National Geographic News
Starting Wednesday morning, sky-watchers will be able to use Jupiter as a guiding light for a few days to help them catch a glimpse of the most distant planet in the solar system.
Never less than 2.68 billion miles (4.3 billion kilometers) from Earth, the gas giant Neptune is too far-flung to be visible to the naked eye.
Spotting the planet can be tricky even for observers using binoculars or backyard telescopes, because it appears as a small bluish dot in the sea of stars.
But as of Wednesday morning, viewers worldwide can see Neptune and Jupiter in conjunction, or seemingly close together in the night sky.
"Jupiter makes a great signpost for casual stargazers to find Neptune," noted astronomer Mark Hammergren of the Adler Planetarium in Chicago, Illinois.
Jupiter will be the brightest "star" in the night sky, a sight Hammergren describes as unmistakable.
Viewers using at least a small telescope or strong binoculars can then search just north of Jupiter for a tiny bluish "star," which Hammergren says will be fainter than Jupiter but still easily visible.
The two gas giants will appear closest to each other by about 2 a.m. local time on Wednesday, but they will stay relatively near for several more days.
The May conjunction will be the first of three this year, with the next two celestial meet-ups starting July 9 and December 21.
Oddly, a Neptune-Jupiter conjunction nearly 400 years ago almost allowed Italian astronomer Galileo Galilei to "pre-discover" Neptune, which was not confirmed as a planet until 1846, Hammergren said.
In 1612, shortly after the invention of the telescope, Galileo famously had his instrument trained on Jupiter and its moons.
"Galileo drew Jupiter along with some of its background stars—and one of those stars was actually Neptune," Hammergren said.
Galileo saw the "star" again near Jupiter in 1613 and noted in his logs that it appeared to be slightly farther away from another previously recorded star.
Stars are usually fixed in the sky relative to each other, while planets seem to wander along their own paths.
"If only he had followed up a little more closely!" Hammergren said.
"If he had, it would have been fantastic, because Neptune would have been discovered centuries earlier."
The Real Thing
Today many people are familiar with what are known as the naked-eye planets—Mercury, Venus, Mars, Jupiter, and Saturn are all bright enough to be visible without telescopes or binoculars.
Still, seeing a planet through the lens of a telescope can be a unique experience.
"A common reaction is amazed exclamations," Hammergren said, recalling people who have seen planets for the first time using a telescope at Adler's Doane Observatory.
"Some people look and then say, You put a photograph up there, that's just a slide!
"And I tell them, No, no I swear! It's the real thing."
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