On September 9, Earth and Neptune were closest for 2019. One day later, on September 10, Neptune reaches opposition, when it is 180 degrees from the sun in our sky. In other words, on September 10, Earth passes more or less between Neptune and the sun, as we do every year in our yearly orbit.
By closest, we don’t mean close. Neptune, the eighth planet outward from the sun, lodges in the outskirts of our solar system. Its current distance is about approximately 2.7 billion miles (4.3 billion km).
For any superior planet – that is, for any solar system planet beyond Earth’s orbit – 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, is not 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 (at least) and a detailed sky chart to see Neptune in front of the constellation Aquarius.
Because we’re more or less between Neptune and the sun around now, Neptune is rising in the east around the time of sunset, climbing highest up for the night around midnight and setting in the west around sunrise. As viewed from Earth now, this world is in front of the constellation Aquarius the Water Carrier, right next to the 4th-magnitude star Phi Aquarii.
Phi Aquarii, though faint, is easily visible to the eye alone on a dark night. However, the moon displays a bright waxing gibbous phase on the day of Neptune’s opposition; and a few days thereafter, on September 13, the nearly full moon swings 4 degrees (8 moon-diameters) south of Neptune. So you’ll have to wait until the second half of the month to view Neptune in a dark sky.
Neptune and Phi Aquarii are so close together on the sky’s dome at present that the two readily fit within a single binocular field. In fact, you can see them together even through the telescope, with blue-green Neptune offering a color contrast to the ruddy tint of Phi Aquarii. Neptune is nearly 30 times fainter than the star Phi Aquarii. You may well be able to view Neptune with this star tonight, despite the lunar glare.
Even with an optical aid, Neptune may look like a faint star. You need to magnify Neptune by about 200 times and have a steady night of seeing to view this distant world as a small disk.
By the way, if Earth and Neptune both orbited the sun in perfect circles and on the same plane, then Neptune would be closest to Earth right at opposition. Yet, the Earth actually comes closer to Neptune on September 9 than on the day of its September 10 opposition. That’s because, on September 10, the Earth is a bit closer to the sun (and, therefore, a bit farther from Neptune) than on September 9. Neptune is also closer to the sun on September 10 than on September 9. But Earth’s change in distance is much more significant than that of Neptune.
Bottom line: We’re closest to Neptune for 2019 on September 9. Neptune’s opposition – when it’s 180 degrees from the sun on the sky’s dome – is one day later, on September 10. You need optical aid to spot it. Links to charts here.