The question
The sky on an anniversary
If you chart the same date every year, do you get the same picture?
Nearly. And the part that does not come back is the part most people would not have guessed.
The stars come back
Chart the fifteenth of June at nine in the evening, from the same town, in 1990 and again in 2040, and you get very nearly the same picture. Not roughly the same — the same to within about nine tenths of a degree, which is less than the width of your little finger held at arm's length. Push it out to a century and it is still under a degree.
The reason is that the calendar is built to track the Earth's orbit. A year is defined as the time it takes to come back round, so the same date returns you to nearly the same place in the orbit, facing nearly the same direction, at nearly the same hour. The small wobble that remains is mostly the leap-year system catching up with itself.
The moon does not
The Moon is on its own schedule, and its month does not divide into the year. Take the same date five years running and it comes back at fifty-four per cent lit, then sixteen, then full, then eighteen, then forty. There is no pattern to it at the scale of a wedding anniversary.
So if two anniversary charts a few years apart look almost identical and you want to know what actually changed, it is the Moon, and after it the planets, which is precisely what the word planet means: a wanderer, something that does not keep its place among the fixed stars.
Which hour to choose
The tool starts at nine in the evening, on the grounds that an anniversary is usually marked in the evening rather than reconstructed to the minute. Unlike a wedding there is often no exact time to be faithful to, and that is freeing: pick the hour when the sky looks best rather than the hour something happened.
Turning the clock forward moves the whole sky westward at fifteen degrees an hour. If a constellation you care about sits awkwardly at the rim, an hour either way will lift it clear.
One a year, if you wanted
Nothing stops you making one every year, and they would be an honest series: the same stars, a different moon, the planets in new places. A row of them on a wall would show you exactly what changes on that date and what does not.
It costs nothing to find out, which is the main argument for doing it that way rather than buying one and wondering.