The moon of Jupiter –
Europa (II) – was discovered in 1610 by Galileo using the newly invented
telescope... Galileo also found Io,
Ganimede and Callisto. He published his observations of those four moons through a homemade telescope in a special report which luckily survived to this
day: ''On the 7th day of January in the present year (...) the planet Jupiter presented itself to my view (...) I noticed a circumstance which I had never been able to notice before (...)
namely that three little stars, small but very bright, were near the planet''. A few nights later he discovered the fourth
moon... Galileo called the Jupiter's moons the "Medicean planets", after the Medici family (since he was seeking their support as the rulers of Florence). However, later on mythological names were officially
adopted: Europa was the daughter of a Phoenician king, seduced by Zeus (one of the continents on Earth was also named after her as
Europe).
The moon by this name is in circular prograde orbit (→eccentricity
e = 0.0094) with a →semimajor axis a = 671,100 km. At pericenter (closest to the planet) Europa is separated from the Jupiter a distance of q = 664,792 km. At apocenter (furthest from the planet) this moon is separated from the Jupiter a distance of Q = 677,408 km. Its mass is huge: ~4.7998×1022 kg – it equals over
6,532,468 mountains like Giewont,
hich makes 66% of the mass of Earth's Moon!
An in teresting feature of this moon is that Europa is the smallest Galilean satellite. Its icy surface is covered in a complex network of cracks (probably formed as a result of some sort of geological phenomena). Only numerous craters have been identified there which proves constant resurfacing. There is a possibility that a liquid mantle might underlie the icy crust. But if Europa could sustain liquid water, could it also sustain some form of primitive life
forms?
You can read more about Europa on the Web pages from the links listed below the table since I would not like to copy any information that is widely
availible...
Its orbital period is almost 4
days, which means that its mean celestial angular velocity in the sky is 101°.
Its brightness is 5.3m
(1.9 times brighter than the faintest stars visible to the naked eye). Despite its substantial brightness, viewing this moon without optical aid is impossible for it always remains to close to the dazzling disk of Jupiter! For
comparison: Jupiter is about –2.7m, so Europa is shining fainter by about 8.0m.
It corresponds to a difference in brightness of over 1,585 times!
Observin four Galilean satellites is easy so it is good to repeat
Galileo's work and follow their configuration for some time. Although nowadays that would be of little scientific value since their positions are known with great precision it would nevertheless be a lot of
fun!
For if you would like to view them even a pair of 10×50 binoculars will do (where 10 is the magnification and 50 is the aperture in milimeters). They are very comfortably viewed in popular 20×50 spotting scopes. Both binoculars and spotting scopes have the advantage of producing a naturally oriented image and not a mirrored one.
Following the movement of Galilean moons gives a lot of in sight in to celestial mechanics (for the Jovian system is like a miniature of the Solar System). Anyway, when one starts observing those four moons one always makes four new friends for life.
I have given some crucial data of Europa below. Table one contains the basic
information. Whereas table two gives more detailed parametrs of its orbit (calculated using the following formulae).
Brightness of the moon as observed from the planet's "surface"** [mag]
–9.6
–9.5
Orbital velocity [km/sec]
13.87
13.61
* This value is calculated for Jupiter at opposition (distance 628.8 million km = 4.2 AU)
** The given value of magnitude is not corrected for some decreasing factors (e.g. the changing phase of illumination)
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