Online Astronomy eText: Orbital Motions
Planetary Orbits

The orbits of the inner planets. From the center out, we have the Sun, Mercury, Venus and the Earth, then Mars and Jupiter. All the planets are orbiting in a counter-clockwise direction, as seen in this view from the North Ecliptic Polar direction. Note that Jupiter, Venus and the Earth have orbits which are nearly centered on the Sun, while Mercury and Mars' orbits are not centered on the Sun. Also, the inner planets' orbits are fairly evenly spaced, but there is a gigantic step outward from Mars to Jupiter.



The "curtate" orbits of the outer planets (that is, as projected onto the Earth's orbital plane). This projection makes the perihelion distance of Pluto look smaller than it is, because of the large tilt of its orbit; but it does still cross inside the orbit of Neptune, near perihelion. The orbit of Comet Halley (P/1) is also shown. All the planets are orbiting the Sun in a counter-clockwise manner as viewed here, but Comet Halley is orbiting in the opposite direction, with retrograde revolution.




The orbits of Neptune and Pluto, showing Pluto's position in various years. The orbit of Pluto has been rotated so that it lies in the same plane as Neptune's orbit. Where it appears to cross Neptune's orbit, it is actually hundreds of millions of miles above it.




A comparison of Pluto's orbit to a circle offset from the Sun. Even with an eccentricity of 25%, the orbit of Pluto has a major axis less than 1.6% larger, and a minor axis less than 1.6% smaller, than a circle of the same overall size. Only very eccentric orbits, such as those of comets, are noticeably elongated.




Relative to the Earth's orbit, both Pluto and Neptune's orbits are tilted upward on the right side of this diagram. For each planet, the line of nodes (the line through the Sun where the planet's orbit intersects our orbit) is shown. Both ascending nodes (where the planet moves upwards, through the orbital plane of the Earth) are at the bottom, but Pluto's orbit is tilted 17 degrees upward, and Neptune's less than 2 degrees. As a result, Pluto's orbit is tilted upward by just over 15 degrees, relative to Neptune's, along a line that would be nearly vertical in this diagram, putting Pluto nearly at its highest point, relative to Neptune's orbit, in the early 1980's.




A side view of the orbits of Pluto and Neptune, showing the angle made by Pluto, relative to Neptune's orbit (and the general plane of the Solar System), and how it places Pluto well above Neptune at perihelion, and well below it at aphelion. See below for a similar view, but rotated 90 degrees clockwise about a vertical axis.



A side view of the orbit of Neptune, along Pluto's line of apsides, showing Pluto's orbit extending above Neptune's at perihelion, below it at aphelion, and well to the side of it, even where near the plane of Neptune's orbit. Dots represent the position of Pluto and Neptune in mid-2005. The vertical line represents the approximate position of the line of apsides, or the major axis of Pluto's orbit. Neptune can never run into Pluto, not only because the orbit of Pluto does not actually come close to intersecting that of Neptune, but also because Neptune's gravitational effects on Pluto's orbit have locked Pluto into a 3:2 resonance with Neptune, causing Neptune to always lap Pluto when it is on the aphelion side of its orbit, at least 1.5 billion miles beyond Neptune.



Above, the inner solar system on April 1, 2007, as seen from the North Ecliptic Pole
Below, the inner solar system on that date as seen from the plane of the Ecliptic



Above, the outer solar system on April 1, 2007, as seen from the North Ecliptic Pole
Below, the outer solar system on that date as seen from the plane of the Ecliptic



Above, the outermost solar system on April 1, 2007, as seen from the North Ecliptic Pole
Below, the outermost solar system on that date as seen from the plane of the Ecliptic