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The Planetary Society Weblog
By Emily Lakdawalla
Earth crosses the Uranian ring plane May 2
Apr. 30, 2007 | 10:34 PDT | 17:34 UTC
The solar system is a playground for geometricians. Slicing up the solar system are myriad planes containing the orbits of every body that circles the Sun. All of these planes intersect at the Sun, but other than that they're not constrained, and no two bodies have perfectly matching orbital planes. However, because of the way that the solar system first formed from a spinning disk of material, most of the planets' orbital planes are quite close to each other, which is why the planets appear to travel across a single arc of the sky, the ecliptic.
Of course, every planet with moons is a mini-solar system unto itself. With a few notable exceptions, most planets' major moons orbit in the planet's equatorial plane. Because of the way angular momentum works, the orientation of a planet's equatorial plane doesn't change as the planet moves around the Sun. So sometimes a planet's equatorial plane is tilted toward the Sun, or away from the Sun, but twice per orbit, the planet's equatorial plane sweeps across the Sun. This semiannual event is the equinox, which has important local significance; at the equinox, spring arrives to a formerly shadowed pole, and autumn darkens the formerly lit pole.
Ordinarily, you don't think of this equatorial plane as having much significance beyond the planet and its moons. But it's actually quite significant if you're an Earth-based observer. For one thing, when the equatorial plane crosses Earth, Earth observers get to view the planet and moon system edge-on. Observers are treated to transits (moons crossing the disk of the planet), occultations (moons disappearing behind the planet), eclipses (moons moving in to the planet's shadow), and mutual events (moons crossing other moons). Observing these events can be quite valuable for making very precise measurements of the geometry and timing of the moons' orbits. The Cassini team had a lot of fun last year observing these kinds of mutual events, during a time when the spacecraft was orbiting in Saturn's ring plane. Mutual event of Janus, Epimetheus, and DioneCassini captured three of Saturn's moons in motion on December 30, 2005. The camera held still on Janus as larger Dione passed behind it (coming from the right) and smaller Epimetheus hovered overhead. Credit: NASA / JPL / SSI / Emily Lakdawalla |
For planets with rings, equatorial plane crossings are even more interesting, because they are also ring plane crossings. (By the way, "ring plane crossing" is often abbreviated "RPX".) Crossing the ring plane does a number of things for you. Because ring systems are so vertically thin, if you don't like rings, ring plane crossings effectively remove the ring system, giving an unobstructed view of the planet. Removing the rings also helps in the search for tiny moons located close-in to a planet.Saturn edge-onIn this crescent view of Saturn taken by Cassini on March 16, 2006, the vast ring system appears nearly to have vanished, because the spacecraft is viewing the planet nearly from within its ring plane; all the rings are compressed into a thin, dark line across the equator. The rings leave their mark, however, in broad shadows painted on Saturn's northern hemisphere. Credit: NASA / JPL / SSI |
If you do like rings, a ring plane crossing allows you to measure the thickness of the rings and search for vertical structures like warps and waves. It also has the effect of "stacking up" ring material as seen from Earth -- ordinarily we are looking straight through a very thin disk, but as we cross the ring plane we get a brief glimpse through the ring in its thick dimension, from front to back. Because of that, it may be possible to see faint rings that have never been seen before; it also gives observers more signal to study the colors and surface properties of ring material. And it treats us to a rare glimpse of the shadowed side of the ring system. Because we're closer to the Sun than the ringed planets, we almost never see the shadowed side of the rings; observations of the shadowed side will greatly add to our understanding of their composition.
I knew that the Uranian equinox was coming up later this year but I had never checked in to the dates of the associated ring plane crossings. Thankfully, Daniel Fischer pointed out to me that the first ring plane crossing is coming up in two days, on May 2. I've now added this and the upcoming dates to my calendar.Keck's Changing View of Uranus
From 2001 to 2004, Uranus's motion around the Sun has changed its orientation as seen from Earth in these images taken through Keck II's K prime filter. The four images also show how the Adaptive Optics system has improved over time. The same astronomers have continued to study Uranus from Keck II as its equinox approaches. Credit: Imke de Pater, Seran Gibbard, Heidi Hammel / W. M. Keck Observatory |
Why isn't the ring plane crossing at the same time as the equinox? If Earth's orbit were coplanar with Uranus', or if Earth and Uranus just happened to lie on the line containing the intersection of their orbital planes, then the equinox would happen at the same time as Earth crossed Uranus' ring plane. But the two orbits are not coplanar, so we actually get to cross Uranus' ring plane three times. Right now we are looking at Uranus' southern hemisphere, and the Sun is also lighting the southern hemisphere (and the south side of Uranus' rings). With Uranus very close to its equinox, our slightly tilted orbit will take us across the ring plane to the north -- and the shadowed side of the rings -- on May 2.
In a few months, with Uranus getting even closer to its equinox, our orbit will carry us back across to the south and sunlit side of the rings, on August 16. Then the equinox will happen on December 7, and the Sun will come to Uranus' north; we'll see the rings lit edge-on and then go in to darkness. Finally, on February 20, Earth will cross back to the north, and as Uranus' seasons advance we'll stay on the north side, seeing sunlit rings, until the next set of ring plane crossings comes with the 2049 equinox. (I figured out these dates using the ephemeris generator at the PDS Rings Node -- check it out, it's interesting to play with.) The Rings Node also has a website with links to further information for the ring plane crossings. There's also an excellent summary of the science goals for observations of the event by Planetary Society Board member Heidi Hammel here (6 MB, PDF format).Uranus Ring Plane CrossingThis graph shows how Earth's view of Uranus changes during the 2007-2008 ring plane crossing season. For decades, Earth's view has been of the southern hemisphere. But as Uranus approaches its equinox, Earth will briefly -- from May 2 to August 16 -- cross the ring plane to the shadowed side of the rings (gray part of the curve) and the northern hemisphere. Then Earth recrosses the ring plane on August 16. On December 7, Uranus reaches its equinox, and the Sun goes to the north side of the rings, leaving the southern side of the rings -- the side that Earth sees -- in shadow (again, gray part of the figure). Finally, on February 16, Earth recrosses to the north side of the ring plane, and the sunlit side of the rings, until the next equinox and ring plane crossing in 2049. Credit: Mark Showalter and Mitch Gordon of the PDS Rings Node |
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