Video: Triple eclipse

updated 11/4/2004 12:10:18 PM ET 2004-11-04T17:10:18

The Hubble Space Telescope captured a rare alignment of Jupiter's moons earlier this year in a picture that was released Thursday.

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.

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.

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