For the last few nights there has been a brilliant light that drew attention low in the eastern sky around 7:30pm local daylight time. It gleams with an even, silvery glow, and a few hours later, having climbed noticeably higher in the east-southeast part of the sky, it appears to be attracting attention.
That would make sense, of course, since you’re looking at the object named after the king of the gods and also the king of the planets: Mighty Jupiter.
And this is a pretty promising week, as we’ll be seeing Jupiter as big and as bright as possible from our Earth vantage point as it nears perihelion: the point in its 12-year orbit that brings it closest to the Sun.
Related: Jupiter has been closest to Earth for 59 years, NASA says
Jupiter now appears 11% larger and more than one and a half times brighter than it did in April 2017 when it was near the aphelion (the point of its orbit farthest from the Sun). Even 7x binoculars held steady show Jupiter as a tiny disk. A small telescope will fare much better, while in larger instruments Jupiter will be resolved into a range of reds, yellows, tans and tans, as well as a wealth of other telescopic detail. Amateur astronomers have been photographing this large planet all summer as it nears Earth. On Monday (September 26) the opposition occurs when it will be in the sky all night from sunset to sunrise.
On Sunday (September 25) at 10 p.m. Eastern Time, Jupiter will make its closest approach to Earth since 1963. It will then be 367,413,405 miles (591,168,168 km) away. That may not seem exactly “close,” but Jupiter is so large and bright that not only is it easily visible to the unaided eye, but it appears as large as the moon to the unaided eye through a small telescope at only 36x magnification.
A giant among giants
With a width of 88,846 miles (142,984 km), Jupiter has a diameter almost 11 times that of Earth. A journey around the sun takes almost 12 years. But if Jupiter’s year is long, its day is short. The big planet rotates once in just under 10 hours. For a planet of this size, this rotation speed is amazing. A point on Jupiter’s equator moves at a speed of 22,000 mph compared to 1,000 mph for a point on Earth’s equator. This rapid rate of rotation gives Jupiter the appearance of a slightly flattened sphere. It has a rocky core surrounded by a thick mantle of metallic hydrogen enveloped by a massive atmospheric mantle of multicolored clouds of ammonium hydrosulfide.
Jupiter is a gigantic giant of a planet, with a mass more than twice that of all seven other planets combined. It has one of the most mysterious blemishes on a planet’s surface: the Great Red Spot, which comes and goes unpredictably and is as wide as Earth. There is also evidence that Jupiter loses more thermal energy to radiation than it receives from the Sun, and may therefore be producing its own energy – an activity more typically characteristic of a star than a planet.
Jupiter also has a faint ring system, although unlike Saturn’s famous rings, which are highly reflective because they are made of ice, Jupiter’s rings are mostly made up of countless tiny dust particles.
And like Earth, Jupiter has a magnetic field; a huge annular belt of electrically charged particles surrounding the planet – a ring similar to the Van Allen belts of charged solar particles trapped by Earth’s magnetic field.
The Dance of the Moons
Let’s not forget the four large moons of Jupiter discovered by Galileo 412 years ago. You are a telescopic treasure. The four – Io, Europa, Ganymede and Callisto – engage in a merry race, orbiting Jupiter so fast (1.68 days for Io to 16.7 days for Callisto) that they change their appearance hour by hour and night by night Night changes and casts its shadows on the planet, disappearing behind its huge disc or plunging into its shadow.
For example, on Sunday (September 25) you can see three moons on one side of Jupiter (Io, Europa, and Callisto) with the fourth moon (Ganymede) all alone on the other side. On Monday (September 26th) Ganymede will be joined by Europa and Io; now it is Callisto who will be all alone on the other side of Jupiter. Finally, on Tuesday (September 27) you will see two moons on one side (Europa and Ganymede) and two (Io and Callisto) on the other side.
Indeed, on Wednesday (September 28) at 00:08 EDT (September 27 21:08 Pacific Time) Ganymede will appear to pass in front of Jupiter in what is called a transit. In addition to “The Big Four”, Jupiter has 76 other satellites. Many of them are exceptionally small and were discovered courtesy of spacecraft that flew near Jupiter in the decades of the 1970s, 80s and 90s.
Orbiting Jupiter?
When Jupiter appears high in the sky, it might appear to some as moving in a circle or spiral. Over the years, I’ve received emails from people who claim to have seen Jupiter do just that: move back and forth.
Why does it seem to move? Probably those who saw this strange movement experienced the autokinetic effect. This is a human visual perception phenomenon in which a stationary, small point of light appears to be moving in an otherwise dark or featureless environment. Many UFO sightings have also been attributed to the action of the autokinetic effect on stars or planets. Psychologists attribute the perception of motion where there is none to “small, involuntary movements of the eyeball.” The autokinetic effect can also be amplified by the power of suggestion: if one person reports a light moving, others will be more likely to report it.
Currently, Jupiter shines in the constellation of Pisces, a star pattern composed mostly of faint stars. Under a clear, dark sky with no nearby moon, Jupiter appears to glow with little if any competition from other nearby stars. If a person keeps staring at Jupiter for a period of perhaps 15 to 30 seconds, it’s quite possible that the autokinetic effect will kick in, causing Jupiter to spin or perhaps make a small circle.
During the late evening hours this week try staring at Jupiter and see if it moves for you.
If you want a great view of Jupiter opposite, don’t miss our guides to the best binoculars and the best telescopes to spot Jupiter or other objects in the night sky. To capture the best Jupiter images, read our recommendations for the best cameras for astrophotography and the best lenses for astrophotography.
Editor’s note: If you take a picture of Jupiter and would like to share it with Space.com readers, send your photos, comments, and your name and location to [email protected].
Joe Rao serves as an instructor and visiting professor at New York University Hayden Planetarium (opens in new tab). He writes on astronomy for Journal of Natural History (opens in new tab)the Peasant Almanac (opens in new tab) and other publications. Follow us on Twitter @spacedotcom (opens in new tab) and further Facebook (opens in new tab).
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