Happy Perihelion!

Today is quite a special point in the year. Obviously, it isn't the start of the calendar year (that was Jan 1st) and it isn't an equinox (those are on March 20th and June 21st this year). It is actually the point when the Earth is closest to the Sun in its orbit. In fact, according to the US Naval Observatory, that is due to occur at roughly 15:00 Universal Time (currently the same as GMT) which is … right now!

The Earth's orbit is slightly elliptical, so for some of the year it is closer than the average distance from the Sun and at other points it is slightly further away. The point in the orbit where the separation is greatest is known as aphelion and the closest approach - happening right now - is perihelion.

At this moment in history, the Earth is closest to the Sun during winter in the northern hemisphere which makes it slightly warmer than it would be if we were on a circular orbit. By the same reasoning, the southern hemisphere experiences slightly hotter summers too. But all this will change in time because of the precession of the Earth's axis (it tilts at 23.5 degrees). In about 13,000 years the northern hemisphere will be tilted towards the Sun during perihelion and summers (for us northern hemisphere folk) will be a bit hotter and the winters a bit colder.

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Posted in astro blog by Stuart on Wednesday 04th Jan 2006 (15:25 UTC) | Permalink
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