Even you can find cool, blue Neptune – here’s how

Earth and Neptune compared. Through the telescope, Neptune looks like a bright blue-colored dot. Voyager 2 photo. Credit: NASA

Point a laser at the planet Neptune tonight and it’ll take 4 hours for the beam to arrive. For light itself, the fastest thing in the universe, to need so much time to get there, you’d guess the planet is far, far away. How about 2.7 billion miles? Yet it’s closer now than at any other time this year because Neptune’s at opposition to the Earth.

To begin your trek to Neptune, head out the next clear night around 10-10:30 p.m. and face southeast. Start with the bright star Altair in the bottom of the Summer Triangle. Shoot a line about two outstretched fists long to the lower left to Beta Aquarii. Continue “sliding” in that direction to Theta and then use the map below. You can also use the Y-shaped asterism nicknamed  the Water Jar below the Square of Pegasus and use it to navigate to Theta.  All maps: Stellarium

Today the outermost planet and Earth will line up on the same side of the sun. Seen from your home, Neptune will appear directly opposite the sun in the sky, rising at sunset and setting at sunrise. Opposition is traditionally the best time to observe a planet. Since they’re all pretty tiny even in a telescope, the closer they come the better.

Neptune and Earth slowly dance apart in the coming months with the two worlds some 200 million miles further apart come early next year. Since it won’t be near any bright stars this season, I’ve prepared a series of maps you to get you there in steps. If your sky is moderately dark you should be able to spot Neptune with a pair of ordinary 7×35 or 10×50 binoculars.

Once you’ve found Alpha and Beta Aquarii or the Water Jar, locate the triangle formed by the stars Theta, Sigma and Iota then use the map below to pinpoint Neptune.

So what does Neptune look like? In binoculars, it’s identical to a faint star of magnitude 7.6. With one difference. This “star” isn’t glued to one spot in the sky but creeps slowly westward among the real stars. You can easily see its progress if you look one night, note the planet’s position, and look again a few nights later.

OK, you’re finally there. This map shows stars down to about 8th magnitude or a little fainter than Neptune. Four stars near the planet are labeled with their magnitudes. Positions are shown around midnight every 10 days. and 5 degrees is about one binocular field of view.

Through a small to medium-sized telescope, Neptune looks like Earth seen from afar. That’s right – a pale blue dot. On Earth, blue means water, but on Neptune blue is the color of bitterly cold methane gas in the planet’s cloud tops.

Triton revolves around Neptune every 5.9 days. Shining at around 13.5 magnitude it’s not too tough a catch with a 10-inch telescope magnifying around 200x and up. The night-to-night motion is easy to see.

Discerning Neptune’s tiny disk will require at least a small telescope and magnification of around 100x.  Even then it’s itty bitty. I like 250x, when on good nights I can also spot its brightest, largest moon Triton as a companion “star”. The guide above shows Triton’s position around midnight on the dates shown. To know the moon’s whereabouts at any time and date, check out Sky and Telescope’s Triton Finder.

Neptune became the most distant planet in 2006 when Pluto was demoted to dwarf planet status. It’s so far away it takes 165 years to make one orbit around the sun. That’s a long time to wait for your first birthday party. What am I talking about? You’d never even see your first birthday.

Galileo drew a faint “star” on the left in this sketch of Jupiter and its moons made on December 28, 1612. Turns out, it was the planet Neptune!

I hope you’ll take time out some night in the next to pursue Planet #8 in binoculars or telescope. Hopefully the charts here will prove useful. While you’re pondering this distant orb, consider the following:

* Neptune was seen first by none other than Galileo in his tinker toy telescope on December 28, 1612 and again on January 27, 1613, but he marked it as a star on his chart and never returned for followup observations.

* The planet’s existence and position were calculated based on oddities in Uranus predicted position in the sky by mathematicians Urbain LeVerrier and John Adams in the mid-1840s. LeVerrier sent a letter with a position of the planet to Berlin Observatory astronomer Johann Galle which he received on September 23, 1846. That very evening Galle used the observatory’s refracting telescope to spot the new planet within one degree of the predicted position.

* Neptune has the strongest winds of any planet in the solar system. They’ve been clocked at over 1,300 mph (2,100 km/hr). To reach such incredible speeds, scientists speculate the planet’s extreme cold coupled with the fluidity of the gases in its atmosphere reduces friction to very low levels.

* Neptune is the coldest planet in the solar system with temperatures dipping to -366 F (-221 C) in its upper atmosphere.

* With a diameter less than 1/30 the width of the full moon, the sun would be too small to see as a disk with the naked eye from Neptune but would still shine with the brightness of several hundred full moons.

* Neptune’s 30,600 miles in diameter or about 4 times the size of Earth. Much of that is pure atmosphere, composed mostly hydrogen and helium, with traces of methane and other gases. Beneath its airy blue exterior lies a core of ice and rock.

* At the bottom of the atmosphere in the core’s mantle, the temperature is between 3,140 and 8,540 F (1,725-,4725 C). Research suggests that the combination of high temperatures and pressures causes methane to transform into diamond dust that gently “rains” down onto Neptune surface.

