June 17, 2021: At mid-month, bright Jupiter and Saturn are in the southern sky before sunrise. Saturn is retrograding in Capricornus. Jupiter’s eastward direction is slowing as it turns westward to retrograde beginning June 21.
by Jeffrey L. Hunt
On the next clear morning step outside about an hour before sunrise. Bright Jupiter is in the south-southeast. It is brighter than all the stars in the morning sky. Saturn, over 19° to the lower right of Jupiter, is not as bright as the Jovian Giant, Arcturus, and Vega.
The star Fomalhaut – “the mouth of the southern fish” – is to the lower left of Jupiter.
Jupiter and Saturn seemingly have become staples in the morning sky. Each week they appear farther westward at the same time interval before sunrise. Earth is revolving around the sun and we see the morning duo farther west.
Each day the sun, moon, stars, and planets seem to move eastward from Earth’s rotation.
The planets are revolving around the sun. We see them move among the constellations behind the plane of the solar system. The center line is known as the ecliptic.
Their motion compared to the stars is a combination of their revolution around the sun and Earth’s annual trek around the central star.
For the planets farther away from the sun than Earth, sometimes called the outer planets or the superior planets, our planet catches and overtakes them. The superior planet seems to stop moving eastward compared to the stars and reverses its direction. The westward journey compared to the starry background is known as retrograde motion. This is an illusion.
Eventually, the planets begin to move eastward again compared to the background stars.
Saturn’s retrograde began nearly a month ago. Jupiter’s begins on June 21. For the past several days, Jupiter seems to have been slowing its eastward motion in Aquarius in the starfield that includes Iota Aquarii (ι Aqr on the chart), Theta Aquarii (θ Aqr) – named Ancha, “the hip” – and Sigma Aquarii (σ Aqr). Use a binocular to see the stars. After Jupiter begins to retrograde it does not seem to move much for several days compared to that stellar trio.
The constellations Capricornus, where Saturn is located, and Aquarius are famous in name because of their association with the ecliptic and the motions of the solar system bodies, but the stars are relatively dim in comparison to Orion or the Big Dipper.
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