Level 1 - Absolute Beginner
Jupiter is a very big planet. It has many moons. Two of them are Io and Ganymede.
NASA shared a new video on Wednesday. In the video, Ganymede moves in front of Io. For a short time, you cannot see Io.
This does not happen often. It happens when Jupiter is in a special place in its path. That is about every six years.
Telescopes can watch these moons. The moons look like small lights next to Jupiter.
- planet
- A big round world that goes around a star.
- moon
- A small world that goes around a planet.
- telescope
- A tool that makes faraway things look bigger.
- video
- A moving picture.
- path
- The way something travels.
- special
- Not like the others.
- often
- Many times.
- hide
- To stop someone from seeing something.
Level 2 - Elementary
NASA's Astronomy Picture of the Day for October 28 shows a rare event at Jupiter. The moon Ganymede passes in front of the moon Io and hides it for a short time. Astronomers call this a mutual occultation.
These events happen in a season. Jupiter's moons go around the planet in an almost flat path. About every six years, Earth sees that path edge on, because Jupiter is near its equinox. Then the moons often pass in front of each other or move through each other's shadows.
The two moons are very different. Ganymede is the largest moon in the solar system. Io is the most volcanic place we know, with many active volcanoes. Io goes around Jupiter in less than two days, while Ganymede needs about a week, so Io often catches up and passes it.
You do not need a big telescope to see the moons. A small telescope shows them as tiny points of light in a line beside Jupiter.
- occultation
- When one object passes in front of another and hides it.
- equinox
- A time when a planet's tilt is sideways to the Sun.
- shadow
- A dark shape made when something blocks light.
- volcanic
- With volcanoes that erupt.
- astronomer
- A scientist who studies space.
- edge on
- Seen from the side, as a thin line.
- season
- A part of time when something happens often.
- solar system
- The Sun and everything that goes around it.
Level 3 - Intermediate
NASA's Astronomy Picture of the Day for October 28 is a short video of a celestial coincidence: the moon Ganymede sliding in front of the moon Io, briefly hiding it from view. Astronomers call such an event a mutual occultation, and it is one of the pleasures of a rare observing season at Jupiter.
The season exists because of geometry. The four large Galilean moons orbit close to Jupiter's equator, and Jupiter's axis is tilted. Roughly every six years, as Jupiter passes its equinox, Earth sees those orbits edge on, so the moons line up with one another and either pass in front of each other, an occultation, or slip through each other's shadows, an eclipse. Outside such a window, the moons rarely overlap from our point of view.
The pair in the video makes a good contrast. Ganymede, about 5,270 kilometres across, is the largest moon in the solar system, larger than Mercury. Io, a little bigger than Earth's moon, is squeezed by Jupiter's gravity so strongly that it has hundreds of volcanoes and is the most volcanically active body known. Because Io circles Jupiter in under two days and Ganymede in about a week, Io regularly catches up and passes it.
Amateurs have made the most of earlier seasons with small telescopes and cameras, stacking thousands of video frames to reveal the moons' disks. Careful timing of such events also lets astronomers refine the moons' orbits, which is one reason professionals value the observations.
- celestial
- Relating to the sky or outer space.
- coincidence
- Two events happening together by chance.
- geometry
- The arrangement of shapes, angles and positions.
- axis
- The imaginary line a planet turns around.
- eclipse
- When an object passes into another's shadow.
- active
- Currently doing something, such as erupting.
- overlap
- To cover part of something else.
- refine
- To make more exact.
Level 4 - Advanced
Among the quieter marvels of the solar system, few reward patience as handsomely as the mutual events of Jupiter's moons. NASA's Astronomy Picture of the Day for October 28 captures one in motion: Ganymede, the largest satellite in the solar system, drifting across the face of Io and extinguishing it for a spell. The term of art is a mutual occultation, and it is only possible during a narrow window that recurs about every six years.
The window opens because of orbital geometry. The Galilean moons circle close to Jupiter's equatorial plane, and the planet's axis is inclined to its orbit. When Jupiter reaches equinox, Earth views that plane edge on, so the moons' paths cease to be spread out like a ladder and instead overlap, producing a run of occultations, in which one moon covers another, and eclipses, in which one drifts into another's shadow. For the intervening years such coincidences are all but impossible.
The two protagonists could hardly differ more. Ganymede, roughly 5,270 kilometres across, outsizes even Mercury and probably harbours a metallic core and a hidden ocean. Io, locked in a gravitational tug of war between Jupiter and its sibling moons, is flexed so relentlessly that its interior melts, driving the most intense volcanism anywhere known. Io's orbit takes under two days to Ganymede's seven, so Io overtakes its neighbour again and again.
The scientific payoff extends beyond spectacle. Precisely timed contacts between moons are sensitive tests of orbital models, and decades of such timings have helped refine the moons' motions. That an amateur with a modest telescope and a video camera can contribute to that record is a quiet argument for the value of simply looking up.
- satellite
- A natural object that orbits a planet.
- extinguish
- To put out or make disappear, especially light.
- term of art
- A specialised phrase used by experts.
- inclined
- Tilted at an angle.
- protagonist
- A main character in a story.
- relentlessly
- Without ever stopping.
- volcanism
- Volcanic activity.
- spectacle
- A visually impressive event.
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