THE SOUTHWORTH PLANETARIUM




*207-780-4249 <207-780-4249>       www.usm.maine.edu/planet
<http://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.*