2 January 2013

Once again the skies are a solid block of low clouds – so no Moon, Jupiter or Quadrantid photographs.

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I mentioned yesterday that perihelion occurred this morning. Earth’s orbit isn’t a perfect circle and the elliptical path that we follow doesn’t quite have the Sun at the centre.

Perihelion at 04:38 today was the theoretical point at which the centre of the Earth was closest to the centre of the Sun. As the orbit continues, its eccentricity means that the separation increases slightly each day until early July when aphelion is reached.

The eccentricity of Earth’s orbit is a different matter to the tilt of the axis. Currently the south pole is tilted towards the Sun near perihelion so the southern hemisphere benefits from slightly increased Solar radiation and a smaller ground area for that radiation to cover. Meanwhile the northern hemisphere’s opposite tilt sees the radiation spread out as it strikes the ground at an angle.

Because of the way that the calendar is organised to keep the months aligned with the seasons, the dates of perihelion and aphelion slowly move backwards. In about 12,000 years the situation outlined above will be reversed and northern summers will occur around the time of perihelion.

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Although the Quadrantid meteor shower is visible in clear skies for almost a week, its maximum takes place in the early hours of tomorrow morning – so tomorrow night’s entry will explain a little more about Quadrantids.

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