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Astronomy 2021: Closer to Home – The Solar System
“Uncle Bob”


Finding Jupiter.
Even after decades of following the night sky events, I’m still sometimes startled by Jupiter’s apparition. This is due in part to the fact that the other bright planet Venus never appears more than 30 degrees or so up from the western evening or the eastern morning horizon. Jupiter will menace you from higher up.

In July, the planet will rise in the east just after evening twilight. Its transit, or highest point in the sky, follows in about 6 hours. These events happen progressively earlier through the autumn and for the balance of the year.

At June’s end Jupiter and Saturn appear in the south, about halfway up, hovering around Capricorn just before dawn. In July Jupiter begins a slight retrograde motion as all the outer planets do – east to west at the trailing end of the Goat, and it reaches opposition on August 19. On that night it will menace from its highest position overhead.

Hearing Jupiter.
We’ve been featuring Gustav Holst’s segments devoted to each of the planets. Here is a link to one of the more recognizable and spectacular movements.


Galilean Moons
In 1610 Galileo discovered the first moons other than our own. He used a homemade telescope with 2 two-inch lenses. More: see June Uncle Bob’s Saturn article.

Rather than paste up some eye-popping pictures of these moons, I’m sending you to NASA’s site for a greater treat – animated moon views compiled from the data of missions that have visited them. You’ll be able to manipulate their size and rotation, and see them in context with other orbits and with the parent planet Jupiter. And of course, there will be much more information about each of them than the brief sketches I give below.

Io. Slightly larger than Earth’s Moon. Io is the most volcanic object in the solar system. Some eruptions can be seen with large instruments from Earth.

Ganymede. The largest moon in the solar system and the only one to have a magnetic field as evidenced by its regular aurorae. The surface indicates there were violent upheavals of rock and/or water in the past. Ridges as much as 2000 feet high run for thousands of miles.

Callisto. The most heavily cratered in the solar system. The bright patches among the rocky surface are thought to be ice which has not yet eroded away.

Europa. Under a coating of water ice perhaps 15 miles thick, Europa houses more water than that of Earth. It is one of the most likely places to search for life forms similar to those of our planet. But of course, we have had a warning about going to this moon: “ALL THESE WORLDS ARE YOURS – EXCEPT EUROPA. ATTEMPT NO LANDING THERE.” [from Arthur C. Clarke’s novel 2010, Odyssey Two].

NASA’s Offerings.
Callisto at https://solarsystem.nasa.gov/moons/jupiter-moons/callisto/overview/

Europa at https://solarsystem.nasa.gov/moons/jupiter-moons/europa/overview/

Ganymede (N.B. the second image can be manipulated) at


Io at https://solarsystem.nasa.gov/moons/jupiter-moons/io/overview/

Next month: Mars

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