Figure: A triple transit of Jupiter by 3 of its Galilean moons

Caption: NASA's Hubble Space Telescope (HST) images of a rare triple transit and shadow transit by the Galilean moons of Jupiter (the largest gas giant planet in the Solar System). This event happened 2015 Jan24 UT. The images were taken with the HST Wide Field Camera 3 in visible light.

The non-course-required, non-lab-required silent, non-authoritative video Triple transit by the Galilean moons, 2015 Jan23 | 1:12 shows this rare triple transit and shadow transit by the Galilean moons.

Further explication:

1. The three Galilean moons in question (and their mean orbital radii in units of Jupiter equatorial diameters and orbital periods in units of days) are Io (2.94928, 1.7691) Europa (4.69307, 3.5512), and Callisto (7.48624, 16.689). The largest Galilean moon, Ganymede (7.48624, 7.1546), was the odd man out---it wasn't doing anything special---just off somewhere orbiting outside of the image frame.

2. The Galilean moons---collectively named after the 17th century scientist Galileo Galilei (1564--1642)---who discovered them with an early telescope in early 1610 or just before (see Wikipedia: Galilean moons: Discovery)---go in nearly circular orbits around Jupiter in the same orbital plane to 1st order approximation which is also nearly Jupiter's equatorial plane.

3. The Galilean moons and their shadows often transit Jupiter---the transits of the shadows are called (by yours truly anyway) shadow transits. However, seeing three Galilean moons transiting Jupiter at the same time is rare, occurring only one or two times per decade.

4. Note in both the HST 2 images, the Sun light rays are beaming in from the lower left. The image on the right was taken ∼ 42 minutes after the image on the left.

5. The HST image on the left shows the beginning of the full triple transit. Left to right are Callisto and Io. Also left to right are the Europa shadow (Europa itself is off the image to the lower left), the Callisto shadow (underlapping Io), and the Io shadow.

6. The HST image on the right shows near the end of the full triple transit. Europa has entered the frame at lower left. Slower-moving Callisto is above and to the right of Europa. Fastest-moving Io is approaching the eastern limb of Jupiter. The Europa shadow is on the left side of the image, the Callisto shadow is on the right, and the Io shadow is past the transit phase: i.e., it is no longer being cast on Jupiter.

7. The Galilean moons have distinctive colors in these images. Io's volcanic, sulfur dioxide (SO_2) surface is yellowy-orangey. Europa's smooth icy surface is whitish-greyish. The Callisto's ancient, cratered surface is dark brownish.

8. A shadow actually has two zones: umbra and penumbra.

   In the umbra, the light source is completely occulted and you have dark shadow.

   In the penumbra, the light source is only partially occulted and you have less dark shadow.

   The ideal umbra is completely dark and the ideal penumbra continuously varies from completely dark to completely illuminated depending on how much of the light source is visible.

   Spherical light sources and shadow-casting obstacles give rise to conical umbras penumbras. The umbra is a closing/opening cone going away from the source if the shadow-casting obstacle is smaller/larger than the light source. The penumbra is an opening cone going away from the source in all cases. In either case, the farther away from the shadow-casting obstacle the more penumbra relative to umbra and the fuzzier the overall shadow is. Point sources of light do NOT have any penumbra and only an opening cone umbra.

   All this description is just to explain why the farther the Galilean moon is from Jupiter, the fuzzier its shadow since the shadow has relatively more penumbra.

   This explains why Callisto's shadow is noticeably fuzzier than the shadows of closer-to-Jupiter Galilean moons Io and Europa.