Pole Stars Of Other Planets?

When I wrote about Philip Latham’s juvenile science-fiction novel Missing Men Of Saturn (1953) recently, I pointed out that Latham had made an astronomically well-informed guess about a possible pole star for Saturn’s moon Titan. Latham (a professional astronomer) knew the orientation of Saturn’s rotation axis, which would have allowed him to deduce the location on the celestial sphere around which stars would appear to rotate in Saturn’s sky, in the same way they appear to rotate around our pole star, Polaris, in Earth’s sky. And it was a reasonable guess that Titan’s rotation axis would be moderately well aligned with Saturn’s, leading Latham to have his protagonist use the star Gamma Cephei to orientate himself during his exploration of Titan.

So I thought this time I’d write about the location of the celestial poles of other planets—that is, the two apparently stationary points on the celestial sphere around which the sky of each planet appears to rotate, as a result of the planet’s rotation. Whether or not a star will turn out to be close enough to these points to function as a “pole star” is another matter. We’re in fact astronomically lucky (literally) to have a bright star situated so close to Earth’s north celestial pole; there’s no corresponding star in the south. And, because of the slow precession of Earth’s rotation axis, the alignment with the star Polaris is only a temporary one.* At present the celestial pole is moving slowly closer to Polaris, but Jean Meeus tells us, in his book Mathematical Astronomy Morsels (1998), that this motion will soon end, and pole and star will start to drift apart again—the closest approach will occur in February 2102, which is pretty close to being tomorrow, in astronomical terms.

Latham’s assessment that a major moon of a gas giant would tend to have its rotation axis aligned with its parent planet has proved to be correct. We now know that the major satellites of Saturn are so aligned (out as far as Titan), as are those of Jupiter, Uranus and Neptune. Even the two tiny satellites of Mars share their rotation axis with their parent. So in what follows, you can consider that the pole position of a planet, as marked on my small-scale sky maps, also indicates the pole positions of many of its satellites.

But before showing you my sky maps of the various celestial poles, I need to clarify what is meant by “north pole” and “south pole”.

If we look at the Earth from above its north pole, it appears to rotate anticlockwise, like this:

This gives us two, not entirely consistent, ways of defining “north” for other planets. The first would be to define the north pole as being that rotation pole from above which the planet appears to rotate anticlockwise. This is sometimes called the “right-hand rule”, because if we imagine wrapping our right hand around the planet’s equator, with our fingers pointing in the direction of its rotation, then a “thumbs up” sign from this position will point towards the planet’s north pole. This is a nice generalizable rule, and I’ll come back to it at the end of this post, but it’s not the one adopted by the International Astronomical Union.

In 1970, the IAU defined a sort of “solar system north”, using the Earth’s north pole as the criterion. The hemisphere of sky that lies on the north side of the plane of the solar system, as judged by the orientation of Earth’s north pole, defines the north poles of all the other solar system planets and their major satellites. The “right-hand rule” and “solar system north” produce consistent results when a planet rotates anticlockwise as viewed from solar system north. But they produce opposite results if the planet appears to rotate clockwise when viewed from solar system north. The IAU defines such planets as having retrograde rotation, because they turn backwards when compared to the Earth. The opposite of retrograde is prograde or direct, and most (but not all) of the major bodies in the solar system have prograde rotation.

You might be wondering how the IAU defines “the plane of the solar system”. Though all the planets orbit in roughly the same plane, they interact gravitationally with each other, so the precise angles between the various orbits varies. But the total angular momentum of the solar system stays the same, and that defines an axis and corresponding plane called (appropriately enough) the Invariable Plane of the Solar System (henceforth, the IPSS).

So there’s a “north pole” in the northern sky associated with the IPSS, and it, with its southern counterpart, are the only poles that have an absolutely constant position on an astronomical time scale. Nearby is another north pole associated with the plane of the Earth’s orbit around the Sun—the ecliptic pole. It’s near the IPSS pole because Earth’s orbit is tilted only slightly relative to the IPSS. It will move very slowly as the Earth’s orbit slowly shifts under the gravitational disturbance of the giant planets. And we know that the Earth’s axis of rotation is tilted at 23½º relative to the plane of its orbit, so the Earth’s north celestial pole lies 23½º away from the ecliptic pole, in the constellation of Ursa Minor, near the star Polaris. As I’ve already mentioned, this pole too is in motion—it describes a wide circle around the ecliptic pole every 26,000 years.

