Dimming of outdoor lights is more common than has been discussed on this thread.
Some countries don't have power 24/7; there are no minimum light levels and they want to "stretch" all the power they can. In Haiti, for example, many areas only get power for an hour per day and it's not the same time each day.
Some places have ordinances regarding outdoor lighting, there are eagle nesting areas, sea turtles laying eggs a certain time of the year, observatories as golddigger speaks of (we have that same issue at the university here), parks where you want some lighting at night without blinding the neighbors, and many other valid reasons to dim outdoor lights. One city near here even outlaws outdoor lights altogether (Sanibel, FL).
We are a manufacturer of outdoor controls and we've been asked for dimmers by some of our larger customers. It's not something we presently offer but it is something we've been working on.
As with indoor dimmers, outdoor dimmers need to be compatible with the lighting technology you are using.
For more information waterproof light fixture, please get in touch with us!