Today is World UFO Day, celebrated each year since 2001 on the anniversary of the crash in 1947 of a supposed flying saucer near Roswell, New Mexico. Back in the days of film cameras, blurry snaps of UFOs were taken by some as evidence that aliens were already among us. In recent years, they seem to be visiting more often; more likely, strange lights in the sky are easier to capture on ubiquitous camera-equipped smartphones. Yet they seem to be choosy about when they come. According to data gathered by the National UFO Reporting Centre, an American non-profit organisation, UFO sightings occur mainly in summertime, in the evening, and especially on Fridays, which suggests aliens may be timing their visits to coincide with alcohol consumption. And those two bright lights close together above the western horizon tonight? Not UFOs, but Jupiter and Venus, which are in conjunction (astronomer-speak for a close encounter) this week.