On July 2nd avid watchers of the skies celebrate World UFO day—the anniversary of the supposed crash of a flying saucer near Roswell in 1947. Helpfully, the National UFO Reporting Centre, a non-profit, has catalogued almost 90,000 reported sightings of UFOs, mostly in America, since 1974. It turns out that aliens are considerate. They seldom disturb earthlings during working or sleeping hours. Rather, they tend to arrive in the evening, especially on Fridays, when folks are sitting on the front porch nursing their fourth beer, the better to appreciate flashing lights in the heavens (see chart). The state aliens like best is Washington—a finding that pre-dates the legalisation of pot there. Other popular destinations are also near the Canadian border, where the Northern lights are sometimes visible. UFOs tend to shun big cities, where there are lots of other lights, and daylight hours, when people might think they were just aeroplanes.