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 never appears brighter than magnitude 7.6.
Thankfully, this is not incredibly faint so, when you know where to look, it’s not too tricky to spy this cold, giant planet.
Finding Neptune in the Night Sky
Named for the Roman god of the sea, Neptune takes a leisurely 165 years to orbits 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.
Neptune Visibility in 2019
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 2019.
- Shines at its brightest from mid-July – November
- Magnitude – +7.8
- Morning sky (an hour before sunrise) – June to September
- All Night – September – October
- Evening sky – November and December
- Opposition Date – September 10, 2019
Is Neptune Visible in the Night Sky Tonight?
To find Neptune, you need to locate the constellation of Aquarius. More specifically, you need the star Phi Aquarii, which is a relatively bright star around magnitude 4.2. Neptune is within close vicinity of this star throughout 2019 and practically next to it at opposition in September.
To find Phi Aquarii, start with the Great Square of Pegasus – the large square of stars that shine with almost equal brightness.
To find the Great Square, begin with the ‘W’ shape of Cassiopeia, one of the constellations that are visible all night long. Draw an imaginary line from Polaris (the Pole Star) to the star Caph in Cassiopeia (see diagram below). Next, double the length of the line and it will point to Alpheratz, which marks one corner of the Great Square.
Now you have Alpheratz, we’ll use two other stars forming the Great Square, Markab and Scheat to get you to Neptune.
Set off from Sheat in the direction of Markab. Now, extend that line a further 1.5 times and it will end at Phi Aquarii with Neptune nearby.
When you’re in the right spot, it’s time to point your telescope and glimpse the blue-green disc.
Finding Neptune With a Small Telescope
For the final details in tracking down Neptune when you’ve found Phi Aquarii, we recommend downloading some astronomy software Stellarium (which is free) or Sky Safari, which is paid. Both of these let you put in your observation time and date to see exactly where to see the planet.
Up until late summer, Neptune is best viewed in the early morning before dawn breaks. Over the course of the season, the planet rises earlier and earlier and is higher in the sky by dawn.
Neptune reaches opposition in September this year, when it will be visible all night long because it is directly opposite the sun from our perspective. It is highest in the sky around midnight, when it will be midway between the zenith (overhead) and southern horizon.
As fall deepens at the end of 2019, Neptune can be found lovely and high above the southwest horizon in the late evening.
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 that 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 that many backyard astronomers can claim.
Can You Spot Neptune’s Moons?
Neptune has a moon called Triton, which you have probably heard of. It’s 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, in fact. 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.
Questions About Neptune
Neptune is a cold, blue, and incredibly distant world which naturally makes us astronomers curious. In this FAQ section, we answer some of the most common questions about the planet.
How Many Moons Does Neptune Have?
Neptune has 14 known and named moons altogether. The biggest – by some margin – is Triton. They are all names after water deities of Greek mythology, including Nereid, Proteus and Larissa.
The brightest is Triton, but you’ll need a very serious telescope to see it from Earth.
How Far Away is Neptune?
Neptune orbits the sun at a distance of approximately 30AU (astronomical units). Since the orbit of Earth defines an astronomical unit (93 million miles, approximately) we can see that Neptune is 30 times further from the sun than we are, or 2,790,000,000 miles away.
How Long to Travel to Neptune?
It took the second Voyager craft 12 years from blast off to its fly-by of Neptune in 1989.
What is Neptune Made Of?
Although it’s smaller than Uranus, Neptune is a dense planet and actually has 17 times the mass of Earth (Uranus is 14x). It is the most dense of the gas giants.
Its core is thought to be rock made of silicates and nickel-iron. Neptune’s mantle is believed to be similar to Uranus, comprised of ammonia, water and methane ices.
Does Neptune Have Rings?
It does, but they are thin and tenuous and you aren’t going to see them in a telescope in your backyard.
How Long is A Day and Year on Neptune?
The further a planet is from the sun, the longer its year – which is just a measure of how long it takes to complete a single solar orbit.
Neptune’s year is 165 Earth years long.
A day is measured by the time it takes for a planet to rotate once on its axis. Unlike its year, the Neptunian day is much shorter than an Earth one, measuring a little over 16 hours.
Written By Sharmila Kuthunur