The Sweaty Stargazer: Case Study: Jupiter Ascending, sort of.

There was an impromptu observing session at Bishan Park yesterday at 8.30pm. Dave, Ava, Jason, and I were there. The main object we were keen to see was Jupiter. At about 9.30pm, I spotted a black semi-circle bobbing at the edge of Jupiter's equatorial band and alerted the rest to look through the telescope. It was Ganymede, one of Jupiter's moons shadow in the process of transiting the planet.

Shortly after, the entire black, round shadow begin to cast on Jupiter and it was a sight to behold. Now, shadow transits are not uncommon. Backyard astronomers see them often.

At 10.20pm, Dave alerted us to something strange which he saw. There was a very faint, dark object accompanying the shadow. At first, he thought there was a speck of dirt on this spectacles, but after wiping his specs, the object was still there. Then he thought there was dirt on his telescope lens but no, the object followed the planet as it drifted off from view. So that cancelled the possibility of dirt. All 4 of us were puzzled by the phenomenon.

Then we thought there might be another moon transiting Jupiter, hence that object is another shadow from a different moon, but yet again, no. The other 3 moons were too far away to cast their shadow on the gas giant. We started to check our astronomy apps and there were no other shadow transits other than that of Ganymede's. We even thought that a comet or an asteroid had hit Jupiter.

Using an app called 'Mobile Observatory' to check if there was a second shadow transit on Jupiter.

It wasn't a UFO, and it certainly wasn't Mila Kunis.

So, what the hell was that object?

Usually, whenever there's a shadow transit on Jupiter, the moon either appears to be a small starlight object because of the sunlight reflecting off the bright surface of the moon or the moon is totally hidden from view as it blends into the whitish equatorial band of Jupiter.

Finally, Gary sent us a picture from his app that made sense to us.

The screenshot Gary sent us.

Never have we thought that mysterious object we have been seeing was Ganymede itself. We used that picture and tallied it with what we saw and the gears suddenly clicked.

Wow.... chibaboom... mind-blown!

Then we thought, maybe we were seeing it as a faint, dark object is because Ganymede's surface is rather black as compared to Europa's icy surface and Io's yellowish surface, which reflected a lot of light. Therefore, its albedo is rather low, meaning its surface doesn't reflect much light. So, it appears as a faint and dark shadow-like object that was drifting pass Jupiter. Also, maybe the whitish equatorial band played a part visually in contrasting Ganymede's dark surface.