THE SYSTEM OF SATURN

...necessarily appears, according to its varying distance from the eye, should it not also be added at what distance a viewed circle would be seen as equal to the disk of Jupiter? Indeed, unless this is added, it seems that nothing certain can be designated by such a comparison. And yet, there is a reason why one particular magnitude, rather than another, is attributed to a seen image, and often the same one by many spectators. But perhaps more about these matters elsewhere. For the present, it will suffice to have shown that an estimation made in this way is entirely deceptive.

This is first clear from the fact that when the Moon or some celestial sign, such as Orion, is seen near the horizon, the eye judges it to be far larger than when it stands high and almost above the zenith, although it is certain that it is perceived at no smaller angle here than there. Huygens is describing the "Moon Illusion," a psychological phenomenon where the Moon appears larger near the horizon than when high in the sky, even though its physical size on the retina remains the same. In these things which we view through a telescope, however, an even greater error occurs. For when, for example, Jupiter appears three times wider in diameter when viewed with one eye through our telescope than the Moon does with the other naked eye—so much so that if both these images were forced to coincide, the Moon would be widely covered by Jupiter—nevertheless, when Jupiter is viewed on its own, it is estimated by most spectators to equal a width of only about three fingers. original: "trium digitorum". A "digit" or finger-breadth was an ancient unit of measurement for the sky, roughly equivalent to 3/4 of a degree. Yet I have found some who made it equal to the Lunar disk, and thus they attributed to it at least a third part of the magnitude that actually appears.

For this reason, it is certain that the magnification original: "multiplicatione" of a telescope is poorly investigated in this manner. It will be done correctly, however, by Galileo's method, which he passed down in the Sidereal Messenger original: Sidereus Nuncius, Galileo’s 1610 book which first announced the discovery of the moons of Jupiter.; or, because this presents difficulty in very large telescopes, by investigating the focal distances in each of the glasses and comparing them with one another. By this reasoning, we have found that an increase of nearly a hundredfold is achieved in our tubes.

What has been observed regarding the other planets and fixed stars.

Furthermore, I believe those who read this will be glad to understand what kind of things we have also observed with the help of these telescopes regarding the other planets besides Saturn and the fixed stars; therefore, concerning these things, briefly: