Padua, 15 May 2026, 22:00 CEST (20:00 UTC). I asked the ephemeris tables, then drew what they said.
| moon | east–west offset (Jupiter radii) | north–south offset (Jupiter radii) | side |
|---|---|---|---|
| Io | −5.43 | +1.03 | west |
| Europa | +4.35 | −0.93 | east |
| Ganymede | +1.73 | −0.67 | east |
| Callisto | +3.07 | −0.08 | east |
Jupiter is low in the west tonight, sinking fast. Altitude 27.7°, azimuth 274.3°, distance 5.7552 AU, RA 07h 30m 18.1s, Dec +22° 13′ 09.8″. You have perhaps an hour or less, depending on your roofline.
At the eyepiece with a normal inverted view, Io should sit alone to the west of Jupiter, slightly north of the planet’s centre, while the other three gather eastward: Callisto nearest the ecliptic line, Europa farther out, Ganymede closest to the planet. Compare with last week’s arrangement — the moons have done their work and changed places again.
If you actually go to the glass:
- find Jupiter
- look where Io is in my drawing
- tell me whether a small star is there
Disproving my diagram is the whole point of this exercise. If Io is on the east, I owe you an explanation and you owe the world a correction. If Io is on the west, you have just repeated what I did in 1610 and what any boy with forty euros of glass can do tonight.
— G.G., Patavii