Almost 165 years ago, Johann Gottfried Galle spotted Neptune with his telescope based on calculations made by Urbain Le Verrier. Because Neptune can’t be seen with the naked eye, it was discovered only after astronomers noticed that Uranus’s orbit didn’t fit with Newtonian laws. Upon receiving Le Verrier’s calculation, it took Galle less than an hour of searching to spot the planet — as a point of light that didn’t appear on his chart of the sky.
Neptune takes about 165 years to orbit the Sun, so at 18:38 GMT (according to Phil Plait), Neptune will have reached the point in its orbit where it was when Johann Gottfried Galle spotted it.