In the 20th century, a fragment of Gan's work, in a later compilation of astronomical texts, was identified by Xi Zezong as describing a naked-eye observation of either of the two largest and brightest moons, Ganymede or Callisto in summer 365 BC.
"The historian and astronomer Xi Zezong published a paper in 1981 in Acta Astrophysica Sinica identifying the "small reddish star" with one of the Galilean moons of Jupiter, an interpretation hitherto unrecognized.
[10][5] Xi used the Beijing Planetarium to simulate the brightnesses of Jupiter and its moons in their relative positions from the earth as reported in the astronomical publications of Bryant Tuckerman and Clabon Allen's Astrophysical Quantities.
[5] Writing in Nature in 1982, the astronomer David Hughes pointed out that William Henry Smyth had recorded reports of Jupiter's moons visible with the naked eye in exceptional conditions in particular places.
Smyth wrote in 1844 that sightings of the moons typically mentioned the Apennine Mountains and Mount Etna in Italy and the Caribbean "and various other fine-climate places as the spots where such a feat is frequently done" by those endowed with "visual organs of extraordinary power".
[10] Galileo Galilei, describing his discovery using refracting telescopes of "four planets swiftly revolving about Jupiter at differing distances and periods" (the Galilean moons), was therefore unlikely to have been correct to write in his Sidereus Nuncius, published in 1610, that these bodies were "known to no one before the Author recently perceived them".
[10] By occluding Jupiter itself behind a high tree limb perpendicular to the satellites' orbital plane to prevent the planet's glare from obscuring them, one or more of the Galilean moons might be spotted in favorable conditions.