THE SOUTHWORTH PLANETARIUM
207-780-4249 www.usm.maine.edu/planet
70 Falmouth Street Portland, Maine 04103
43.6667° N, 70.2667° W
Founded January 1970
"The robbed that smiles steals something from the thief.
He robs himself who spends a bootless grief."
-W. Shakespeare ("Othello")
THE DAILY ASTRONOMER
Thursday, April 7, 2016
"Invisible" Saturn
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SOUTHERN MAINE ASTRONOMERS MEETING TONIGHT
at the Southworth Planetarium.
Thursday, April 7 at 7:00 p.m.
Free and open to the public
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Remember when that happy-go-lucky Nietzsche said, "When you look into the abyss, the abyss looks into you?" Well, such a staring contest can also be waged between an innocent, well-meaning person and a jar stuffed with neglected parchment paper. You look at it guiltily, and it glowers at you reproachfully and doesn't turn away, even if you attempt to ignore it. Even if you smile pleasantly at it. Even if you promise you'll be back eventually....even if, in a desperate moment of guilt induced self loathing you scream that the new Star Wars trailer has just been released and you've only managed to watch it about a dozen times.....
So, in order to insure domestic tranquility,...we cheerfully take another question (and a splendid one, at that) from Pandora's Jar
"Is it true that Saturn wouldn't be visible to the unaided eye if it didn't have a ring?"
-S.P, Bridgton, Maine
While it is true that Saturn's ring contributes to the planet's apparent brightness, Saturn would still be visible to the unaided eye even if it didn't have a ring. For instance, at present, Saturn's brightness is increasing as it approaches its June 3rd opposition. By the end of April, Saturn will be almost as bright as Vega (Lyra the Harp) the fifth brightest star in the night sky. Were Saturn to be sans rings, it would still be as bright as Spica (Virgo the maiden), the 16th brightest star in the sky.
The rings' contribution to the planet's brightness varies considerably, as its ring angle (called its "aspect") also varies from 0 degrees (when the rings are edge on and therefore not visible) to 26 degrees. This maximum tilt angle occurs abut every 15 years, equal to half of Saturn's orbital period. The wider the angle, the greater the surface area from which it reflects the Sun's light back out into space.
This ring aspect makes the calculation of Saturn's brightness quite complicated. The other gas giants (Jupiter. Uranus and Neptune) also have rings, but they do not make their planets much brighter at all. Saturn's maximum brightness is about -0.24, about 2.5 times dimmer than Sirius, the night sky's brightest star. Saturn would only be this bright when its rings were inclined at a high angle when the planet was at perihelic opposition (when it was at its closest distance from the Sun at the same time that it was at opposition from the Sun relative to Earth and therefore closest to our planet.) Saturn's minmum brightness is about 1.5, about as bright as Leo's brightest star, Regulus. Only when Saturn's rings were seen edge on while Saturn was at its greatest distance from Earth would Saturn ever appear this dim.
Saturn does have periods of invisibility, which occur when it is too close to the Sun in our sky. However, when Saturn is far enough from the Sun to be present in the night, it will always be visible.