Here on Earth, the occasional alignments of the Sun and Moon with our planet are greeted with much fanfare. Solar and lunar eclipses are often spectacular sights. However, our solar system undergoes many other more subtle alignments that come under the general name of occultations. From Earth's ever-changing viewpoint, planets can be occulted by our Moon and other moons can be occulted by their planets. Consider the chances for such occultations on Jupiter, which has four of the seven large moons in the solar system. In this episode, we examine Hubble's observations of these otherworldly and somewhat poetic events.
Hubble press release:
Hubble Catches Jupiter's Largest Moon Going to the 'Dark Side'
Hubble Spots Rare Triple Eclipse on Jupiter
Rare Hubble Portrait of Io and Jupiter
Eclipses of the Sun can be seen from some point on Earth about every 6 months. Total solar eclipses occur, on average, about once every 18 months. The next total solar eclipse is on July 22, 2009, and is visible in India, through China, and across the southwest Pacific Ocean. A great place to get dates, maps, and other info about solar and lunar eclipses is the NASA Eclipse Web Site
The similar apparent size on the sky of our Moon and the Sun does not occur for any of the other large moons of the solar system. For Jupiter, Saturn, and Neptune, the Sun is much farther away, and therefore much smaller in their skies. The large moons are similar in size to our Moon, though their orbital distances can be up to five times larger.
Still, the combination of sizes and distances always produces moons that are much larger in the sky than the Sun. The closest is Callisto, which appears about 1.5 times the size of the Sun from Jupiter's view. The amazing views of the solar corona we get during total eclipses are a unique treat for our planet.
In addition to the image from Bernd Nies (link below) used in the video podcast, I found several other very nice images of the Moon occulting Saturn. For example, here are two others by DJLand Job Gehenia
Hubble has also seen occultations on Saturn. It is interesting that while Jupiter has four large moons, the rest of its moons are rather small. Not so with Saturn. Saturn has one large moon, Titan, and several medium-sized moons as well. These medium-sized moons cast shadows that Hubble can see, setting up this opportunity for an observation of four shadows on Saturn. Another cool observation occurred when Saturn's rings were edge-on to the Sun, and Hubble observed moon shadows across Saturn's rings.