Tomorrow the planet Neptune will be at opposition and at its closest to Earth for the year. Neptune is one of the the “ice giants” or planets beyond Jupiter and Saturn that have slushy, ice-filled cores. The other is Uranus. Neptune doesn’t register on too many amateur astronomers’ radars, because it can be tricky to find even though it’s within range of binoculars on a dark night.
Once a year, Neptune and Earth line up on the same side of the sun. From our perspective on the ground, the planet is directly opposite the sun in the sky, rising at sunset and setting at sunrise. That’s the origin of the term “opposition”. Like some sky watchers I know, it’s up all night long. You pick the time for viewing and Neptune will oblige. The 8th and outermost planet is in eastern Capricornus right next door to Aquarius. This region of sky rises high enough to make the planet worth seeking from 10:30 p.m. on in late August.
The typical faintest star visible to the naked eye from a dark site is 6th magnitude (the brightest are 1st and 0 magnitude). Neptune is magnitude 7.8 or nearly two levels below the naked eye limit. Not to worry. A pair of 7×35 or 10×50 binoculars can reach down to 8th magnitude and show the planet as a small “star”. The question is: how do you get there?
It’s always a good idea when hunting for a dim object to start with the brightest thing you can find. In this case, we’ll use the Summer Triangle, that long triangle of sky defined by the three prominent stars Vega, Deneb and Altair. At our chosen time, the Triangle is due south and high in the sky making it an easy target. Drop one outstretched fist at arm’s length below Altair to spot the pair of Alpha and Beta Capricorni. From there, move another fist to the left or east to Delta Cap. Now point your binoculars at Delta and use the more detailed charts below to pinpoint Neptune. The planet will be in the same field as Delta and Iota in Aquarius and appear as a dim star. The two binocular stars in the white boxes are similar in brightness to Neptune and will serve as excellent guides for watching the planet’s slow crawl to the west in the coming months.
With the moon out and growing toward full, seeing Neptune in binoculars will soon become challenging, but later this month and into September it will become easier to see and rise higher in the sky. If you get to know where the planet hangs out now, you’ll be ready for the next dark night.
Telescope users using high magnification — 100x and up — can discern the planet’s tiny pale blue disk. Neptune is a perpetually cloud-covered world like Venus but instead of carbon dioxide and sulfuric acid, Neptune’s clouds are composed of methane ice chilled to 350 below zero. This is a bitterly cold place, no surprise given its great distance from the sun. That’s the same reason why its orbital period is also large — one Neptunian year (a single trip around the sun) takes 164 Earth years. As if to make up for this molasses-like movement, the planet rotates on its axis once every 16 hours compared to Earth’s 24.
If you’ve never seen the solar system’s most distant planet, at least according to the current definition of planet, consider a foray in Neptune’s direction this season.