Learn Something New Every Day Department:
Had you asked me up till five minutes ago whether Saturn's largest moon was ever visible to an Earth-bound naked eye, I would have said no. Now, thanks to The Astronomy Picture of the Day, I'm not so sure.
Tip: Let your mouse hover over the image if you don't see it at first.
I don't know whether the visibility is due to a prolonged camera exposure, but it's still pretty amazing.
5 comments:
I read somewhere that back in the 1700s mariners used to navigate by the phases of the moons of Jupiter. They must have had a much clearer sky back then. I don't believe that anyone can see a moon of Saturn with the naked eye at least not here in the industrialized part of the world. All of Saturn just looks like a pinpoint of light similar to a star to most people. With a fairly low powered telescope say 20 or 50 power you might. I doubt if one could see it with a pair of the usual 6 power binoculars. Even the rings around Saturn or the Casini division in the rings are dubious with the naked eye.
TC:
I'm certain that even the Galilean moons are invisible except with a telescope. Ditto, the rings of Saturn. In fact, Galileo's first telescope was unable to resolve the rings; he could only see a mysterious equatorial bulge, IIRC.
So, thinking more about Titan, even though it's a lot bigger than Jupiter's moons, and how much further away Saturn is, I guess I think that the camera used to take that APOD picture must have been set to a long exposure time. Given the apparent size of the moon in the picture, it didn't seem like a telescope was used, though.
Check with Gail?
A camera using a long exposure is hardly an Earth bound naked eye though. I can't remember off hand what the power was on Galileo's first telescope. I seem to have a vague recollection that it was 30 power, but it's an ancient memory. Being a refractor there was a limit to how much power it could have had in any normal size instrument and given the fact that lens grinding was in its infancy at the time it probably had all kinds of spherical and chromatic distortion. It's probably a wonder that he could even see the rings as a bulge, much less a moon even if he knew to look for a moon which I doubt at the time.
A camera using a long exposure is hardly an Earth bound naked eye though.
Yes, I agree. I should have said so more explicitly.
JPL has a nice page on the history of observing Saturn. Short version: both of our memories were sort of right.
Galileo first looked at Saturn in 1610 using a 20x telescope. He could not resolve the rings, but he could see something weird. He reported that there appeared to be three bodies. By 1612, Saturn was oriented so that the rings were edge-on to the Earth, so he couldn't see anything. In 1616, he could see something again, and reported "two half ellipses with two little dark triangles in the middle of the figure and contiguous to the middle globe of Saturn, which is seen, as always, perfectly round."
It wasn't until 1655 that Christaan Huygens, using a 50x telescope of his own design, properly identified the rings. Huygens was also the discoverer of Titan.
This elucidation from my amateur astronomer friend Gail:
According to my reference book, Saturn's moon Titan is 8th magnitude. Under the best conditions, very good human eyes can see stars as faint as 6th magnitude. Each increase by 1 in mag is about 2.5 times less light. So Titan is 6 or 7 times too dim to be seen without optical aid, even in dark clear skies.
Could Galileo see it? Well, his telescopes had objective lenses about 1" in diameter, which would collect about 15 or 20 X more light than the eye, so in theory he might have seen it. His magnifications went up to 30X, which is plenty, but his optics were poor quality because of primitive glass available at the time.
Titan is relatively far (760,000 mi) from Saturn, which is roughly 800 million mi from here, and Saturn's magnitude is about 0.
Compare those figures to those of the Galilean satellites of Jupiter. Jupiter is about 400 million mi from us, and Ganymede, its largest moon is 666,000 mi from Jupiter. Jupiter has a magnitude of -2.5, while Ganymede is mag 5. Most people cannot see Ganymede, but a few can. The dazzle of Jupiter's bright -2.5 mag light make a 5th mag companion hard to see if there are any vision defects, but Ganymede is easy to see in 6X or 7X binoculars when it is well separated from Jupiter.
For purpose of discovery, the rapid orbital period (few days) of Jupiter's inner moons made it easy for Galileo to notice that something was going on. But Titan takes half a month to circle Saturn. My book says Titan was discovered by C. Huygens in 1655, decades after Galileo's observations in 1610. Certainly glassmaking and lensmaking had progressed a little during those 45 years, and Huygens used a much better eyepiece design than Galileo had.
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