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NASA’s Astronomy Picture of the Day for Oct. 28, 2026, features a video of Ganymede briefly blocking Io from view during an event observed on Oct. 20. The alignment is part of a cycle of mutual occultations that becomes visible from Earth about once every six years.
NASA’s Astronomy Picture of the Day on Oct. 28, 2026, features a video of Ganymede briefly blocking Io, two of Jupiter’s large moons, from view as seen from Earth. The event took place on Oct. 20 and is a mutual occultation, an alignment that offers a view of one moon passing in front of another.
The APOD explanation, written by astronomer Michael Wong, says Ganymede and Io met in the sky on Oct. 20. In the accompanying video, Ganymede passes across Io’s apparent position and briefly obscures it. The NASA page describes the event as a mutual occultation: one astronomical object passes in front of another and blocks it, either partly or completely, from the observer’s line of sight.
The video also shows Ganymede’s shadow on Jupiter, on the planet’s right side in the image. The explanation notes that a hypothetical observer in that shadow on Jupiter would experience a solar eclipse. This is a description of the geometry shown, not a report of an observation from Jupiter.
The APOD entry is an astronomy image-and-video feature, not an announcement of a new mission or discovery. NASA credits the video to Michael Wong; the page lists Keighley Rockcliffe, Robert Nemiroff, Jerry Bonnell and Cecilia Chirenti as authors and editors. Its stated purpose is to explain the celestial event captured in the video.
A Rare View of Jupiter’s Moons
Mutual occultations let observers see the changing positions of Jupiter’s moons through a direct alignment: one moon appears to cross in front of another. The Oct. 20 event gives viewers a clear example of that geometry, while the visible shadow on Jupiter adds a second alignment effect to the scene. The result is useful as a visual illustration of how the moons, planet and Sun can line up from our viewpoint.
These events are not visible in the same way every year. Wong’s APOD explanation says mutual occultations of the Galilean moons are rare to witness because their orbits must appear nearly edge-on from Earth. According to the page, this viewing geometry occurs about once every six years, when Jupiter reaches equinox and appears less tilted to observers. That cadence makes the video a record of a particular window for observing the moons’ apparent crossings, rather than a routine view available at any time.
The feature also makes a distant orbital event accessible to readers who did not observe it directly. It connects a brief apparent disappearance of Io with the larger arrangement of Jupiter’s system, without implying that either moon physically collided with or changed its orbit around the other.
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Jupiter’s Moon Alignment Cycle
Jupiter has many known natural satellites. The APOD explanation gives the count as 115 discovered moons and compares it with Saturn’s 293. These figures are the counts stated in the Oct. 28 entry; the page does not provide a separate discussion of how moon counts are established or updated.
Io and Ganymede are two of Jupiter’s four Galilean moons, the large satellites highlighted in the APOD. Their apparent close approaches and crossings depend on the viewing angle from Earth. During the period described by Wong, the moons’ orbital planes were positioned so that one could appear to pass in front of another. The same alignment can also place a moon’s shadow on Jupiter, as the video shows.
The date distinction matters: the recorded meeting occurred on Oct. 20, 2026, while NASA featured it eight days later, on Oct. 28. The APOD presents the footage with a short professional-astronomer explanation rather than a detailed observing report. It does not identify the video’s equipment, location or recording method.
Details Beyond the APOD Video
The NASA page does not specify the exact duration of the occultation, whether Io was completely or only partly hidden, or the time and location from which the video was recorded. It also does not provide technical details such as the camera or telescope used. Those particulars should not be inferred from the short explanation alone.
The once-every-six-years description refers to the favorable viewing geometry described by Wong; the entry does not give a schedule of individual occultations or say that every alignment in that period will be equally easy to observe. The page also does not provide a separate confirmation or analysis from an observatory. Its account and interpretation are presented as part of the APOD feature.
The APOD’s Next Feature
For this item, the next development identified on the NASA page is the following day’s Astronomy Picture of the Day. The Oct. 28 entry lists the next feature as “a planet grows horns”; it does not announce further reporting or follow-up observations of the Io-Ganymede occultation.
Readers seeking more about this particular video can consult the APOD page, which credits Wong and names the editorial team. The source material does not say whether additional footage, measurements or a detailed observing account will be released.
Key Questions
What happened between Ganymede and Io?
On Oct. 20, 2026, Ganymede passed in front of Io from Earth’s point of view and briefly blocked it from view. NASA’s Oct. 28 APOD features a video of the event.
What is a mutual occultation?
It is an alignment in which one astronomical object passes in front of another and blocks it partly or completely from the observer’s line of sight. The term describes an apparent crossing, not a collision.
Why are these events relatively rare to see?
According to APOD author Michael Wong, Earth must have a nearly edge-on view of the Galilean moons’ orbits. The page says this favorable geometry occurs about once every six years as Jupiter reaches equinox.
What else appears in the video?
The video also shows Ganymede’s shadow on Jupiter. Wong notes that an observer hypothetically located in the shadow on Jupiter would experience a solar eclipse.
Where can readers find more technical details?
The APOD page provides the video, its explanation and credits, but does not list recording equipment, location or the occultation’s precise duration. Those details are not specified in the supplied source.
Source: science.nasa.gov
