If you are interested in taking on what will be a challenging observation, I recommend beginning by reading a short article published in Sky and Telescope magazine in 1976, by the late Dr. Denis Dutton. The linked article contains a bit of information on the history of naked eye observations of the Jovian moons, as well as references to other historical articles and observations. In addition to the topic at hand, I particularly enjoyed his words at the end of the article when he wrote "The concept of true scientific discovery involves grasping the meaning or importance of what is revealed." Contained in these words is the often unspoken wisdom that guides those of us who spend hours at the eyepiece, straining to tease out the faintest hints of light from the dark background. What we are looking at is quite frequently not impressive at all, even to our fellow amateurs; it is only with an understanding of what we are looking at does the observation take on meaning and significance.
|12.22.13/TEC 140 APO/Lost Pleiad Observatory|
Today, January 5th, Jupiter is at opposition (exactly opposite the Sun in our sky) and rises at sunset. Perhaps most remarkable about this opposition, is that if seen from Jupiter, the Earth would appear to transit the Sun! The result is that the phase angle is quite small and the opposition effect on Jupiter and her moons will be quite strong. Les Cowley has an explanation of the opposition effect on his website, and essentially it is a result of objects appearing brighter when they are near the anitsolar point in the sky. As he identifies, "The Moon's brightness at full is greater than can be accounted for by the increase in its illuminated area compared with its partial phases."
How much brighter? Due to the opposition effect, Jupiter's moons may appear as much as 30% brighter tonight! As it happens, the crescent moon sets approximately 10:40 PM and is far from Jupiter in the sky. Below is a graphic representation of Jupiter and the location of it's moons (created with Winjupos) at 10:30 PM MST (0530 UT Jan. 6th)
Click the graphic to enlarge
Diameter Visual magnitude
Io 1.190" 5.0 mag
Europa 1.029" 5.3 mag
Ganymede 1.725" 4.6 mag
Callisto 1.573" 5.7 mag
While all of the moons are intrinsically bright enough to be seen by most observers at a reasonably dark site, it is the glare of Jupiter that typically prevents their observations. Tonight, Ganymede will be the easiest given that it is the brightest and will sit nearly 15 Jupiter diameters from the gas giant herself (one Jupiter diameter at tonight's opposition is 46.76 arcseconds). Io, while the second brightest, will likely be lost in the glare of the planet, and Europa will be exceedingly difficult. Callisto will be interesting as it is the faintest moon, yet will be furthest away at approximately 19 Jupiter diameters.
One recommended trick for attempting to see the moons is to stand (or sit) in a location where one can block Jupiter with the edge of a wall (or pole, tree, etc). Doing so may reduce the perceived glare and make spotting one or more moons a bit easier. Personally, I will attempt this observing challenge tonight, and would love to hear from others who attempt to see the Galilean moons without optical aid. Drop me a note, firstname.lastname@example.org (or comment below). Good luck!