Neptune, the outermost planet in our Solar System, is far too faint to be seen with the unaided eye, despite its ice giant status. However, with a magnitude of 7.8, it is not difficult to find Neptune with binoculars providing you know where to look and have clear, dark skies.
Neptune currently (2017) lies in the constellation of Aquarius. It will remain in this same general area of the sky for the next few years because it takes 164 years to make a single trip around the Sun.
It becomes visible in the evening sky in the summer months and is currently best seen in the autumn (fall) and winter months. Those observing in the pre-dawn hours will be able to see it from the spring.
Our first chart shows a wide-angle view of this general area of sky, with a streak marking the track of Neptune over time. The second chart shows the planet’s track in Aquarius more clearly.
Aquarius lies south of the celestial equator, so Neptune will be higher in the sky and so easier to spot for observers in the southern hemisphere. If you are a southern observer, remember that these charts have north at the top, and will need to be inverted to match your own view of the stars.
What can I see of Neptune?
Because Neptune lies nearly 5 billion miles from us, it shows only a tiny disk, half the diameter of that displayed by Uranus. You will see the planet as nothing more than a speck of light with binoculars.
You will need a telescope with a primary lens or mirror at least 100mm, or 4 inches, in diameter to see a hint of its disk, which is just over 2 arc seconds wide. A telescope twice that size will make things easier. Don’t expect to see any detail with a small or standard telescope. But you should be able to note Neptune’s bluish tinge.
You can also enjoy watching, over the weeks and months, how the planet gradually moves against the starry background. If you do this regularly, you will note how Neptune like the other outer planets, changes direction for a time every year. This so-called retrograde motion is due to the Earth overtaking Neptune as we make our own journey around the Sun.
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