October 18, 2021: Jupiter’s illusion of retrograde motion ends today. Arcturus is appearing in the morning sky along with Mercury. The planet pack – Venus, Saturn, and Jupiter – and the moon are in the evening sky.
by Jeffrey L. Hunt
Chicago, Illinois: Sunrise, 7:07 a.m. CDT; Sunset, 6:04 p.m. CDT. Check local sources for sunrise and sunset times for your location.
Jupiter’s retrograde ends today. This illusion is from a faster moving Earth passing between the sun and Jupiter.
On the chart above, Earth and Jupiter are shown in their respective places on three key dates:
- June 21 – When Jupiter appeared to stop moving eastward compared to the stars;
- August 19 – Jupiter’s opposition. Earth is between the sun and the planet; and
- October 18 – When Jupiter seems to stop moving westward compared to the stars.
As seen from high above Earth’s north pole, the planets revolve around the sun in a counter clockwise direction. As seen from our backyards this is to the east.
When retrograding compared to the stars, the outer planets seem to move westward.
Note the three dates on the chart above. On June 21, a line of sight runs from Earth through Jupiter and extends to “A” in the distant starfield.
When Earth moves between the sun and Jupiter, opposition, the extended line points to “B.” From A to B, the line of sight has moved westward, but Jupiter’s movement around the sun is eastward.
On October 18, when retrograde ends, the extended line points to “C.” From opposition to this point, Jupiter appeared to move farther westward compared to the stars, but was moving eastward around the sun.
During the time from A to C, Earth moved nearly one-third of the way around the sun.
Retrograde motion is the illusion from our faster moving planet passing the slower moving outer planets.
Jupiter is at opposition – the middle of the retrograde interval – nearly every 399 days. Saturn is slower, and its opposition is every 378 days. The farther away from the sun, the shorter the interval. Neptune’s interval is nearly every 368 days.
On the other hand, Mars is closer to Earth and it revolves around the sun at less than twice (1.88) the interval of our planet. The Red Planet’s opposition is every 780 days.
Saturn’s retrograde ended over a week ago. It is moving slowly eastward against the distant starfield. Saturn’s eastward speed is slow. Jupiter does not appear to move much during the next several evenings. As it picks up eastward speed compared to the distant starfield, Saturn closes to within 15.36° on October 23. Jupiter picks up speed and moves farther away and catches the Ringed Wonder on November 5, 2040, the next Great Conjunction.
Each year until then, both display the illusion of retrograde motion as our planet passes by.
In addition to Jupiter’s retrograde ending, Spica rises at sunrise this morning, known as its cosmic rising. By month’s end, it is low in the southeastern sky before sunrise.
Morning Sky
The star Arcturus is making its first morning appearance as well as Mercury. Mercury is a binocular object, but brightening quickly, during the next several mornings. Find it about 8° up in the east about 35 minutes before sunrise. At the same time, Arcturus is nearly 6° up in the east-northeast.
Evening Sky
The bright moon, Jupiter, Saturn, and Venus are in the evenings. The moon is nearing its full phase. It is low in the east-southeast as night falls. Bright Jupiter and Saturn are to its upper right.
As night falls, Venus is over 10° above the southwest horizon. It is 2.6° to the upper left of Antares – the rival of Mars – and 2.9° to the upper right of Tau Scorpii (τ Sco on the chart). Venus continues to step through Scorpius.
As the sky darkens further, use a binocular to find the starfield behind Jupiter and Saturn. The moon’s brightness this evening blots out the dimmer stars.
By two hours after sunset, Jupiter and Saturn are low in the southern sky.
Jupiter is in eastern Capricornus. The two brighter stars – Deneb Algedi and Nashira. Saturn is in front of the stars of western Capricornus. It is to the lower right of Upsilon Capricornus (υ Cap on the chart).
Detailed Daily Note: Spica rises within a minute of sunrise this morning. Just one day after its solar conjunction, the star is at its cosmic rising. It makes its first appearance in the morning sky before month’s end. Forty-five minutes after sunset, Venus is over 10° above the southwest horizon, 2.6° to the upper left of Antares, and 2.9° to the upper right of Tau Scorpii (τ Sco, m = 2.8). Bright Jupiter is nearly 28° up in the south-southeast, 15.4° to the left of Saturn (m = 0.6). The bright moon (12.5d, 97%) is over 12° above the east-southeast horizon. Two hours after sunset, the moon is over 25° up in the east-southeast. Jupiter is over 32° up in the south. Its retrograde ended today and it resumes its eastward trek. The planet is 3.8° to the lower right of μ Cap, 2.1° to the upper right of Deneb Algedi, and 1.4° above Nashira. Saturn, nearly 29° up in the south and west of the meridian, is 1.4° to the lower right of υ Cap. Use a binocular to see the starfields with the bright moonlight.
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