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.
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.
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.
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.
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.
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.