Perhaps you’re familiar with Neptune, the Roman god of the sea. Or maybe you know him by his Greek moniker, Poseidon. Despite the ancient mythos, the planet that bears the name as a result of its cerulean color — evoking the image of a great ocean world — was only discovered in 1846. Before that scientists thought Neptune wasn’t a planet at all, but a star.
It is still a “star” in the sense that it is famous in our night skies, however — but not for the reason you might think. Neptune is not showy like Jupiter or Venus. Rather, it is the farthest known planet from the Sun, so its fame stems from its sheer elusiveness.
This godly planet is an ice giant, meaning that it has a small rocky core enveloped by a mass of hot, dense water, methane, and ammonia. It’s not every day that a god-tier planet enters a celestial event, so mark your calendars to catch a glimpse of mighty Neptune while you can.
Neptune “in opposition”
When a planet is “in opposition,” that means it’s at its closest point to the Earth. In fact, it’s in alignment with the Earth and Sun, with the Earth smack in the middle of both — only planets more distant from the Sun than Earth can be in opposition.
The planet’s position in orbit closest to the Earth is called its perigee, while its position farthest from Earth is called its apogee.
For Neptune, this is a special moment. Neptune is our Solar System’s furthest planet from the Sun. When it reaches opposition, it will be the closest it may ever come to us Earthlings — only a mere 2.7 billion miles away.
Mars, Jupiter, Saturn, and Uranus are the other planets that can be in opposition. Opposition is an annual event for Jupiter, Saturn, Uranus, and Neptune. Mars only enters opposition about once every two years, however.
When will Neptune be in opposition?
Neptune will be in opposition on Tuesday, September 14 at 5:12 a.m. Eastern.
Fittingly, it will be in front of the constellation Aquarius the Water Carrier.
The planet reaches its highest point in the sky around midnight Eastern (or in any time zone).
Neptune’s opposition occurs annually, but every year the exact date shifts forward by several days in part because of how large the planet’s orbit is in comparison to how fast the Earth flies around the Sun.
One year on Neptune is equivalent to 165 Earth years, although a day is short — only 16 Earth hours. The Earth realigns with Neptune every year, but by that time Neptune has moved so little in its own orbit that the opposition date shifts forward only a little bit. Next year, Neptune’s opposition will be September 16.
Technically, Earth and Neptune will be at their closest point to each other 20 hours before the planet is in opposition, but that’s because, in opposition, the perfect alignment with the Sun and the Earth occurs.
There’s still one more planet set to enter opposition later this year, and that’s Uranus on November 4 at 7:49 p.m. Eastern
How to see Neptune in opposition
Usually, the celestial orbs that grace our night skies are at their biggest and brightest in opposition, but Neptune is far away that it will still be hard to pick out with the naked eye.
Neptune is the only major planet in our solar system that isn’t visible to the naked eye. Compared to the dimmest visible star on a completely dark night, Neptune is about five times fainter.
The Moon on September 14 will be a waxing gibbous, so there may be some light interference — although, after midnight, moonlight interference should become less of a problem.
To see Neptune, you will need either a low-powered telescope or binoculars — plus a sky chart.
There are a few points in the sky that can help you triangulate Neptune’s position. The planet will be in front of the constellation Aquarius, so it will be near the star Phi Aquarii. While this star is dim, it’s still visible to the unaided eye — but beware: Neptune is about 30 times dimmer than Phi Aquarii. To help you find the elusive planet, the star and planet will appear fairly close and will fit in the same binocular field. Neptune will also buddy up with nearby star HR 8924, which could help you spot it, too.
A small telescope with 100x magnification and direction from a sky chart will help you pinpoint what will appear as a disc-shaped body with a bluish hue. The famous blue color comes from all the methane in its atmosphere, which absorbs red light and reflects blue light.
Neptune does have five rings — but unfortunately, those won’t be visible. Neptune also has 14 moons, but even its largest — Triton — can’t be captured using small, amateur telescopes.
When Neptune is visible at night
Neptune won’t get any closer to the Earth than this night, but it does rear its faint, icy head on other nights. Any time when the constellations Pisces and Aquarius are in the night sky, Neptune tags along.
Between June and November, Neptune is visible between evening and dawn, rising and setting earlier and earlier in the night as the year goes on before fading entirely from view in the winter.
Generally, you have the best odds of actually glimpsing Neptune after midnight. Take your binoculars or telescope to a remote area with little light pollution and few sky obstructions such as tall buildings and trees. You can also try going up to a roof terrace or balcony, so long as it is safe to do so. Good luck!