Neptune comes closest to Earth today – on August 26, 2013 – and reaches opposition a little more than one-half day thereafter. Neptune is said to be at opposition – opposite the sun in Earth’s sky – whenever our planet Earth in its orbit passes in between the sun and Neptune. On the day 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.
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 sytem 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, and 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.
Even at its closest, Neptune lodges way out there, in the outskirts of the solar system. At opposition, Neptune lies 29 times farther away from Earth than the Earth lies from the sun. You’ll still need good binoculars or a telescope to spot Neptune, the solar system’s most distant planet, even when it’s at its closest and brightest for the year.