As Jupiter?s four large moons circle the planet, they oftencast their shadows on the cloud decks below. These are readily visible as inkyblack spots in small telescopes.
On rare occasions two of Jupiter?s moons cast shadowssimultaneously. A series of such double shadow transits begins this week onJupiter involving the two innermost moons, Io and Europa.
Io has an orbital period of 1.769 days and Europa?s periodis almost exactly twice that, 3.551 days. As a result, the two moons return toalmost the same position every 3.5 days. Because the ratio isn?t exactly 2, themoons gradually draw closer until Europa overtakes Io.
The first double shadow crossing takes place Sunday, March7, with the moons about as far apart as they can be: Io and its shadow on theeastern limb and Europa and its shadow on the western limb. Because Jupiter isalmost exactly on the opposite side of the Sun from the Earth, the moons andtheir shadows are very close together.
At the next shadow crossing, 3.5 days later, Io and Europaare closer together. And so it goes through March: at each event, the two moonsdraw ever closer.
By March 28, the date of the image here, Europa hasactually passed Io. Strangely enough, the shadows are actually the reverse ofwhat you would think: Io?s shadow is on the left and Europa?s shadow is on theright. That?s because of the relative positions of the two moons, Europa beingcloser to us than Io. Within a few minutes the shadows appear to merge: Europahas eclipsed Io.
Although these events are well shown in Starry Night, theyare not actually visible in the sky because Jupiter is too close to the Sun tobe observed for the whole month of March. Fortunately, software like StarryNight allows us to view such events.