Todd J. Barber, Cassini lead propulsion engineer
Happy Valentine's Day from JPL, Cassini, and Saturn! As couples are hopefully able to bask in lovely first-quarter moonlight this evening, the waxing lunar phase reminds us of an exciting week ahead in astronomy and space science. On Wednesday, Feb. 20, the moon will effortlessly glide into the deep shadow cast by Earth from the sun. This total lunar eclipse favors most of North America very well, though the moon will already have begun its game of hide-and-seek by the time it rises on the west coast. You may wonder what this has to do with Cassini, definitely a fair question within a column named "Insider's Cassini!" There are at least two Saturn and Cassini connections with next week's eclipse, largely due to chance geometric lineups in the heavens.
First off, the requisite condition for a total lunar eclipse is a full moon, and this only occurs when the moon is in opposition. Basically, the term "opposition" means that the body in question is on the opposite side of the Earth as the sun. That is why a full moon always rises at sunset and vice versa. Lunar oppositions occur roughly every 29.5 days, but other orbital parameters have to line up just right to enable lunar eclipses, partial or total. This is interesting enough, but the twist this time is that Saturn is nearly at opposition as well! The time between oppositions of Saturn is slightly over one year, so they are rare and special events. I'm sure this is not news to the amateur astronomers in the crowd, especially Mars buffs. If you think about the planets as marching around nearly circular orbits about the sun (a pretty good approximation), then oppositions of planets enable the closest approaches between Earth and the other planets. That means that Saturn, its tiny moons, and glorious ring system never look better in a telescope than they do at opposition. Moreover, the fact that Earth's moon and Saturn are both near opposition on Feb. 20 implies that they must be in the same part of the sky -- and indeed they will be! Even though Saturn is about 3000 times farther away than the moon, to our eyes they will inhabit the same celestial real estate! Saturn will appear as a bright, slightly yellowish star about three degrees east of the eclipsed moon.
On the day of the eclipse, almost as if not to be forgotten during this orbital alignment, the Cassini spacecraft will pass behind Saturn and its rings as seen from the Earth (and the moon, and the sun!). Rather than being times of communication blackout, these periods often enable us to pass the radio signal from Cassini through the rings themselves or even the upper reaches of Saturn's atmosphere before and after occultation. In this way, we use Cassini's radio signals as science instruments themselves, telling us about ring particle distribution and structure, physical and chemical properties of Saturn's atmosphere, and so much more. So, tonight, we'll leave this first quarter moon for romance, but next Wednesday night, I encourage you to take a few minutes to watch the sunset and moonrise, and then to watch Saturn -- and Cassini -- as Earth's moon dips into sublime, shadowy beauty.