Saturn without its rings is like the Taj Mahal without the dome, and yet that is what a telescope will show for a few days at the end of this week. The rings are not going anywhere. They are simply turning edge-on to us, and a sheet of ice a hundred thousand kilometres wide but only tens of metres thick, seen exactly from the side, is a line too thin to see.
Why it happens
Saturn’s axis is tilted 26.73 degrees from the vertical, and it keeps that tilt as it circles the Sun once every 29.4 years, just as Earth keeps its own 23.5-degree lean. So from Earth we see the rings from above for roughly half of Saturn’s year, then from below for the other half — and twice per orbit, as our line of sight crosses the ring plane, we see them from precisely the side. Astronomers call it a ring plane crossing. From Earth it comes round every 13 to 15 years; the last was in 2009 and the next will be in 2038.
“The rings literally almost disappear,” says astronomer Sean Walker. “Usually you see the rings around Saturn. Occasionally you see a single edge, and the rings look like a hair-thin line of light.” Ground-based telescopes have watched the rings thin steadily for months; this week they reach the exact angle.
What to look at instead
The compensation is the moons. With the glare of the rings gone, four to six of Saturn’s moons are routinely visible in a modest telescope, and during a crossing a patient observer under a dark sky can pick out close to a dozen. Titan, larger than Mercury, is the easy one; Rhea, Tethys and Dione line up along the invisible ring plane like beads on a string.
Seeing it from Bangladesh
Saturn is currently low in the eastern sky before dawn, which is the honest problem: it rises only an hour or so before the Sun, and the late-March haze over the delta does not help. A 100 mm telescope from a rooftop in Dhaka will show the planet as a pale oval with a line through it; from Sylhet’s or Bandarban’s clearer skies, the moons too. The Bangladesh Astronomical Society and the university astronomy clubs typically announce public viewing sessions for events like this. If you miss it, the rings will slowly reopen over the coming months, and the reverse view — rings wide open, at their most spectacular — arrives around 2032.
Source: Space.com, via Prothom Alo.
