Once you’ve found the brightest five planets – Mercury, Venus, Mars, Jupiter, and Saturn, and perhaps even taken on the challenge of Uranus, the last and furthest planet in the solar system, Neptune, awaits.
Neptune is such a tricky beast because it is the only solar system planet that is beyond naked-eye visibility from Earth. It is only slightly smaller than Uranus, but is 50% further away and never appears brighter than magnitude 7.6.
Thankfully, magnitude 7.6 is not incredibly faint so it’s not too tricky to spy this cold, giant planet, so long as you know when and where to look.
Finding Neptune in the Night Sky
Named for the Roman god of the sea, Neptune takes a leisurely 165 years to orbit the sun once. Its cloud tops are famed for being one of the coldest temperatures in the solar system at just 55K. In everyday language, that’s -218°C / -361°F.
Is Planet Neptune Visible Tonight in 2021?
Since Neptune takes so long to orbit the sun, it appears to move very slowly against the background stars. For us, this means we’ll find it in Aquarius throughout 2021, although it comes very close to the border of Pisces in the summer.
- Shines at its brightest: late July to early November
- Magnitude Range: +7.8 to +8.0 (dimmest)
- Visible in the evening sky: January, October – December
- Not visible: February – May
- Visible in the morning sky: June and July
- Visible all night: August and September
- Opposition Date – September 14, 2021
At the start of 2021, Neptune is best viewed in the evening after true darkness takes hold. This is a short window of opportunity because Neptune sets before 9pm at the end of January.
The planet moves too close to the Sun to be usefully spied from February to May, before reappearing in the predawn darkness of June’s sky. It stays in the morning sky, gradually getting higher during hours of darkness, into July and August.
This ice giant is so distant that its visibility changes very little, even as it reaches its closest approach to us and opposition on 14 September. Its disc size and brightness increase a fraction, but not enough to be noticeable at the telescope.
A few weeks on either side of its opposition date, the planet is visible all night long. But, as 2021 progresses, Neptune becomes an evening planet visible after dusk gives way to night and setting around midnight at the beginning of November. By year’s end, Neptune will set before 10pm.
Where to Find Planet Neptune
As we said earlier, Neptune doesn’t move against the background of stars very much in a year, so it can always be found in the same small patch of sky in 2021.
The image below, from SkySafari 6, shows where it is at opposition on 14 September this year. The stars are shown to magnitude 5.0 and you can see Neptune midway between the horizon and zenith at 1am. The planet is nestled between the constellations of Pisces and Aquarius.
However, you don’t need to look for Neptune on just this one night. Whenever Aquarius and Pisces are in the night sky, Neptune will be in approximately the same place.
To find it locate the stars Phi Aquarii and Hydor in Aquarius. These are relatively bright stars, shining at magnitude 4.2 and 3.8, respectively.
Use an imaginary line to join Hydor to Phi Aquarii, this points to the patch of sky that Neptune spends all of 2021 inside, as shown in the image below.
What Does Neptune Look Like Through a Telescope?
Neptune’s apparent size is a tiny 2.4 arcseconds at its closest approach to us. Even with big magnification in a decent-sized scope, the best you’ll see is a recognizable disc (rather than a pinprick of light, like a star) with a bluish hue.
Use even a small telescope at a magnification of 100x or to see color which is caused by methane in Neptune’s atmosphere absorbing red light and reflecting blue.
Swap up your magnification to 200x and your telescope will reveal that this is not a point of light like a star, but a disc. This is your first confirmation that you are looking at a planet.
Sadly, there are no surface features apparent with small telescopes, but just seeing the planet is a bigger achievement than many backyard astronomers can claim.
Can You Spot Neptune’s Moons?
Neptune has a moon called Triton, which you have probably heard of. Triton is slightly odd because it is the only moon in the solar system which orbits against the direction of the planet’s spin.
Triton is also a big satellite, the seventh-largest in the solar system. However, it is such a distance from us that it shines below magnitude 13 and is effectively out of reach of average amateur telescopes.
There are 13 other moons known around Neptune, but they are all considerably smaller than Triton.
More on Neptune…
If you have a desire to learn more about Neptune, check out our article Neptune: All you need to know for fascinating information about the planet.
Written By Sharmila Kuthunur