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It could be a weather vane...but they tend to be able to turn in the wind.
Our guess is that it's a sun dial. You would use it to tell the time of day and season, by the shadows and settings.
I agree with Sally/Ernie. A sundial, that is also a garden scupture.
Not a weather vane or sun dial, it's a mariner's navigational device known as an Astrolabe; it pre-dates the sextant. Can't tell if yours is authentic and old, or not. Here's what Google has to say: https://www.google.com/search?q=navigation+device+astrolabe&tbm...