Neptune: The Blue World

Jim Hendrickson’s September article on locating Uranus prompts me to submit this piece dealing with how to easily find the famously blue outer planet, Neptune. Although it reached opposition back on August 20, Neptune will be favorably positioned close to a fifth magnitude star in northeastern Capricornus in early October, making it a fairly easy find. Absence of any background stars of comparable brightness close to Neptune during this time will also work in your favor. The planet’s current apparent direction of motion on the sky is retrograde (westward); it resumes direct, eastern motion on November 7. The New Moon of October 7 will be a big help, too, as well as Neptune’s transit (culmination) times in mid-evening during early October-the best time of night for viewing works out to a convenient schedule for most people.

A simple and straightforward star-hop to Neptune begins with spotting the brightest star in Capricornus, magnitude 2.9 Delta Cap, also known as Deneb Algedi. This white star marks the northeast tip of the roughly heart-shaped main outline of Capricornus and is paired with magnitude 3.7 Gamma Cap (Nashira), which lies about 1.75° to the WSW. You can’t miss noting this conspicuous, very wide pair, the brighter of which is your “jumping-off” point for this star-hop. Delta Cap is at RA 21h 47m 02s, Dec -16° 08’. The next star involved in forming a triangle for navigating to Neptune lies 5.2° to the ENE of Delta, in Aquarius: magnitude 4.3 Iota Aqr, also listed as 33 Aqr. This star qualifies as naked-eye if local sky clarity and light pollution will enable you to spot it; the position is RA 22h 06m 26s, Dec -13° 52’.

Consider the 5.2° line joining Delta Cap with 33 (Iota) Aqr as being the base of a “coathanger”-shaped triangle; the star marking the “hook” is close to Neptune and lies 3° to the NNE of Delta Cap. The star in question is magnitude 5.1 Mu Cap, also listed as 51 Cap, located at RA 21h 53m 18s, Dec -13° 33’. Using binoculars is the best way to note the triangle I’ve described, especially when you must deal with light pollution and/or a less than ideal sky condition. Once you’ve isolated 51 (Mu) Cap, finding Neptune shouldn’t be difficult-on October 5, it’ll be just 0.4° northeast of the star at RA 21h 54m 26s, Dec -13° 15’, and on the night of the 10th the position will be slightly closer to 51 (Mu), still to the northeast.

Neptune has a distinct bluish hue as opposed to nearly all stars seen anywhere in its vicinity, a factor which greatly helps in positive identification but only to a certain point-you must use high telescopic magnification (greater than about 150x; 200+x is even better) to resolve the planet as being a tiny dot or disk. Neptune’s angular diameter on October 10 is only 2.32” of arc, but if you can determine, at sharpest focus, that you’re seeing a very small disk that’s blue in color, you’ve found it. The magnitude is 7.9 and there won’t be any stars as bright or brighter than Neptune in its immediate area northeast of 51 (Mu) Cap during the dates I’ve indicated. If we’re lucky enough to get clear skies in early October, I hope some of you who may not ever have glimpsed the 8th planet can finally manage to do so. By the way, Neptune will transit locally at about 9:25pm on October 10-not too early or too late for most of us!

(Remember: If it’s a small disk as opposed to a star-like point of light, you’ve found it. Use lower power to isolate it, then high power to verify.)

Neptune is slightly smaller in diameter than Uranus, but is still nearly 3.9 times the size of Earth. The tiny visual size is due to its vast distance from us, which on October 10, will be 29.384 AUs, or over 2.73 billion miles. Neptune’s lovely blue color is attributed-in part-to the presence of a small percentage (1%) of methane in its upper atmosphere. Exactly why this is relevant to the perceived color makes for a good trivia question to look into, if some readers might be so inclined.