Driftless Dark Skies: Giant Planets

Our solar system’s two largest planets will be wonderful this summer. Jupiter is closest to Earth on June 10 and Saturn on July 9. During these oppositions, they will be at their biggest and brightest and visible all night long. How do you tell a planet from a star? Planets don’t twinkle. These two are brighter than stars. And they follow the same path the Sun makes in winter rising in the southeast in the evening, passing low in the south, and setting in the southwest in the morning.

As June begins, Jupiter is rising around 9:12pm here in the Driftless. By the end of the month, it’s rising two hours earlier at 6:57pm. June 15 and 16 are especially great evenings to enjoy Jupiter. On that Saturday, look for Jupiter to the left of the almost full moon which rises in the southeast around 7:08pm. Together they form an equilateral triangle with Antares, the brilliant red star in the heart of Scorpius. On that Sunday, Jupiter is to the right of the full moon which rises in the southeast around 8:12pm. This full moon is known as the Strawberry Moon (now in season) or the Honey Moon (because of its golden color as it travels low across the sky). This is also a great time to appreciate the relative size of the moon and Jupiter. Jupiter is 40x the diameter of the moon but is 1600x further away, so it appears 1/40 the diameter. A view through a small telescope reveals the intricate cloud bands in Jupiter’s atmosphere and lets you spot the four largest of Jupiter’s 79 known moons–Io, Europa, Ganymede, and Callisto. Watch as they change position from evening to evening as they orbit about Jupiter. On June 11 (9:22pm-12:40am), you can watch Io and Ganymede pass in front of Jupiter and cast dark shadows on the cloud bands below. You might even glimpse the Great Red Spot. You can watch the Moon and Jupiter together again on July 13 and August 9.

Saturn rises about two hours after Jupiter, 11:14pm at the beginning of June and 9:09pm at the end. It’s not as bright as Jupiter, but still brighter than the surrounding stars. June 18 is a great night to enjoy Saturn. Watch for it very close to the dark side of the waning gibbous moon which rises in the southeast around 10:04pm. If it’s cloudy that Tuesday or beyond your bedtime, Saturn and the moon will be together again on July 16 and August 12. Saturn is 33x the diameter of the moon but is 3300x more distant, so it appears 1/100 the diameter. Also a great time to appreciate light speed. The sunlight reflected by the moon reaches our eyes in just over a second, but the sunlight reflected by Jupiter has been traveling 36 minutes and the light from Saturn 76 minutes. A small telescope reveals the amazing rings circling Saturn. You should also be able to spot Titan, the largest of Saturn’s 62 observed moons. Like Ganymede, Titan is larger than the planet Mercury.

Enjoy the planets this summer and look forward to Winter Solstice 2020 when these two wonderful worlds will be together in the evening sky for a Great Conjunction after sunset.

John Heasley is an astronomy educator and stargazer who enjoys connecting people with the cosmos. He volunteers with NASA/JPL as a Solar System Ambassador and the International Astronomical Union as a Dark Sky Ambassador. For more information about stargazing in southwest WI, like Driftless Stargazing LLC on Facebook and find out whenever there’s something awesome happening in the skies. Driftless Dark Skies appears monthly in the Voice of the River Valley.