As Jupiter?s four large moons circle the planet, they often cast their shadows on the cloud decks below. These are readily visible as inky black spots in small telescopes.

On rare occasions two of Jupiter?s moons cast shadows simultaneously. A series of such double shadow transits begins this week on Jupiter involving the two innermost moons, Io and Europa.

Io has an orbital period of 1.769 days and Europa?s period is almost exactly twice that, 3.551 days. As a result, the two moons return to almost the same position every 3.5 days. Because the ratio isn?t exactly 2, the moons gradually draw closer until Europa overtakes Io.

The first double shadow crossing takes place Sunday, March 7, with the moons about as far apart as they can be: Io and its shadow on the eastern limb and Europa and its shadow on the western limb. Because Jupiter is almost exactly on the opposite side of the Sun from the Earth, the moons and their shadows are very close together.

At the next shadow crossing, 3.5 days later, Io and Europa are closer together. And so it goes through March: at each event, the two moons draw ever closer.

By March 28, the date of the image here, Europa has actually passed Io. Strangely enough, the shadows are actually the reverse of what you would think: Io?s shadow is on the left and Europa?s shadow is on the right. That?s because of the relative positions of the two moons, Europa being closer to us than Io. Within a few minutes the shadows appear to merge: Europa has eclipsed Io.

Although these events are well shown in Starry Night, they are not actually visible in the sky because Jupiter is too close to the Sun to be observed for the whole month of March. Fortunately, software like Starry Night allows us to view such events.

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This article was provided to SPACE.com by Starry Night Education, the leader in space science curriculum solutions.