Today's the shortest day of the year, the winter equinox. You'd expect that it is because sunrise is later than on any other day and sunset is earlier. This isn't quite true. For the next week, sunrise will get later and later, but sunset will also get later, by a little bit more, so overall the day length will increase. The reason is that whilst day length is a result of the tilt of the Earth's axis, day and night is caused by the rotation of the Earth around that axis. As the Earth travels a little further around the Sun, the Earth has to travel slightly more than 360 degrees to get back to the same point. This causes both sunrise and sunset to seem slightly later. After a week or so, the effect of the tilt of the Earth will have swamped thie effect.
It was put to me that this is why the mornings seem darker in January, but it's over by then anyway. And you'd be struggling to notice the effect against variations in weather due to cloud cover.
