Two people on a work trip, sitting outside a rented house on the edge of Gaborone in the last light, waiting for dinner and talking about nothing in particular. The subject of satellites comes up. One of them has never seen one. The other says it is easy, you just look for a star that moves, and tips his head back to find one.

His colleague, meanwhile, is looking somewhere else. Not up, he says, but over there, just above the horizon. What’s that?

What it was, according to the report one of them filed with the American reporting centre NUFORC a few weeks later, was an object with lights on it: red, green and yellow, arranged so that the whole thing read as a triangle. As they watched, it slid to the left, then abruptly to the right, then upwards, then stopped dead and hung there. The first thought was a helicopter. But there was no sound, and helicopters at night do not, in the writer’s experience, dart and reverse like that. Every thirty seconds or so a separate flash of light went off a little distance from it, like a camera. They watched for a good half hour while the thing wandered, paused, wandered, and then went in to eat. When they came back two hours later it was gone.

That was 14 May 2008. The remarkable part is the second report.

On 7 July, eight weeks later, the same two colleagues (the reports are written in the same voice, on the same business trip, at the same house) were again outside before dinner, again talking about satellites, again with one of them never having seen one. Again the newcomer asked what the object just above the horizon was. Again it was a triangle of green, yellow and red, silent, moving in all directions and stopping now and then to hover. It was not over the town, the writer noted, but out over the dark country beyond it. After forty-five minutes they went in for dinner, and afterwards it had gone.

At the foot of the second report the centre’s director added a one-line note of his own. Possible sighting of a twinkling star, he wrote, with a question mark.

He had a point. A bright star or planet sitting just above the horizon, seen through a lot of warm, turbulent evening air, does exactly this. It flashes red, green and yellow as the atmosphere splits its light, and it seems to jump about because the eye has nothing nearby to fix it against. Sirius is the classic offender, and the early evening sky over Botswana in the winter of 2008 also had Mars and Saturn sitting low in the west after sunset, close together in July. A helicopter that is silent, stationary for minutes at a time, and back in the same spot eight weeks later is a hard thing to be. A star manages it every night.

The two colleagues never said what they thought it was. They just went in for dinner both times, which is its own kind of verdict. And they never did find the satellite.