* Triton is probably a captured asteroid. The moon revolves around Neptune in the wrong direction compared to its other 12 moons, meaning it was probably lured into Neptune’s gravitational well and trapped.


26 Responses

  1. Edward M. Boll

    I may take my 20 x 60 binoculars out. I should have enough power. My star charts only go down to magnitude 8.0. I hope that that is enough to identify it. Your charts may help me get the job done. I would like to look for it when it is near Jupiter or Saturn. But that does not happen often. And when it is near Venus, it is usually during twilight.

      1. Tyler

        Why people keep saying that Neptune is the coldest planet in the solar system is beyond me. Uranus is the coldest planet in the solar system….

  2. Troy

    This is a great article. I recently made my first decent photo of Neptune (I just got an 8″ SC). I was waiting for a clear, moonless night to go back and see if I can see Triton. Using your great charts someone might be able to actually see it and identify it with binoculars, but really the key to identification is the dusky blue color .

    1. Profile photo of astrobob

      Hi Troy,
      Thank you. I agree that Neptune’s blue color is a helpful identifier, but it’s too dim in typical binoculars for most people to pick up on its color, hence the need for a good chart. This is especially true when the planet happens to be near a group of stars of similar magnitude as it is now. Through the telescope, once I’m in the general area, I’ll sometimes go by color, or more often, by Neptune’s slightly non-stellar appearance.

  3. randy

    I too thought this was a great post. I printed out the star charts and went looking for Neptune last night with my binoculars. It took a while to find the right area to look; I couldn’t have done it without the charts. I’m pretty sure I spotted Neptune. It was super faint, and I couldn’t look directly at it or it would disappear (I live in the suburbs). It was a lot of fun; I’ll try again tonight. Thanks for the great blog!

  4. jayashree

    Thanks for the information. My 9 year old son was asking me about this. The sky was cloudy that night. I can’t find anything. Was the event happened after 260 years? I actually don’t know but my son was saying so. He was very very sad as he could not see that. Please tell me whether it was possible to see if it wouldn’t be a cloudy sky? Also suggest me the technical specification of a telescope to purchase if I want to see all this things.

    1. Profile photo of astrobob

      Hi Jayashree,
      Neptune will be visible in Aquarius for months, so you have lots of time. Tell me how much money you can spend on a telescope and I will make a suggestion.

  5. caralex

    Bob, you say Triton may be a captured asteroid. However, how does that explain its apparent ‘bullseye’ orbit in the diagram above? It seems like a polar orbit. I would have though it would pass in front of and behind the planet. According to Wiki, its orbit is almost circular. But what is its inclination to the plane of Neptune’s orbit? Is it highly inclined?

    1. Profile photo of astrobob

      Good observation Carol! Yes, Triton is inclined 157 degrees to Neptune’s equator (hence the retrograde orbit). Combined with Neptune’s 28 degree axis axis tilt, Triton orbits Neptune in a fairly steep though non-polar orbit. Here’s a diagram to make it clear: http://bit.ly/12Nqp6u

  6. Giorgio Rizzarelli

    Nice post as always Bob, especially the “diamonds rain” topic.
    About Triton, I didn’t notice that it may be possible to see it in a amateur scope. At vmag 14 it’s just beyond the visual mag of my C8, but I’ll try with photography.
    By the way, rather than “captured asteroid”, I’d find more appropriate, and interesting as well, saying “captured dwarf planet”, since its round shape (by the way it shares with Pluto a similar composition and size). This explains why we can see in amateur scope such a distant satellite.

    1. Profile photo of astrobob

      Hi Giorgio,
      Calling it a captured dwarf planet is an interesting idea, but since the capture concept is theoretical and not proven, I’ll stick with asteroid, an accurate if more generic term. We can also look at Saturn’s moon Mimas, which is quite round but much smaller than Triton. Had it been captured instead of Triton I doubt it would be considered a dwarf planet.

  7. Great writeup! Using your easy-to-follow info, I went out and found Neptune. I live in LA, where I can barely see Alpha and Beta Aquarii. I turned my DSLR to that region and started snapping away, arriving at the images in the URL http://imgur.com/a/4sNQ1 . You can use Imgur to locate the full rez 6000×4000 photos by clicking on the gear in the upper right of each photo. Next month I’m going to the desert with excellent visiblity, and no light pollution so I’ll get some better pics. Thanks so much!

    1. Profile photo of astrobob

      Hi Victor,
      That’s great that you found it with your camera – congrats! For those living with very light-polluted skies, Victor’s idea of using a camera to record the camera instead is an excellent one. Easy to do too if you have the ability to make a 30-second time exposure and a tripod.

  8. Andres Rojas

    Thanks for the great guide. I was able to find Neptune with an old telescope. I couldn’t see a disk, but the blue light was unmistakable. Maybe it’s my cheap telescope, but I could see a little bit of red and some white too. Very beautiful. It helped me to use the moon as a guide too, which right now is up and to the right of Neptune in the evenings.

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