Jupiter and Saturn are easy to spot in September’s evening sky.
by Jeffrey L. Hunt
Bright Jupiter and Saturn shine from the south-southeast sky this evening. The two giant planets are well-placed for viewing during the early evening.
The planets are 8.3° apart in eastern Sagittarius. Both are retrograding – moving westward compared to the starry background.
As the planets revolve around the sun, they move eastward compared to the starry background. While they rise in the east and set in the west from Earth’s rotation, each night they appear farther eastward compared to the stars.
As Earth catches up to and passes between the planets and the sun, they appear to move backward compared to the stars. This illusion is similar to the classic train impression where the passenger cannot at first determine which train is moving, the one they are occupying or the one adjacent to them. One seems to be moving compared to the other and the background through the windows.
Both planets end their retrograde this month. Jupiter ends its apparent backward motion on September 12, followed by Saturn, September 28.
Then Jupiter somewhat quickly closes the gap between them until the Great Conjunction of the two planets on December 21, 2020. This is the closest conjunction since the meeting in 1623. While other Jupiter – Saturn conjunctions have occurred during the following centuries, this year’s promises to be spectacular in its closeness.
Follow the progress of the planets in the starfield with a binocular. Jupiter is near Pi Sagittarii (π Sgr on the photo) and 50 Sagittarii (50 Sgr). Saturn is near 56 Sagittarii (56 Sgr).
If held steadily, up to four of Jupiter’s satellites are visible. In the photo at least two are captured. They appear as dim stars to each side of the planet.
This chart shows the positions of the planets compared to the stars mentioned above on September 15, 2020.
Mars is well-up in the eastern sky by 11 p.m. If you are outside during morning twilight, spot it high in the south. It’s the brightest “star” in the southern sky. On the evening of September 5 and the morning of September 6, the moon and Mars are close together in the sky.
Venus is low in the eastern sky before sunrise.
December 30, 2021: As the year ends and the new one opens, the night sky’s brightest star – Sirius – is in the southern sky at the midnight hour.
December 31, 2021: This morning before sunup, the thin waning crescent moon appears near Mars and the star Antares. Four planets – Venus, Mercury, Saturn, and Jupiter – are on parade in the southwest after sundown.
December 30, 2021: The morning crescent moon seems to be captured in the Scorpion’s pincers to the upper right of Mars. Four Evening Planets – Venus, Mercury, Saturn, and Jupiter – are in the southwest after sundown.
December 28, 2021: The Great Andromeda Galaxy is nearly overhead at the end of the evening twilight.
December 29, 2021: The morning crescent moon approaches Scorpius and Mars. In the evening sky, four evening planets – Venus, Mercury, Saturn, and Jupiter – are lined up in the southwest. Venus is rapidly leaving the evening sky.