Ian Clarke

While you may believe you’ve been enjoying summer for some weeks now, it officially arrives this Friday, June 21, at 11:54 a.m. EDT. That’s the moment the sun reaches its northernmost position in the sky for the year, and since we live in the northern hemisphere, it’s our longest day.

Then July 4 brings another important point in the earth’s annual trip around the sun: aphelion, the day the earth is farthest from the sun in its elliptical orbit. As you can tell, the changing earth-sun distance is not what determines our seasons. Instead it is the tilt of the earth’s axis and the resulting variations in the length of daylight and in the sun’s altitude in the sky. If you saw a scale drawing of the earth’s orbit on a piece of paper, you wouldn’t even be able to tell, without careful measurement, that it was an ellipse and not a circle.

Jupiter and Saturn are both near their best for 2019 and are well-placed in the evening sky. Jupiter, the brightest object in the night sky besides the moon, will be shining in the south after dark. Look for Scorpius and its brightest star, Antares, to the right of the planet. Recently I’ve seen a number of headlines along the lines of this one: “This month, you can see Jupiter and its largest moons with just your binoculars.” That’s true enough; you can see with binoculars the same four moons that Galileo saw over 400 years ago. They look like stars close to and on either side of the planet. (All four may not be visible on any given night.) The only problem with that headline is that you can see them with a decent pair of binoculars any time Jupiter is in the night sky. Back in January 1610, when Galileo was looking, Jupiter was not at its closest, and he had equipment far inferior to a good pair of 10x50 binoculars. Nevertheless, it was in part Galileo’s observations of these four moons orbiting Jupiter (and not the sun) that provided the evidence to upend the earth-centered model of the solar system.

As for Saturn, it’s currently rising about 10 p.m. in the southeast, about two hours after Jupiter and among the stars of Sagittarius. It is fainter and more yellowish than Jupiter. While its brightest moon Titan is sometimes visible with 10X50 binoculars, the planet’s famous rings are not. Galileo was unable to see them clearly in his early telescope, calling Saturn a ball with “ears.” The rings of Saturn do make a great target for the owner of a modern small telescope. Wait closer to midnight when the planet is higher above the horizon for a clearer sight. When you see Saturn you’ll be following in the footsteps of Dutch astronomer Christiaan Huygens, who first described the rings in 1655 as “a thin, flat ring, nowhere touching” the planet. He thought they were solid. Now we know the rings to be made up of particles ranging from specks of dust to large boulders. But he was right about their thinness. Though 70,000 miles across, the rings are only about 70 feet thick. Little could Galileo or Huygens have imagined, but we have visited Saturn with robotic probes four times, NASA’s Pioneer 11 in 1979, followed by Voyagers 1 and 2 in 1980 and 1981. The Cassini-Huygens mission, a collaboration among NASA, the European Space Agency, and the Italian Space Agency, operated at Saturn from 2004 to 2017. Will we be back? Time will tell.

Ian Clarke is the director of the Hatter Planetarium at Gettysburg College. More information available at www.gettysburg.edu/hatterplanetarium.

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