Rare Triple Eclipse on Jupiter
The Hubble Space Telescope captured a rare alignment of Jupiter's moons earlier
this year in this picture, which was released today.
Three of Jupiter's largest moons -- Io, Ganymede, and Callisto -- cast dark
shadows on Jupiter in this near-infrared image. In these mini-solar eclipses,
the moons block the sunlight that otherwise reflects off Jupiter. Io's shadow
is located just above center and to the left; Ganymede's on the planet's left
edge; and Callisto's near the right edge.
Two of the moons are directly visible in the image. Io is the white circle
in the center, and Ganymede is the blue circle at upper right. Callisto is out
of the image and to the right.
The image was taken March 28, 2004.
Seeing three shadows on Jupiter happens only about once or twice a decade.
Io, Ganymede, and Callisto orbit Jupiter at different rates, Hubble astronomers
explained. Their shadows likewise cross Jupiter's face at different rates.
For example, the outermost moon Callisto orbits the slowest of the three satellites.
Callisto's shadow moves across the planet once for every 20 shadow crossings
of Io.
Add the crossing rate of Ganymede's shadow and the possibility of a triple
eclipse becomes even more rare. Viewing the triple shadows in 2004 was even
more special, because two of the moons were crossing Jupiter's face at the same
time as the three shadows.
Jupiter and Venus are easily
visible this week in the morning sky.
-- SPACE.com Staff
Credit: NASA, ESA, and Erich Karkoschka (University
of Arizona)
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