This Java applet shows the location of the four large moons of Jupiter as viewed from the earth in real time seen through a pair of binoculars or a telescope.
One of the frustrations of looking at astronomical objects is to know their orientations. This depends on whether you are looking at them with a pair of binoculars or a terrestrial telescope, which do not invert the image, or with an astronomical refractor, a Cassegrain or Newtonian reflector which do invert the image. Moreover, if the refractor or the Cassegrain has a diagonal, you will see a mirror image, whose orientation will depend on whether you are looking down through the eyepiece or sideways. In addition to all that, if you are in the southern hemisphere looking to the north, everything is inverted.
This applet was originally created by G. Nugent. As the original source code has been lost, it was recovered by decompiling the compiled Java code, which was then modified to allow the view to be reflected about the horizontal or vertical axes (or both), allowing Jupiter and its moons to correspond to how they would appear through different telescopes from different hemispheres. The default view has north at the top and east to the left, corresponding to a view through binoculars or a terrestrial telescope in the northern hemisphere, or a view through a refractor, Cassegrain or Newtonian reflector in the southern hemisphere. If you want a different view, click the buttons below.
Key: NH = Northern hemisphere. SH = Southern hemisphere. T = Terrestrial telescope without a diagonal or binoculars. R = Astronomical refractor or Cassegrain without a diagnonal or a Newtonian. TDD = Terrestrial telescope with a diagnonal looking downwards. TDS = Terrestrial telescope with a diagnonal looking sideways. RDD = Astronomical refractor or Cassegrain with a diagonal looking downwards. RDS = Astronomical refractor or Cassegrain with a diagonal looking sideways.