Winter light on a north window: readings and a lamp on a timer
I had been assuming that moss, which grows in the shade, would be fine on a north-facing shelf in winter. Then two jars started browning on the side away from the window within a fortnight of the clocks changing, and I decided to measure rather than guess. The meter is a free light meter app on an old phone, not a calibrated instrument, so these numbers are only good for comparing with each other.
At noon on a bright day in June the shelf had read between 2,800 and 3,200 lux at jar level. On the 3rd of December, also at noon and with a clear sky, the same spot read 850 lux. On an overcast day two days later it read 410. The jars at the end of the shelf, half hidden by the curtain, read under 300. Moss can survive very low light but it stops being bright green and starts drawing down its reserves, and the thin creeping mosses seem to go first.
The fix was a small lamp with a warm-white bulb, the kind sold for reading, clamped to the shelf above so the bulb sits about 30 centimetres over the lids. Directly under it the meter reads 1,900 lux; at the ends of the shelf it reads around 1,100. It runs on a plug-in timer from eight in the morning until four in the afternoon, which with the natural light either side gives the jars something like a ten-hour day. I was wary of heat, but with the bulb at that distance the jar lids are only a degree or so warmer than the room.
Results after three weeks. The browning on the two affected jars has stopped spreading, and the cushion moss has new green tips on the brown side. Condensation has increased, which makes sense, since the lamp warms the glass slightly and drives the cycle. One jar fogs so heavily by mid-afternoon that I have moved it to the end of the shelf. Nothing has recovered its June colour, and I do not think it will until spring, but nothing is getting worse. I will keep the timer at eight hours until the end of February and then see what the real daylight does.