The moon of Jupiter – Callisto (IV) –
was discovered in 1610 by Galileo using the newly invented telescope... Galileo also found Io,
Europa and Ganymede. 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: Callisto was beloved of Zeus transformed in to a bear by Hera in a rage of
jealousy... The moon by this name is in almost circular prograde orbit (→eccentricity
e = 0.0074) with a →semimajor axis a = 1,882,700 km. At pericenter (closest to the planet) Callisto is separated from the Jupiter a distance of q = 1,868,768 km. At apocenter (furthest from the planet) this moon is separated from the Jupiter a distance of Q = 1,896,632 km. Its mass is huge: ~1.0759×1023 kg – it equals over
14,643,240 mountains like Giewont,
making it almost 1.5 times more massive than the Earth's Moon!
An in teresting feature of this moon is that being the furthest Galilean satellite it is also most heavily cratered... The most unusual one is
Valhalla. It consists of a central region of lighter coloring that is 600 kilometers in diameter (also called a
palimpsest which is a latin word for a manuscript of parchment, that has been written on more than once, with the earlier writing erased) and outer concentric rings that extend to 2000 kilometers in diameter! It results from an ancient collision and has been raised and smoothed by
→isostatic
forces...
You can read more about Callisto 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 17
days, which means that its mean celestial angular velocity in the sky is 21°.
Its brightness is 5.7m (1.31 times brighter than the faintest stars visible to the naked
eye). It is sometimes possible to see Callisto without any optical aid when it is at its greatest separation from the planet. For comparison: Jupiter is about –2.7m,
so Callisto is shining fainter by about 8.4m.
It corresponds to a difference in brightness of almost 2,291 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 Callisto 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]
–7.0
–7.0
Orbital velocity [km/sec]
8.26
8.14
* 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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