2023, August 18: Jupiter’s New Fifth Bright Satellite?


August 18, 2023: Through a telescope, Jupiter appears to have acquired a new Galilean satellite.

Photo caption – 2019, November 25: Venus is one day past its conjunction with Jupiter.


by Jeffrey L. Hunt Chicago, Illinois: 

Sunrise, 6:02 a.m. CDT; Sunset, 7:47 p.m. CDT.  Check local sources for sunrise and sunset times. Times are calculated by the US Naval Observatory’s MICA computer program.

Here is today’s planet forecast:

Morning Sky

Chart Caption – 2023, August 18: Jupiter is in the southeast during morning twilight.

An hour before sunrise, bright Jupiter is high in the southeastern sky.  It is slowly moving eastward in front of Aries, 13.3° to the lower left of Hamal, the pattern’s brightest star, and 11.3° to the upper left of Menkar, in Cetus.

Notice the Pleiades to Jupiter’s left and Aldebaran, with the Hyades star cluster, to the planet’s lower left.

Chart Caption – Through a telescope the star Sigma Arietis (σ Ari) appears to take the role of a new Galilean satellite.

Through a spotting scope or telescope, notice that Jupiter appears to have added a fifth bright satellite.  This morning, an object slightly brighter than the other moons, appears to the east of the Jovian Giant, in the same orbital plane as Callisto, Europa, and Ganymede.  In the Americas during morning twilight, the moon Io is behind the planet.  The planet’s four largest and brightest moons were first observed by Galileo in 1610 and aptly named the Galilean satellites.

This apparently bright, new Galilean satellite is a distant star that happens to appear in the same line of sight as other moons. The star, Sigma Arietis (σ Ari on the chart) is nearly 500 light years away, compared to Jupiter’s 435-million-mile distance.  During the next week, Jupiter slowly moves past the distant star, so, in this brief encounter, the “new moon” seems to revolve toward the west.

Robert C. Victor, retired staff astronomer at Abrams Planetarium, Michigan State University, and frequent collaborator on the Venus rising and setting charts that appear here, first noted this line-of-sight event in his August astronomy column.

At approximately 80x magnification through a telescope, Jupiter’s cloud bands are visible, along with the Galilean satellites.  The clouds appear in lines parallel to the equator, whipped in that fashion by the planet’s rapid rotation.  Depending on Jupiter’s location in the sky, the bands can be tilted.

Photo Caption – This Hubble Space Telescope view of Jupiter, taken on June 27, 2019, reveals the giant planet’s trademark Great Red Spot, and a more intense color palette in the clouds swirling in Jupiter’s turbulent atmosphere than seen in previous years. The colors, and their changes, provide important clues to ongoing processes in Jupiter’s atmosphere.

Through the telescopic eyepiece, the moons’ orbital paths are lined up with the equator, making it reasonably easy to identify them from distant stars that appear in the field of view regardless of the angle we see the planet in the sky.  Because the orbital planes are along our line of sight, we see the moons seem to shuttle back and forth during several nights, centered on Jupiter. Simply draw an imaginary line along the planet’s equator and extend it to both sides of the planet.  Any dimmer starlike body along that line is likely one of the brighter moons.

Callisto, Jupiter’s outer-most Galilean satellite, revolves around the planet in about seventeen days, while Io, the inner-most bright moon, motors around the planet about every two days.

During the next week Sigma Arietis appears, while much farther away, along Jupiter’s equatorial plane, giving the illusion of a fifth bright Galilean satellite.

Jupiter begins to retrograde on September 4th

When Jupiter appears to move past the star again, September 11th through September 22nd, the star comes into view again through the telescopic eyepiece, but it is clearly not aligned with the equatorial plane.  Jupiter passes 0.1° south of the star on the 18th, so the illusion does not work the second time. When Jupiter resumes its eastward trek during 2024, it passes the star again March 21st, 0.5° to the north.  The pair tightly fits into that 80x magnification eyepiece, without the Galilean satellite illusion that occurs beginning tonight.

Chart Caption – 2023, August 18: Saturn is in the southwest, with Skat and Lambda Aquarii (λ Aqr) before sunrise.

This morning during twilight, Saturn is in the southwestern sky.  Its retrograde continues. Find the Ringed Wonder less than 20° up in the southwest.  From regions that suffer from the perpetual glow of outdoor lighting, Aquarius’ stars are likely washed out by that veil.  With a binocular find Saturn, 7.9° to the right of Skat, the Aquarian’s leg, and 7.5° to the lower right of Lambda Aquarii (λ Aqr on the chart).  The trio nearly makes an equilateral triangle. After inferior conjunction, about a week ago,

2020, June 16: Venus is low in the east-northeast during morning twilight. Use a binocular to see its tiny crescent phase.

Venus is racing into the morning sky.  This morning it rises nineteen minutes before the sun and appears over 3° above the eastern horizon.  Anxious sky watchers with exceptional views of the eastern horizon can attempt their first view with a binocular this morning. For others, the first easy view occurs on the 22nd.  Venus gains six to seven minutes of rising time each morning and by then, it rises forty-nine minutes before daybreak.

Evening Sky

Chart Caption – 2023, August 18: Through a binocular, Mercury, Mars, and the crescent moon are a challenging view thirty minutes after sunset.

Mercury retreats into bright twilight, setting forty-one minutes after the sun.  Thirty minutes after nightfall, the speedy planet is about 2° above the horizon, a difficult location to find it.  Attempt to see the crescent moon, 6% illuminated, with a binocular.  Mercury is 6.1° below the moon.  This twilight is bright.  Find a clear horizon and attempt this view. 

Chart Caption – 2023, August 18: Through a binocular, can you see Mars near the crescent moon, forty minutes after sunset.

If the sky is exceptionally clear, Mars is 1.1° to the lower left of the crescent, for those sky watchers already missing it.  If the Red Planet is not visible, wait another ten minutes for another attempt.  It is fainter than might be expected. Try again at five-minute intervals, until the planet sets seventy-two minutes after sunset.  The sky grows darker, but Mars and the moon are lower at each interval.

When the moon comes through next month, Mars sets about forty-five minutes after sunset.  This evening might be the last opportunity to see the Red Planet with the moon acting as the celestial marker to find it.

Chart Caption – 2023, August 18: Saturn is in the east-southeast, two hours after sunset.

As opposition nears, Saturn rises twenty-two minutes after sunset this evening.  This countdown continues until the night of the 26th/27th, when Earth passes between the Ringed Wonder and the sun.  Then each evening, the planet is higher in the sky as night falls.

By two hours after sundown, the planet is over 15° above the east-southeast horizon.  Look for Lambda Aquarii, 7.6° to Saturn’s lower left.  Skat is not high enough to see easily. 

By tomorrow morning Saturn and the three stars are in the southwestern sky again.

Jupiter, rising before midnight in most areas, appears above the eastern horizon less than three hours after Saturn rises.  By tomorrow morning, the Jovian Giant is in the southeastern sky.  Take a look to where that “extra” Galilean satellite appears.



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