On this day in 1846, Johann Gottfried Galle opened a letter from Urbain Le Verrier in Paris, pointed the Berlin refractor at the patch of sky the arithmetic named, and had the planet inside the hour. Less than a degree from the predicted spot. John Couch Adams, working alone in England, had arrived at much the same figures. Nobody saw Neptune first and explained it after; the explanation went first, and the universe kept the appointment.

It is a long way to keep an appointment from. Neptune sits about thirty times farther from the Sun than we do, so the sunlight arriving there is roughly a nine-hundredth of what falls on your windowsill this morning: bright enough to read by, about like the inside of a refrigerator with the door open. One of its years runs about 165 of ours, which means it only completed its first full orbit since discovery a few years back.

The photograph below shows the disk close up, blue-cyan with pale streaky cloud bands and a dark oval near the left edge. We have that view from a single spacecraft passing through in 1989, and nothing since. Neptune is near its closest to us around this time of year, though you will want binoculars and patience; it looks like a faint blue-grey star that refuses to twinkle. Worth a look up, if the sky is clear.