The north IPSS pole and ecliptic pole lie in the constellation of Draco, within the curve of the “neck” of the imaginary dragon that is sketched out by the constellation’s stars. A little cluster of other north poles is gathered around them. Venus and Jupiter follow orbits that are minimally tilted relative to the IPSS, and have rotation axes that are only slightly inclined to their orbits, so their poles sit close to the IPSS pole. Mercury also has a minimal tilt relative to its orbital plane, but its orbital plane is tilted by more than six degrees relative to the IPSS, moving its pole a corresponding distance from the IPSS pole. In the constellation diagram below, I’ve also marked the north pole of the Sun, which is tilted by around six degrees relative to the IPSS. So all the north poles mentioned so far lie in Draco, in or around the loop of the dragon’s neck, but none of them near any particularly bright stars.

On the maps that follow, planets with direct rotation are marked in green, and retrograde rotators in red.

North celestial poles of various solar system bodies
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(You’ll notice that I’ve deviated from my habit of using Celestia to generate my astronomical and astronautical illustrations—all my plots this time are superimposed on star charts generated at In-The-Sky.org, which are clearer for my purposes today.)

Also plotted above are the poles of Saturn, Mars and Neptune, all significantly tilted relative to their orbital planes and the IPSS. Saturn’s north pole lies in a dark corner of Cepheus, actually a little closer to Polaris than to Gamma Cephei (marked with its name Errai on the chart). Mars has its pole inconveniently placed in the dark sky between Cepheus and Cygnus. Neptune’s pole has perhaps the most navigationally convenient location, on the left wing of Cygnus the Swan, almost on the line between second-magnitude Sadr and third-magnitude Delta Cygni.

In contrast to the other bodies plotted above, Earth’s Moon is marked with a dashed circle rather than as a single point. This is because, like Earth, the Moon’s rotation axis precesses around the ecliptic pole—but it does so in a mere 18.6 years. So on any given date the Moon’s north pole will be located somewhere around the circumference of a circle about three degrees across, centred on the ecliptic pole.

In the southern sky, the pattern of poles is the same, but inverted relative to their northern counterparts.

South celestial poles of various solar system bodies
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The poles of Mars and Jupiter lie in dark parts of the constellations of Vela and Puppis, respectively. Mercury has a good, but relatively faint, southern pole star in third-magnitude Alpha Pictoris. Saturn’s pole star, Delta Octantis, is even fainter. This leaves the cluster of poles around the ecliptic pole, all of which lie in the faint constellation of Dorado. They all have a good pole “star” in the form of the Large Magellanic Cloud (not shown on my chart) which extends south from Delta Doradus and into the neighbouring constellation, Mensa.

One planet is missing from the charts above—Uranus. This is because Uranus notoriously orbits lying pretty much on its side, the inclination of its equatorial plane to its orbital plane being variously given as 98º or 82º and retrograde. This puts its poles in the vicinity of the plane of the ecliptic, rather than the ecliptic pole.

On my charts below, the brown line is the ecliptic, as a stand-in for the IPSS. Uranus’s south pole (retrograde rotation) lies in a dark area of sky, but is positioned between the two bright constellations of Orion and Taurus, making it relatively easy to find, at least approximately.

South celestial pole of Uranus
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Its north pole lies in Ophiuchus, conveniently close to the second-magnitude star Sabik (Eta Ophiuchi).

North celestial pole of Uranus
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Finally, I promised I’d come back to that “right-hand rule” definition. While the “solar system north” standard works well for planets and their large satellites, the International Astronomical Union realized that smaller bodies can have rotation axes that precess or migrate so quickly that they could easily shift back and forth across the IPSS within a few years. Under the “solar system north” rule, this would mean that the body would swap its north and south poles, and shift between direct and retrograde rotation, over the same period. So in 2009 they officially fell back on the good old “right-hand rule” for dwarf planets, minor planets, their satellites, and cometary nuclei. To avoid running two conflicting definitions of “north” in parallel, they designate the right-hand-rule pole of these bodies the positive pole, with the negative pole lying in the opposite direction.

Which brings us to Pluto, everyone’s favourite ex-planet. Until 2009 it was officially a retrograde rotator, with its north pole in Delphinus and its south pole in Hydra. Now, under the new definition, its positive pole is in Hydra, and its negative pole in Delphinus.

Location of positive celestial pole of Pluto
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Location of negative celestial pole of Pluto
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These poles will serve for Pluto’s huge moon, Charon, too. But sadly, both poles lie in rather dim and undistinguished parts of the sky.

* When the Phoenicians were setting out on their voyages of discovery, three millennia ago, they had no pole star to guide them. The north rotational pole of the Earth lay in a fairly empty bit of sky north of the bowl of the Little Dipper.

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