Galileo saw Neptune!

He had deduced this by observing 4 moons around Jupiter, using his improved but still primitive refracting telescope. His first home-made telescope was like an opera glass nowadays, the main lens convex and the eyepiece a concave lens, magnification 3 ;-) His later improvements gave his best telescope a magnification of about 20, albeit with a narrow field of view and lens defects causing blurry images. Nevertheless he saw Jupiter as a disc and the four larger moons of Jupiter. In order of decreasing size, these are Ganymede, Callisto, Io and Europa. The first three are even bigger than Pluto, which was why Pluto was recently degraded to the status of a dwarf-planet, leaving Neptune as the planet farthest out in our solar system.

In 1612 Galileo was observing Jupiter again when he observed what he at first thought was a fifth moon but later wrote that it was a fixed star although documenting its movement relative to the background stars. Here are his notes made at the time :-

Nowadays, every decent amateur astronomer has a solar-system-simulator running on his or her PC. By setting its date to winter of 1612, it can calculate what the night sky looked like back then. Galileo's notes show the 5th object, a faint blue dot, in the plane of Jupiter's moons. The simulation places Neptune - the outermost planet of the Solar system - exactly where he saw the 5th object. So I conclude : Galileo saw Neptune!

Now we can test if this was indeed feasible. Tonight and tomorrow night Neptune is right next to Mars making it very easy to find in the sky. So I shall use my small portable reflecting telescope - Galileo would have eaten his heart out for one of these - choose an eyepiece to set the magnification to about 20x, and look at the red disc of Mars. Right next to it I shall expect to see a pale blue dot, Neptune, thus "confirming" that Galileo could have seen Neptune with a 20x magnifying telescope. You can do this too, so give it a try! You can see Mars with the naked eye, it's really close right now, just 8 light-minutes away. But you will need either high-power binoculars (20x) or a telescope to see Neptune. Just like Galileo did :-)

In all fairness, mine is the better instrument, a reflecting telescope like the one invented by Sir Isaac Newton in 1688. His primary mirror had a 2 inch aperture and magnified about 35x. Mine has a 3" aperture and so a maximum useful magnification of 150x, viewing permitting. Still not enough to see Neptune as a disc though (needs 200x+ AND good viewing).

Getting off-topic here, but if you want a semi-portable scope that will show ALL the planets as discs, you will need something like this one shown on the left. This has an eight inch aperture (and so could reach 8*50=400x magnification with best viewing), and when dissembled into its 4 main pieces will JUST fit in my car (a VW New Beetle). The tube goes across the back seat, the mounting in the trunk/boot, the box of different eyepieces behind the passenger seat and the sight behind the driver seat. The sight has no magnification, it just projects a red spot into the sky so you can aim the main tube. Mind you, you'll need to drive (in winter!) with the car's heating off or you'll find it will take the main mirror ¾ hour to cool down after arrival at your dark-sky observation site ;-)

Now let's just hope the skies are clear on 6th and 7th :-)

Comments (4)
Brian (UK) asks "What do those scopes cost?" They're cheap. The small tabletop one is a Celestron First Scope and cost just 73€. The bigger one is an 8 inch aperture Galaxy d8 on a Dobsonian mount. An equivalent Orion XT8 model costs just 380€. Add 4*30=120€ for four different Plössl eyepieces of your choice of focal lengths. That's how I got started. But don't expect to see the quality of satellite-based imagery like Hubble or Kepler photos.
Petra (A) asks "What else did Galileo see?". He also saw sunspots, the phases of Venus, craters & mountains on the Moon, and Saturn. But the lenses on the first refractors were so bad that early observers thought Saturn had ears rather than rings. There was even a publication by a Vatican astronomer - in Latin - suggesting that the rings of Saturn - I translate from the title of the publication - were "...the foreskin of our dear Lord..." ;-)
Brian (UK) replied, asking "So, did it work for you? Did you see Neptune at 20x ?" Sadly no, we had 8/8 cloud cover both nights until after Mars set. The best laid plans of mice and men gang aft aglay :-(
Jack Kinker (USA) corrected my introduction, as forwarded by Pergelator, writing " Wasn't this the Ptolemaic claim? Copernicus moved the Sun to the center...?" Yes indeed Jack, my bad! In 1543 Copernicus published De revolutionibus orbium coelestium putting the sun at the centre of the solar system, although Aristarchus of Samos had initially suggested this around 250 BC. The model with the Earth at the centre and everything else in nested spheres around it was due to Ptolemy (around 130 AD, in his book Almagest). I have corrected the initial blogpost, thankyou Jack!

Copyright ^© Ole Phat Stu on December 6, 2018 permalink Comments Email