The planet Saturn will reach opposition on Sunday, April 3, and be not only among the brightest planets in the sky but the only one visible for most of the night.
Opposition is when a planet appears on the opposite side of the sky from the Sun. As Saturn takes 29 years to make a single circuit of the Sun, this happens once a year (it actually takes a year and a few extra days because Saturn moves against the background sky a bit -- the last opposition was on March 22, 2010).
But this time opposition is a bit different. Saturn happens to be one of the only planets visible in the sky for most of the night. Mercury is lost in the Sun's glare until late in the month when it will be visible before sunrise. Venus rises at about 5:23 a.m. (Eastern) on April 1, and will stay visible in the predawn sky for the whole month. Mars, like Mercury, is too close to the sun in the sky to see, as is Jupiter.
Through a telescope, Saturn's moon Titan is visible just to the south of the planet itself (in most telescopes that will be towards the top of the image). The rings are also at their brightest, as the planet is tilted in such a way that Saturn is facing the Earth and we see it from an angle north of the planet's equator.
The opposition comes just as NASA scientists have found, using the data from the Cassini spacecraft orbiting the planet, that there are tell-tale ripples in Saturn's rings caused by collisions with comet fragments and small asteroids.
The ripples in the rings were probably caused by cometary debris that went through its system of moons in the early 1980s. What's cool is we're finding evidence that a planet's rings can be affected by specific, traceable events that happened in the last 30 years, rather than a hundred million years ago, said Matthew Hedman, a Cassini imaging team associate and a research associate at Cornell University, in a statement.
A team that included Hedman and Mark Showalter, a Cassini co-investigator based at the SETI Institute in Mountain View, Calif., realized that grooves in the D ring appeared to wind together more tightly over time. Playing the process backward, Hedman demonstrated the pattern originated when something tilted the D ring off its axis by about 100 meters (300 feet) in late 1983. The scientists found the influence of Saturn's gravity on the tilted area warped the ring into a tightening spiral.
The Cassini scientists found more evidence when the Sun shone right along Saturn's equator and lit the rings edge-on in August 2009. The lighting conditions highlighted ripples not previously seen. That showed that what happened in 1983 was not a local event. The collision had tilted a region more than 19,000 kilometers (12,000 miles) wide, covering part of the D ring and the next outermost ring, called the C ring. No spacecraft were visiting Saturn then, and the planet was on the far side of the sun, so earth-based astronomers wouldn't have been able to see what happened.
The same team looked at data from the Galileo mission to Jupiter and found a similar pattern. Jupiter also has rings, though they are not visible to ground-based observers.