A bonus astronomy piece by David Vincent
We are still waiting for the blaze star – the anticipated nova that I told you about in the spring. It could go off at any time. Now, however, we would need to get up in the early morning to see it. Search T Coronae Borealis for more details.
The winter solstice is at hand – at last. This is the point in time – December 21st – when the Sun is at its lowest elevation at noon, and the duration of daylight is the shortest. It’s a good time to point out some lesser known features of the Earth’s orbit. The orbits of the planets, including that of the Earth, are close to being circular, but are all elliptical. This means that there is a point in the orbit when the planet is closer to the Sun (called perihelion), and a point when the planet is furthest from the Sun (aphelion). The Earth’s closest approach to the Sun is in early January, when we will be about 5 million kilometres closer to the Sun than we will be 6 months later, in early July. That’s about a 3% difference, and easily measurable to astute observers of the apparent size of the Sun’s disk in photographs! So, the Earth receives more warming solar radiation during the northern hemisphere winter than it does in summer, ameliorating our winters. Can I tempt you into researching the work of my favourite Serbian: Milutin Milankovic, and how changes of the time of perihelion (on a twenty-six thousand year cycle) have contributed to climate change and ice ages in the past?
We measure day length by the Sun – one day is the time it takes between one noon and the next. This is caused by the Earth’s rotation, of course. The Earth, though, spins once in 23 hours 56 minutes – not 24 hours. After one complete spin, because the Earth has also moved in its orbit around the Sun, the Sun does not quite appear back at its noon position, and another 4 minutes are needed to bring the Sun to its highest point for that day. 4 minutes? Well, that’s an average. Remember perihelion when we are closer to the Sun? All that time since aphelion, 6 months ago, the Earth has been getting closer to the Sun, and therefore accelerating. So in winter, the Earth is moving faster in its path around the Sun than in summer. That greater speed means that the Earth moves through a larger angle each day with respect to the Sun, and therefore needs to turn even more after one spin in order to bring the Sun back to its noon position. This amounts to about fifteen minutes! So a day in December and January is almost 15 minutes longer, measured by the Sun, than in summer! Of course, we use a standard, averaged, day length of 24 hours, because we can, now that we don’t need to rely on sundials. Pity those poor early astronomers trying to make accurate sundials!
Do use the extra 15 minutes wisely – ideally for orienteering, of course [or solving David’s puzzle, which can be found in December’s Club News – Ed.].
