The moon of Jupiter – Elara (VII) – was discovered in 1905 by C. Perrine. The discovery came only a year after the same person had discovered
Himalia. It is also in teresting to note that Himalia was discovered
13 years after Amalthea was found.
The name of that moon is of mythological origin. Elara was the daughter of King Orchomenus and gave birth to a giant Tityas. She was also one of Zeus' many lovers...
The moon by this name is in elliptical prograde orbit (→eccentricity
e = 0.2174) with a →semimajor axis a = 11,741,000 km. At pericenter (closest to the planet) Elara is separated from the Jupiter a distance of q = 9,188,507 km. At apocenter (furthest from the planet) this moon is separated from the Jupiter a distance of Q = 14,293,493 km. In future these parameters may vary due to a large orbital distance from Jupiter and the orbital motion being disturbed by the Sun and other factors. Perhaps it is a captured asteroid (with a
small mass: ~8.6923×1017 kg – it equals over 118 mountains like Giewont
!),
and in future will possibly return to heliocentric orbit.
An in teresting feature of that moon is that being the in nermost
''irregular'' satellite of Jupiter (in a higly inclined elliptical orbit), it revolves in the same direction as the planet is spinning on its
axis.
Due to its similarity in semimajor axis to some of the other satellites of Jupiter, it is considered to be a member of a so called
Himalia (probably the remnants of a single large body).
The orbital period is over 2/3 earthly
year, which means that the average speed of its celestial motion is only 1°23'12''
per day. When this motion was detected and compared to the changes in Jupiter's position, this body was classified as a true planetary satellite (and not a distant celestial body with its position projected near the planet).
Other obstacle in detecting this moon earlier was its brightness of only 16.3m
(almost 13.2 thousand times fainter than the faintest stars visible to naked eye). For comparison: Jupiter's brightness is about
–2.7m,
so Elara is shining fainter by about 19.0m. The corresponding difference in brightness is over
39.8 million times!
I have given some crucial data of Elara below. Table one contains the basic
information. Whereas table two gives more detailed parametrs of its orbit (calculated using the following formulae).
Translated by Karol Pankowski
|
Elara
|
|---|
| Mean distance from the planet [×103 km] |
11,741.0
|
| Mean distance from the planet [planetary R] |
164.2
|
| Orbital period [days] |
259.64
|
| Orbital eccentricity [e] |
0.2174
|
| Orbit inclination [degrees] |
26.6
|
| Mean diameter [km] |
86.0
|
| Main discoverer and year of the discovery |
C. Perrine 1905
|
| Visual magnitude [mag] |
16.3
|
| Mass [kg] |
~8.6923 × 1017
|
Orbital Parameters |
Pericenter
[q] |
Apocenter
[Q] |
| Distance from the planet [×103 km] |
9,188.5
|
14,293.5
|
| Distance from the planet [planetary R] |
128.5
|
199.9
|
| Angular size of the moon's orbit observed from the Earth* [degrees] |
0°50'14.40''
|
1°18'09.39''
|
|
| Angular diameter of the planet's disc as observed from the moon [degrees] |
0°53'29.77''
|
0°34'23.37''
|
| Brightness of the planet as observed from the moon** [mag] |
–11.9
|
–10.9
|
| Diameter of the moon's disc as observed from the planet's "surface" [degrees] |
0°00'01.95''
|
0°00'01.25''
|
| Brightness of the moon as observed from the planet's "surface"** [mag] |
7.1
|
8.1
|
|
Orbital velocity [km/sec] |
4.10
|
2.63
|
* 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)
|
See other related links:
Astronomy for every – Jupiter's moons
Monde
des Monats
– HIMALIAGRUPPE
Views
of the Solar System – ELARA
Solar
System Exploration – ELARA
The
Nine Planets – ELARA
The Astronomy Workshop – Satellite Viewer
Planetary Satellite Mean Orbital Parameters
Natural Satellite Physical Parameters
 |
