Want to see Neptune at its brightest? Good news: we’ve got a chance to see the elusive blue beauty through binoculars tonight, as the planet comes into opposition. This means the planet will be well-lit by the Sun.
Neptune only comes into opposition once a year, on almost the same date – give or take a few days.
Following hot on the heels of Saturn in opposition on 27 August, Neptune will go into opposition on 19 September 2023, but will appear close to opposition tonight. Following Neptune, we’ve then got another two oppositions to look forward to before the year is out: Jupiter on 3 November, and Uranus on 13 November.
But what exactly does it mean when we say a planet is at opposition? Will you be able to spot Neptune in opposition? And, which constellation is Neptune in at the moment?
The best time to see Neptune tonight
Neptune will reach opposition at 11:17am on Tuesday 19 September 2023. As this is during daylight hours, the best time to see Neptune will be the evening of 18 or 19 September. Neptune rises at 7:13pm on 19 September and is up almost all night.
You’ll need good binoculars or a telescope to see it, but the evenings around the date of opposition are the best time to try and view this ice giant.
Being a faint planet to begin with, we won’t see too much of a change in brightness for Neptune. Even at opposition, it will be a faint magnitude of 7.8 – that basically means you’ll need around 100x magnification to see the blue colour of Neptune, and around 200x magnification to discern Neptune as a disc. For even more detail, you’re looking at around 700x magnification.
With the high magnification needed to see Neptune, you’ll want to steady your binoculars with a tripod, or opt for a telescope.
Neptune is, unfortunately, the only planet in the Solar System (excluding dwarf planets, of course) that is not visible to the naked eye, and it’s the most distant planet from the Sun. It’s more than 30 times as far away from the Sun as Earth is, and 1.01 billion miles further out than Uranus. That’s a long way.
How can I spot Neptune in the night sky?
When Neptune is in opposition, it’s opposite the Sun in the sky. The easiest way to determine where Neptune is, is to physically look in the opposite direction from where the Sun sets.
Neptune will rise in the east and will move further toward the south as it rises higher in the sky. It’s trailing behind Saturn, currently in Aquarius, and the blue giant is above the horizon all night.
Neptune is currently located around 7.7 degrees below the Circlet asterism in the Pisces constellation, which makes up the head of one of the two fish.
Through binoculars, you’ll see a pinpoint of light, a small blue-green dot. Make sure your binoculars are steadily supported, and be careful not to touch them (or the tripod) once you’ve brought Neptune into your field of vision... or it will quickly bounce out of view.
It’s a tricky one to spot, but by first locating the Circlet in Pisces, and with the help of astronomy apps you should be able to pick it out.
What is opposition?
In astronomy, opposition is when two objects are in opposite directions in the sky, when viewed from Earth. On 19 September 2023, the two objects are Neptune and the Sun. During opposition, the sunlight that shines on Neptune is fully reflected back to us, and we can see the whole of its planetary disk (with the right magnification).
When the other planets in the Solar System reach opposition, they appear brighter in the sky – and a fantastic target for backyard astronomers. Neptune however, is so faint anyway that opposition doesn’t make a huge amount of difference. But if the skies are clear, it’s as good a night as any – and opposition might make the difference between spotting it and not. So why not give it a go?
The inner planets, Mercury and Venus, do not reach opposition – because the Earth will never be situated between either planet, and the Sun. And for the superior planets (planets with an orbit larger than Earth’s), the effect that opposition has on the brightness of the planet decreases with distance. So for the furthest planet, Neptune, the effects of opposition are minimal.
When are the planets next in opposition?
- Mercury: Never
- Venus: Never
- Mars: 15 January 2025
- Jupiter: 3 November 2023
- Saturn: 7 September 2024
- Uranus: 13 November 2023
- Neptune: 19 September 2023
- The Moon: 29 September 2023
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