Find Neptune with binoculars in 5 easy steps
8/24/2014 3:49 PM
Neptune is just slightly below the dimmest naked eye objects from a dark sky site. Most humans can see down to about magnitude 6.5; at the moment, Neptune glows at magnitude 7.6. That puts it easily within the visibility of most any 7x35, 8x40, 7x50 or 10x50 binoculars from just about anywhere. The key simply knowing where to look.
Five steps might make this star hop sound more difficult than it really is, but because I'd like everyone to be able to see the furthest planet, I am showing this one small step at a time, so no one misses where to go. The first thing to do is get oriented in the sky. The ecliptic - the imaginary line in the sky that the Sun passes through as Earth revolves around it - is close the area where all the planets appear in the sky. And Neptune is currently visible within the boundaries of the constellation Aquarius. To get an idea of where to look in the sky, check out the Aug 25 thru Aug 31 Eyes on the Sky weekly video:
Later in summer and through fall, Aquarius may appear more towards the south or even southwest, but for now, it's towards the southeast. A bit of geometry will help you locate the proper position in the sky. While 6.5 magnitude stars are the dimmest seen from dark sky locations, most of us aren't likely to see much below magnitude 4.5 or so. I've made an animated gif that shows stars to only about magnitude 3.8. That means almost everyone should be able to find these stars, and then locate the geometric location of Neptune near Sigma Aquarii.
But! You will not need to look right there; we will star hop from much brighter Deneb Algiedi in Capricornus, to make the star hop easier. To begin, note where those eastern-most stars are. Deneb Algiedi is 2.9 magnitude, the brighter of the two. Nashira is nearby, less than two degrees away. Most binoculars will show a 5 to 7 degrees field of view, so you can see both of these stars in them at the same time.
The steps to get to Neptune are all pretty simple from here. Use Deneb Algiedi and Nashira to draw a line in the sky - making sure you go east, in the direction of Deneb Algiedi. You will be looking for 4.3 magnitude Iota Aquarii. This is about a full magnitude dimmer than Nashiri is. It is also just over 5 degrees away from Deneb Algiedi. That's nearly a full binoculars field of view, but you should be able to see both stars with one on each side. Then center Iota.
The next step can be tricky; first because the stars get dimmer, and secondly because there's a line of them from low 5-th magnitude to 6-th magnitude. But the ones to look for are the two brighter ones closest to each other, then zero in on the upper one. That is 42 Aquarii, and it is (barely!), the brightest of the bunch here. That's a shorter hop - under three degrees, so you can see Iota and 42 in the same field of view. When you've positively identified 42 Aqr though, center it, and get ready for step 4.
Now we're getting close to our target. Most of the lines we've made so far have been pretty straight along the lines of the stars from our "connected dots." This last one is bent a bit though; it will go up slightly. Fortunately, our target star - Sigma Aquarii - is brighter at magnitude 4.8, making it a more obvious target than 42 Aquarii was. This is a leap of only 4 degrees, so again, both 42 and Sigma can be seen together in the same field - no need to make "leaps of faith" here! You can see exactly where to go on each hop. That's why it's 5 steps.
And lastly, we make it to Neptune! The exact location of Neptune WILL change over time - it is a planet after all, a word whose root meant "wanderer," and planets are indeed wanderers among the stars. For this week - August 25 through August 31 2014, Neptune will largely be visible as per the graphic shown below - just slightly north and east of Sigma Aquarii. Over the next several months, it will move in retrograde towards the west, to the north of Sigma, go past it, then return to prograde motion and pass over it again. (Neptune finder chart here.) But here's an animated look at the Neptune star hop you can use to get to Sigma Aquarii from now through autumn and early winter of 2014.
Neptune isn't going to look large in binoculars; in fact, it will just look like a small, pale blue dot. At magnitude 7.6, it's nearing the edge of what's visible in most binoculars. But you can use this same star hop to find Neptune with a small telescope too. With a magnified finder, use the same steps. For those with a red dot or zero power finder, just estimate the location of Sigma Aquarii using the Enif->Sadalmelik (NOTE! Animated gif at top of page INCORRECTLY state Sadalsuud - it's actually Sadalmelik) line that intersects with the Nashira->Deneb Algiedi line. With a low power eyepiece, find the star, then locate the planet nearby. Use as much magnification as your telescope will allow (usually about 50x per inch of aperture, so don't exceed 120x with a 60mm telescope!).
Neptune is small, just 2.3 arc seconds across - that's about 1/20th the size of Jupiter, and many people remark that even that planet sometimes looks small at a telescope. It's not a feature-rich object - but remember, you're seeing photons that travelled a LONG way to get to you. Just seeing the planet, and knowing you've found it, is often the reward in and of itself.
There's a lot more here on Eyes on the Sky. For example, every week on the homepage there is a new astronomy video about observing objects in the night sky. They're only 5 minutes long - why not check out the latest one right now? For those new to astronomy, don't miss Eyes on the Sky's Ultimate Beginner's Guide!