The moon of Jupiter – Ganymede (III) –
was discovered in 1610 by Galileo using the newly invented telescope... Galileo also found Io,
Europa 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: Ganymede was the son of King Tros of Troy and cupbearer to the gods...
The moon by this name is in almost circular prograde orbit (→eccentricity
e = 0.0013) with a →semimajor axis a = 1,070,400 km. At pericenter (closest to the planet) Ganymede is separated from the Jupiter a distance of q = 1,069,008 km. At apocenter (furthest from the planet) this moon is separated from the Jupiter a distance of Q = 1,071,792 km.
Ganymede is the most massive moon in the Solar System! Its mass is huge: ~1.4819×1023 kg – it equals over 20,167,725
mountains like Giewont,
making it 2 times more massive than the Earth's Moon!
An in teresting feature of this moon is that Ganymede is the LARGEST moon in the Solar System, even larger than Mercury! It is covered by light and dark regions and heavily cratered. The largest dark region is Galileo Regio which spans over 3200 kilometers! Viewing this feature near the north pole of Ganymede is only possible with an aperture of at least 25 centimeters and a magnification of 400×.
You can read more about Ganymede 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
7 days, which means that its mean celestial angular velocity in the sky is 50°.
Its brightness is 4.6m
(3.63 times brighter than the faintest stars visible to the naked eye). It is sometimes possible to see Ganymede without any optical aid when it is at its greatest separation from the planet. For
comparison: Jupiter is about –2.7m, so
Ganymede is shining fainter by about 7.3m.
It corresponds to a difference in brightness of almost 832 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 Ganymede 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]
–9.2
–9.2
Orbital velocity [km/sec]
10.89
10.86
* 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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