2021, August 19: Jupiter at Opposition

Astronomy, Sky Watching
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August 19, 2021:  Jupiter is at opposition this evening.  It shines brightly in the sky nearly all night long.

2021, August 19: As seen from space the faster moving Earth moves between Jupiter and the sun.
Chart Caption – 2021, August 19: As seen from space the faster moving Earth moves between Jupiter and the sun.

by Jeffrey L. Hunt

The giant planet Jupiter reaches opposition this evening.  At 7:28 p.m. CDT, Earth passes precisely between Jupiter and the sun.  This is known as opposition.

Saturn was at opposition on August 2. It appears to the upper right of Jupiter in the evening sky.

Our faster moving planet catches and passes Jupiter every 399 days.  As a lead up to the opposition, Jupiter appears to stop moving eastward in its orbit and it seems to back up or retrograde.  This westward motion is an illusion from our faster-moving planet.

At opposition, Jupiter (and other planets farther from the sun than Earth) rises in the eastern sky at sunset.  It appears in the southern sky around midnight and sets in the west at sunrise.  It seems to appear in the sky at opposite times and opposite directions from the sun.

This striking view of Jupiter’s Great Red Spot and turbulent southern hemisphere was captured by NASA’s Juno spacecraft as it performed a close pass of the gas giant planet. (NASA Photo)

The opposition moment is not important for precise observing.  Telescopes are turned toward Jupiter at any time, because it consistently appears large in a telescope eyepiece.

At about 80x through a backyard telescope, the planet is about the size of the number 2 pencil eraser held at arm’s length.  On the planetary globe, its cloud stripes are visible and depending on its location, the Great Red Spot may make an appearance.  Up to four of its largest moons are easily visible.  If held steady, a binocular can see them as well.

On other occasions, one of Jupiter’s largest moons can be seen in front of the planet or disappear behind it.  Sometimes the shadow of one of those moons is visible on the planet’s cloud tops.

Near the times of oppositions, astronomers have first observed intricate details of the outer planets, such as fainter moons and features in the planets’ atmospheres.

Along with Saturn, Jupiter is one celestial wonder to view through a telescope at a public star party or through the neighborhood astronomer’s telescope.

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