Expert: Tom Whiting - 11/19/2011

Question
Hi Tom,
I have what I think is possibly a fairly simple question to answer but the thought of what the answer might be fascinates me!

If you were stood on the surface of Io (which I believe is the innermost of the Galilean moons?) how big would Jupiter appear? Would the large surface features (i.e. Great Red Spot) be easily visible?

Kind regards,


Gavin

Answer
Hi Gavin,
Since Io orbits Jupiter at about the same distance as our own moon, we can proportion it from the fact that our own moon subtends a 1/2 degree angle in our own sky. (And the Earth being 4 diameters bigger would subtend about a 2 degree angle in the lunar sky).  Jupiter is about 11 Earth diameters (88,000 to 8,000 miles) so Jupiter would subtend about (11 x 2) 22 degrees in Io's sky.  If you use the proportional diameters of Jupiter and the moon you come up with 88,000/2160 miles or about 41, and 41 x 1/2 degree = 20.5 degrees... close enough.

A full hand span at arm's length, from tip of thumb to tip of little pinkie finger, fully extended, is about a 15 degree angle held up against the distant background. So a 22 degree apparent angle would be 1.5 full hand spans at arm's length.  That's about how big Jupiter would loom in Io's sky... big enough to see all the major features with our human... naked eye. for an artist's conception, see
http://www.cosmographica.com/gallery/portfolio2007/content/035_JupiterFromIo_1_l
For more, just Google "Jupiter as seen from Io"
Clear Skies,
Tom Whiting
Erie, Pennsylvania  USA

FOLLOW UP:
Another way to look at it... we see Jupiter at opposition at about 400,000,000 miles distant, and Io sits at only 220,000 miles. So that's the same telescope magnification increase as 400 million divided by 220,000 = ~1800X through a telescope.  And I'm sure at 1800 power in a BIG scope, all the major features, under perfect sky conditions, would be easily visible to the eye.
Clear Skies,
tom