which is why the control above exists and why the caption says how much it is exaggerating by.
"The ecliptic" is Earth's orbital plane specifically, the plane we happen to be in. It is a reference, not a law. The planets are not lined up on it; they are lined up near one another, and we picked our own orbit as the zero. The invariable plane, weighted by the angular momentum of the whole system, lies about 1.6 degrees away and is arguably the better reference. It is not the one anybody uses, because we are not on it.
Neptune's orbit is tilted 1.77 degrees, less than Mars's, Saturn's, Venus's or Mercury's, and reaches 0.94 AU out of the plane, nearly the distance from the Sun to us. Mercury's 7.00 degrees is the steepest and reaches 0.05 AU. Degrees hide the scale; the middle column does not.
In the other direction: Venus's orbit is tilted only 3.39 degrees, yet Venus can appear 8.6 degrees off the ecliptic, because at inferior conjunction it is 0.27 AU away, closer than the Sun, and the same sideways offset covers a much larger angle. Mercury does the opposite. It has the steepest orbit of the eight but never comes closer to us than 0.55 AU, so it never appears more than about five degrees off the line.
A strip 9 degrees either side of the ecliptic holds every planet, all the time. That is where the traditional eight-to-nine-degree width of the zodiac comes from, and it is set by Venus rather than by the planet with the steepest orbit, because the width is a fact about how things look from here and not about the shape of the solar system.
The same band is why planetary conjunctions happen at all, why the Moon can cover the Sun, and why a planet can occult a star. Everything interesting happens along one line, because everything is in one plane.
The zodiac here means a strip of sky and the constellations the ecliptic passes through. Nothing on this page attaches meaning to which one a planet is in front of.
It also means the planets are always somewhere along the same arc of sky, and that arc is the Sun's daytime path shifted by six months. If you know where the Sun sets in June, you know roughly where to look for Jupiter at midnight in December.
| Planet | Inclination | Max height above the plane | Apparent excursion from here |
|---|---|---|---|
| Mercury | 7.00° | 0.05 AU | ≈ 5.0° |
| Venus | 3.39° | 0.04 AU | ≈ 8.6° |
| Neptune | 1.77° | 0.94 AU | ≈ 2.0° |
The Sun and the planets condensed out of one cloud of gas and dust. A cloud with any net rotation at all cannot collapse evenly: it falls freely along the rotation axis but is held up by its own spin in the perpendicular directions, so it flattens into a disc. Everything that formed in that disc inherited its plane, and nothing since has knocked the planets out of it.
An orbit's inclination is measured at the Sun: the angle between its plane and the ecliptic. How far off the line a planet looks is measured from here, and depends on how far away it is at the time. The two are not the same number, and neither one bounds the other.
Eclipses are the clearest case. If the Moon's orbit lay exactly in the ecliptic there would be a solar eclipse at every new moon and a lunar one at every full moon. It is tilted 5.1 degrees, so the alignment only works near the two points where the Moon's orbit crosses the ecliptic. That is what makes an eclipse a season rather than a monthly event.
Related tools: Solar System Orrery · Greatest elongation · Eclipses
Orbital elements from NASA/JPL, "Keplerian Elements for Approximate Positions of the Major Planets" (Standish & Williams, 1992), valid 1800–2050. Positions are geometric — no light-time or aberration — because this is a diagram of the ecliptic plane, not a simulation of a telescope view.