Conjunctions this close are rare, but give humanity an opportunity to view our Solar System’s faintest planet.
With eight planets orbiting our Sun, any two will eventually appear close together from our perspective.
The eight planets of our Solar System and our Sun, to scale in size but not in terms of orbital distances. Note that these are the only eight objects that meet all three of the planetary criteria as set forth by the IAU, and that they orbit around the Sun within just a few degrees of the same plane as one another. (WIKIMEDIA COMMONS USER WP)
All the planets revolve in nearly the same plane — the ecliptic — with each one possessing its own unique speed.
The orbits of the planets in the inner solar system aren’t perfectly circular, but are very close to orbiting in the same plane as one another, with the outer Solar System planets also occupying the same approximate plane: the ecliptic. When two planets appear to approximately line up with Earth’s line-of-sight, a conjunction occurs. (NASA / JPL)
When two planets closely approach one another, we perceive a conjunction: a common but beautiful event.
In 2013, three planets (Venus, Jupiter, and Mercury, from L to R) experienced a mutual conjunction in the night sky. Conjunctions, defined as the closest approach of two planets as seen from Earth without them occulting one another, are relatively uncommon but spectacular astronomical events. When a conjunction occurs with a non-naked eye planet, such as Neptune, it can present an ideal opportunity for spotting and identifying it. (STAN HONDA/AFP via Getty Images)
Multiple naked-eye planets in conjunction are visually spectacular, but Neptunian conjunctions can be even more special.
In April of 1990, the Voyager 2 spacecraft flew by Neptune, snapping a series of incredible images of our Solar System’s outermost planet. 150 years prior, nobody knew that our Solar System would wind up containing 8 planets, but a few scientists suspected, from the evidence of Uranus, that it might be out there. (TIME LIFE PICTURES/NASA/THE LIFE PICTURE COLLECTION/GETTY IMAGES)
Neptune is always too faint to be seen with the unaided eye, so conjunctions provide the best opportunities for spotting it.
The planet Neptune and its largest moon Triton, as photographed by the Voyager 2 space probe in August 1989. Although it requires a very strong telescope to be able to see Neptune’s largest moon, Triton, Neptune itself can be seen with an off-the-shelf pair of binoculars, if you know where to look. With 1846-level technology, discovering its presence was easy and unambiguous, once its location was known. Today, when conjunctions with other, easily visible objects occur, conditions are ideal for spotting Neptune again. (NASA / VOYAGER 2)
Either telescopes or simple binoculars can magnify Neptune sufficiently to appear as an unmistakable blue disk.
The seven extraterrestrial planets of the solar system: Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune. Photographed in 2019 with a Maksutov telescope from Mannheim and Stockach in Germany. The angular sizes and colors shown are accurate, but the brightnesses are not: Venus is some 63,000 times brighter than Neptune, or 12 astronomical magnitudes; the same difference as between the full Moon and a typical bright star like Vega or Capella. (GETTY)
January 27, 2020’s conjunction with Venus provides the perfect opportunity for viewing our outermost planet: Neptune.
In the southwestern portion of the skies after sunset on January 27, 2020, the planet Venus will be clearly visible near a bright but thin crescent Moon. Venus, the brightest planet in the sky, will have the planet Neptune pass within just 0.04 degrees of it at closest approach: at 8:12 PM Central European Time (2:12 PM Eastern Time). The view will still be very good after sundown from North and South America. (E. SIEGEL / STELLARIUM)
After sunset, brilliant Venus outshines all other stars and planets in our western skies.
It only happens once every 11 years, but occasionally, all five naked-eye planets are visible at once. Mercury is always the toughest to spot due to its proximity to the Sun, but sometimes Mars appears even smaller in angular diameter than Mercury. Venus is always the brightest planet, followed by Jupiter, and then usually followed by Mars and then either Mercury or Saturn. Neptune is always too faint to be seen with the naked eye, but conjunctions provide incredible opportunities to ‘discover’ it for yourself. (MARTIN DOLAN)
At 8:12 PM CET (2:12 PM ET),
Venus and Neptune will pass within 0.04° of one another.
Although Neptune and Venus will have an extremely close conjunction at one particular moment in time on January 27, 2020, they will remain close by one another for the entirety of the night of the 27th, for as long as Venus is above the horizon everywhere in the world. (© DOMINIC FORD 2011–2020 / IN-THE-SKY.ORG)
If you can find Venus through binoculars or a telescope, Neptune will appear as a static, clearly blue disk.
For a North American observer viewing Venus through either a telescope or a pair of binoculars at 9:00 PM ET (6:00 PM PT), this will be the approximate view centered on bright Venus. There will be an orange-colored object on one side of Venus (the star Phi Aquarii), while Neptune will be a faint blue disk (or dot, depending on your magnification) on the opposite side. (E. SIEGEL / STELLARIUM)
Although all the surrounding stars will twinkle, Neptune won’t, your surefire planet-hunting signature.
At closest approach, best viewed from Europe, Africa and Western Asia, Neptune and Venus will pass within just 0.04 degrees of one another, making it appear as though Venus has a faint blue ‘moon’ that doesn’t twinkle through a telescope. That’s no Moon, however, that’s the much larger planet Neptune, at a distance some 30+ times as far away as Venus. (E. SIEGEL / STELLARIUM)
The next convenient
Neptunian conjunctions won’t occur until April 2022, more than two years away.
Neptune may be the fourth largest planet in the Solar System and the coldest, but its primary atmospheric composition is hydrogen and helium, which make up more than 95% of Neptune’s gaseous outer layers. Mixed into that atmosphere is a small amount of methane, about 3%, which is the primary agent responsible for Neptune’s blue color. (Hulton Archive/Getty Images)
Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more. Ethan Siegel is the author of
Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.