What's the coldest, windiest place in the solar system? No, it's not Duluth, Minn. It's Neptune, which is also the farthest planet. Today the Earth and Neptune are 2.7 billion miles (4.3 billion km) apart. If you could hop on a 747 jet with a cruising speed of 575 mph (930 kph) it would take only 536 years to get there. Bundle up! — the temperature there hovers around 390 below zero (–200 C) with sustained winds of 1,300 mph (580 meters/sec). On second thought, maybe I should cancel my travel plans and stick with the Earth. Fraught as it is with problems, at least you can solve them in shirtsleeves.
On Sept. 11 Neptune will reach opposition when it's on the same side of the sun as the Earth, and the two planets are closest. Neptune will also shine brightest, which isn't saying much. Right now Mars is almost 10,000 times brighter despite being six times smaller. Poor Neptune is more than 60 times as distant, so it shines at magnitude 7.8, too faint to see without optical aid. No problem — a pair of 50mm binoculars (7x50, 10x50) will nab it from the outer suburbs and the countryside. It's even visible in heavy light pollution with a 3-inch or larger telescope as long as you know exactly where to look.
Neptune is located in the dim constellation Aquarius the water-carrier. I wish I could say that the planet will be near a bright star that you could use as a handy guide. Sadly, that won't be the case until 2022 when Jupiter swings by. But why wait that long when you can see this frighteningly frigid planet right now? To find it, start with a big, easy pattern located halfway up in the eastern sky at 10 o'clock local time called the Great Square or the Square of Pegasus.
Shoot a line from the center of Pegasus to the lower right (southwest) and you'll arrive at the asterism called the Circlet in the constellation Pisces the fish. Once you're there, Neptune awaits your arrival a little more than one binocular field of view further southwest. Use the detailed map to track it down. You can call up both maps on your phone while you're outside the next clear night or use a free phone app like Star Chart to locate the planet. Download the app foriPhone or Android by clicking the links.
Because Neptune is so incredibly far away it appears to travel very slowly in the sky. In fact it remains in the same binocular field of view now through at least Halloween, so once you find the planet you can follow it a long time with little effort.
Galileo was the first person to see Neptune back in 1612 and again in 1613, but he mistook it for a faint star. On September 23, 1846, astronomer Johann Gottfried Galle properly discovered the planet at the Berlin Observatory. Galle used predictions of the planet's position from French mathematician Urbain Leverrier to track it down. Neptune remains the only planet discovered based on a prediction. Irregularities in the motion of Uranus hinted at the existence of a more distant planet and provided the impetus to search for an 8th planet as the cause.
I was thrilled to learn that on that September night, the planet appeared in Aquarius just 1 northeast of Saturn — what a handy reference! As a testament to Neptune's snail-like motion it didn't return to the same spot until 2011, 165 years later, the time it takes the planet to complete a single orbit of the sun.
Neptune is a gaseous planet 30,600 miles (49,250 km) in diameter with an atmosphere composed primarily of hydrogen and helium. Beneath its thick cloud cover the data suggest a super-compressed, hot ocean of water, ammonia and methane. The heat that still radiates from the planet — caused by compression due to gravity — dates back to the time of its formation and is the likely source powering its ferocious winds. Earth's winds in contrast stem from solar heating.
Besides its signature blue clouds Neptune has 14 known moons, including Triton, which orbits backwards around the planet opposite to Neptune's rotation. It's the only large moon in the solar system that does this. Neptune also possesses a set of scraggly, dark rings only visible in the largest telescopes using special filtering and other techniques.
Once you find Neptune, check it in binoculars once a week or so to see it crawl westward over the season. Although it looks exactly like a star, it's exciting to be able to look so far — a cool 2.7 billion miles. A 3-inch telescope will show its blue hue and an 10-inch or larger instrument at high magnification will bring Triton into view.