Neptune comes closest to Earth today – on August 31, 2015 – and reaches opposition less than a day later, on September 1. By closest, we don’t mean close. Neptune lodges in the outskirts of our solar system. At opposition, this giant world lies 29 times farther away from Earth than Earth lies from the sun.
Neptune is said to be at opposition – opposite the sun in Earth’s sky – whenever our planet Earth in its orbit passes between the sun and Neptune. That’s what’s happening over the next couple of days.
On the night of Neptune’s opposition, Neptune rises in the east around sunset, climbs highest up for the night around midnight and sets in the west around sunrise. As viewed from Earth now, this world is in front of the constellation Aquarius the Water Carrier.
Opposition is a special event. When any planet outside of Earth’s orbit is at or near opposition, Earth comes closest to that planet for the year, and that planet, in turn, shines most brightly in our sky. Even at opposition, however, Neptune, the eighth planet outward from the sun, is not all that close and it’s not all that bright.
In fact, Neptune is the only major solar system planet that’s absolutely not visible to the unaided eye. This world is about five times fainter than the dimmest star that you can see on an inky black night. You’ll need binoculars and a detailed sky chart to see Neptune in front of the constellation Aquarius.
Even at that, it’ll only look like a faint star.
Neptune, the fourth largest planet, is just a touch smaller than Uranus, the third largest. You would have to line up four Earths side by side to equal the diameter of either planet. Unlike Neptune, it’s possible – though difficult – to see Uranus, the seventh planet outward, without an optical aid.
Read more: Neptune and its maverick moon Triton
Bottom line: On August 31, 2015, the Earth is about to swing in between the sun and Neptune. On this day, Neptune comes closest to Earth and shines at its brightest for the year. Yet … you’ll still need good binoculars or a telescope to spot Neptune. Uranus and Neptune finder charts for 2015, here.