Well, my "gear change" is complete. Working Tax Credit was paid into my bank account last Monday, and the paperwork came through at the end of the week. (I didn't dare touch the money until I was in receipt of some written confirmation; they are just are so intent on clawing back the money if they've made an error.)

The other half of the equation is not going well so well. I'm still struggling with my ephemerides. It's getting there, but I've massively underestimated the amount of work. If I was paid by the hour, that wouldn't be a problem. Instead, I dream of the minimum wage... :(

Today's graph is a case in point:

that double-humped, bluey-turquoisey line should be perfectly smooth. It was last Wednesday. But I've introduced some, *ahem*, defects. And the 183 tests probing the code’s internals are all “A-okay”. So I've got to pull everything apart and spend time treading water today.

BTW the turquoise line is the Equation of Time; that’s the difference between the time shown on a sundial and that displayed by a clock or your wrist-watch (assuming you're in Greenwich and your wrist-watch is set to GMT, or equivalent). If the Earth’s orbit was a perfect circle then clocks and sundials would perfectly agree. But the Earth’s orbit is somewhat squashed, and so the day is only on average 24 hours long; it can be thirty seconds longer or shorter. For that reason we set our clocks and watches to Mean Time (like Greenwich Mean Time). The Equation of Time describes the difference between mean and "real" time (solar time).

Practical upshot? Well, not only do the days get longer and shorter during a year, but the "window of daylight" wobbles backwards and forwards about noon. In January, for example, the daylight is moving into the evenings. From the Winter Solstice to the end of January about 15 minutes of the extra evening light are due this shift; for me, that's a quarter of the total gain in evening daylight.

Oh, and December the 25th is one of the days when mean time and solar time agree.Sol Invictus indeed. ;)

UPDATE: I got that bit back on track about 1pm:

And in case I over simplified: the curve is the difference between a sundial and a clock over the course of an entire year; each vertical square is 5 minutes.