2023, October 20: Jupiter’s Double Shadows, Mercury at Superior Conjunction

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.
Photo Caption – 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)


by Jeffrey L. Hunt

Chicago, Illinois:  Sunrise, 7:09 a.m. CDT; Sunset, 6:02 p.m. CDT.  Check local sources for sunrise and sunset times. Times are calculated by the US Naval Observatory’s MICA computer program.

Photo Caption – Jupiter’s volcanically active moon Io casts its shadow on the planet in this dramatic image from NASA’s Juno spacecraft. As with solar eclipses on the Earth, within the dark circle racing across Jupiter’s cloud tops one would witness a full solar eclipse as Io passes in front of the Sun. (NASA Photo)

The sky is very dynamic, especially with fast moving planets and those that rotate quickly.  Jupiter is a spectacular sight through a telescope.  A view through the nighttime shows cloud features that appear on the east side of the planet and disappear several hours later on the west side from its rapid rotation.  The four largest satellites seem to shuttle from one side of Jupiter to the other.  On occasion, they pass in front of the planet and project their shadows on the clouds.

Io revolves around Jupiter nearly every two days, while Callisto’s period is nearly 17 days.

For sky watchers with telescopes, shadows of Jupiter’s moons Io and Ganymede are visible simultaneously on Jupiter’s cloud tops.  Io’s shadow is projected on the clouds beginning at 12:40 a.m. CDT. Ganymede’s shadow appears with Io’s at 12:57 a.m.  As Jupiter rotates and the moons revolve around the planet, the shadow’s move together across the clouds until Ganymede’s shadow leaves at 2:43 a.m.

Io is visible in front of Jupiter from 1:02 a.m. through 3:10 a.m.  Ganymede’s time in front of the planet occurs for about an hour beginning at 2:50 a.m. CDT.

Chart Caption – 2023, October 20: Mercury is at superior conjunction. It is lined up with Earth and sun, on the far side of its orbit.

Similarly, Mercury’s quick year is only eighty-eight earth-days long.  Revolving around the sun at a distance less than 40% Earth’s solar range, the planet overtakes and passes our world every 116 days.  It seems to bounce from the eastern morning sky to the western evening sky and back to the morning again, passing near the sun in the sky on the way.

Today, it passes superior conjunction. Earth, sun, and Mercury are aligned with the sun between the two planets.  The planet then enters the evening sky, reaching its farthest separation from the sun, known as greatest elongation, on December 4th.

The planet makes a favorable appearance for southern hemisphere sky watchers, but from the northern hemisphere, the visibility suffers from a poorly inclined ecliptic – or plane of the solar system.  At greatest elongation, the planet is less than 5° above the southwest horizon at the middle of evening twilight from Chicago’s latitude.

Summaries of Current Sky Events
Summary for Venus as a Morning Star, 2023-24

See this week’s highlights article.

Here is today’s planet forecast:

Morning Sky

Chart Caption – 2023, October 20: Venus is in the east-southeast before sunrise.

Brilliant Venus is in the east-southeast before sunrise. Three days before its greatest separation from the sun, the planet is about 30° above the horizon. While not as quick as Mercury, its changing place compared to the distance stars is easily observed.

The planet is over 10° to the lower left of Regulus, the Lion’s brightest star, and 3.8° to the lower left of Rho Leonis (ρ Leo on the chart).  Use a binocular to see Venus and dimmer Rho.

In three mornings, Venus passes Chertan in a wide conjunction that has a gap of nearly 10°.

Through a telescope, Venus displays a phase 49% illuminated.  When the planet reaches greatest elongation, the phase is 50% illuminated and then grows into a morning gibbous phase.

Chart Caption – 2023, October 20: During morning twilight, Jupiter is in the west nearly between Hamal and Menkar.

After its exhibition of moons and shadows earlier this morning, bright Jupiter is in the western sky, about the same altitude – height above the horizon – as Venus.  The planet is retrograding in front of Aries, 12.2° to the left of the constellation’s brightest star, Hamal.

The planet is retrograding, appearing to move westward against the starfield.  In about a week it moves between Hamal and Menkar, Cetus’ nostril, 11.4° to the planet’s lower left.

Evening Sky

Photo Caption – 2007, December 1: Late winter in the northern hemisphere shows clouds above the northern polar cap and some above the southern cap. (NASA Photo)

Mars is slowly moving toward its solar conjunction on November 18th.  This evening it sets twenty-three minutes after the sun.

Chart Caption – 2023, October 20: After sundown, the crescent moon is in front of Sagittarius.

At forty-five minutes after sundown, the crescent moon, 37% illuminated, is less than 20° above the west-southwest horizon.  It is in front of Sagittarius, commonly known as the Teapot.  In this moonlight, use a binocular to see the pot’s outline.

Chart Caption – 2023, October 20: One hour after sunset, Saturn is in the southeast near Deneb Algedi.

Fifteen minutes later, Saturn is nearly 30° above the southeast horizon.  It continues to retrograde in front of Aquarius, 6.9° to the left of Deneb Algedi, Capricornus’ tail, and nearly 20° above Fomalhaut, the Southern Fish’s mouth.

During the night, Saturn appears farther westward, setting in the west-southwest before Venus rises.

Bright Jupiter is less than 5° above the east-northeast horizon when Saturn is in the southeast.  By midnight, it is over halfway up in the southeast.  Tomorrow morning, it is in the western sky.


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