Naked eye Galilean moon challenge
If you look into the eastern sky after sunset over the next few days you’ll see the stunningly bright Jupiter, which at opposition on Sep 21 at 11:00UT was up to 75million km closer than other oppositions (ie directly opposite sides of the Earth compared to the Sun). Its distance about 3.953AU or about 592 million km (1). So it is close and therefore bright but is that all?
Over the next few weeks it may just be possible to see the big Galilean moons naked eye! Mmm you say is that possible? Well it has been a hot topic for some time as was discussed in the 1976 Sky and Telescope article Naked-Eye Observations of Jupiter’s Moons (2). Io can never be seen, not because of its brightness of Mag 4.8 but because it is simply too close to the bright Jupiter. Similarly Europa at Mag 5.2 is too close at 220 seconds of arc to the bright Jupiter making it impossible to see. However Callisto and Ganymede will over the next few weeks reach up to 10 minutes of arc or 600 seconds away.
The 4th century BC Chinese astronomer Gan De is credited with seeing Jupiter’s moons naked eye (3) though many doubt if this is possible, however, if it is ever to be done now is the time as the closeness of Jupiter therefore brightness of the moons and elongation are all, well, cosmically aligned for a good go.
(3) The Astronomy Encyclopaedia RD Press 1987. Pp 165.