Tonight – October 30, 2017 – look for the moon and know that Neptune, the outermost of the major planets in our solar system, is near it on the sky’s dome. Just don’t expect to see Neptune. Even on the darkest of moonless nights, you need an optical aid to see it. What you can do tonight is use the moon to get a feel for the whereabouts of Neptune in our sky. Plus, you might be able to glimpse the zodiacal constellation Aquarius in the starry heavens.
Neptune lies within Aquarius’ boundaries now, and Aquarius lies to the north of the bright star Fomalhaut, which will be visible tonight – despite the moonlit glare. The sky chart of Aquarius below shows Fomalhaut as the bright (though unlabeled) star to the south of Aquarius, at a right ascension of 23 hours and a declination of -30o.
The ecliptic – roadway of the sun, moon and planets in our sky – passes right through the constellation Aquarius. Practiced sky gazers know that any planet of our solar system must reside on or near the ecliptic on our sky’s dome.
And right they are, for Neptune will be close to the 4th-magnitude zodiacal star Lambda Aquarii for the rest of this year.
This star, Lambda Aquarii, is visible to the unaided eye as a faint speck of light in a dark country sky. This chart from skyandtelescope.com provides the telescopic field of view and can help you find Neptune … once the moon moves away.
Neptune reigns as the most distant full-fledged planet in the solar system, since Pluto was reclassified as a dwarf planet by the International Astronomical Union (IAU) in 2006.
Pluto wasn’t denied planetary status because it’s not massive enough to be a planet. Believe it or not, Pluto – in and of itself – has sufficient mass to qualify. Rather, Pluto was reclassified by the IAU because this world doesn’t have the heft to “clear the neighborhood around its orbit.”
Pluto, by the way, is locked into a 2:3 orbital resonance with Neptune. For every two times that Pluto circles the sun, Neptune circles the sun three times.
Beyond Neptune, there are a number of trans-Neptunian objects – sometimes called plutinos – that are also locked into a 2:3 orbital resonance with Neptune.
Once the moon leaves the evening sky, starting at the end of the first week of November, try locating Neptune in front of the constellation Aquarius, and near the star Lambda Aquarii. Be forewarned. You’ll need an optical aid, and probably a good sky chart plus lots of patience to see this faint and distant planet.
Bottom line: On October 30, 2017, the moon passes close to Neptune on our sky’s dome.
Bruce McClure has served as lead writer for EarthSky's popular Tonight pages since 2004. He's a sundial aficionado, whose love for the heavens has taken him to Lake Titicaca in Bolivia and sailing in the North Atlantic, where he earned his celestial navigation certificate through the School of Ocean Sailing and Navigation. He also writes and hosts public astronomy programs and planetarium programs in and around his home in upstate New York.