7th January, 1610. Galileo turned his telescope to the sky, and “I have seen Jupiter accompanied by three fixed stars…”.
Die itaque septima Ianuarii, instantis anni millesimi sexcentesimi decimi, hora sequentis noctis prima, cum cælestia sidera per Perspicillum spectarem, Iuppiter sese obviam fecit; cumque admodum excellens mihi parassem instrumentum (quod antea ob alterius organi debilitatem minime contigerat), tres illi adstare Stellulas, exiguas quidem, veruntamen clarissimas, cognovi; quæ, licet e numero inerrantium a me crederentur, nonnullam tamen intulerunt admirationem, eo quod secundum exactam lineam rectam atque Eclipticæ parallelam dispositæ videbantur, ac cæteris magnitudine paribus splendidiores.
(I am faintly pleased that I understand more than three words of that. Life’s simple pleasures.)
Here’s where it all began, in a way. Four centuries on, we can talk quickly and happily of three-hundred-mile sulphur plumes, ice-bound planetary oceans teeming with hypothetical life, moons larger than planets, or geological features a thousand miles across; we have photographed them from near and from far, mapped them to a point, studied and speculated for lifetimes. They allowed us, less than a century later, to work out a fundamental physical constant for the first time. We have even worked with them so closely that we have begun to worry about the risk of physically contaminating them, amazing as it sounds.
But they’re still four little dots in the sky, clustered around a bigger dot; people can still pick up binoculars for the first time, or a cheap child’s telescope, look up, and feel the same rush. This is the easiest way to see it; they’re the most available sign that the system works, that there are some kind of universal laws out there, and that we ourselves can stand still and watch them played out in front of our eyes, regular as clockwork. I remember years ago, clustered in a telescope dome in a night which felt almost as cold as this one, watching the three moons strung out around Jupiter, the blurry cloud bands of light and dark on its surface… and then a perfect black circle in the middle of the southern hemisphere, as Io swung between it and the sun, letting us see an eclipse from four hundred million miles away. (We were meant to be looking at Saturn, but you don’t forget that kind of opportunity. I still have the slip of paper with the picture in a box somewhere.)
There’s a good set of posts here on the discovery and its context; if you feel up to the Latin, there’s a transcription of the Sidereus Nuncius here, or a scanned copy here (the Jovian moons are from leaf 17 on). Note the diagrams.