Astro2110 – The Solar System

The priests of ancient Babylonia and Egypt were pioneer astronomers – they studied the sky, mapped constellations, and noted movements in both the Sun and the Moon – but, it was a Greek astronomer, Hipparchus, who made the first major new discovery in astronomy: precession. Comparing observations more than a century apart (particularly those done by Greek astronomers Timocharis and Aristyllus) with that of his own, Hipparchus noticed that Spica, and other bright stars in the sky, appeared to be displaced by 2 degrees relative to the autumnal equinox. Hence, after a few calculations, it did not take long for Hipparchus to ultimately conclude that the sky was moving and that the rate of this movement was no less than 1 degree per century (completing a full cycle in no less than 36,000 years) – a relatively accurate result in a time where astronomers still believed in the Geocentric model of our Solar System.

As we all know, the Earth experiences seasons each year. These are due to the tilt of Earth’s rotational axis, which causes certain areas of the earth to receive more direct sunlight during parts of the year, which has obvious effects on yearly climate.

Interestingly, the Earth’s seasons also shift around on the calendar year. Eventually, summer in the Northern Hemisphere will take place during December and January and the winter solstice will take place in July. This phenomenon is known as the Milankovitch Cycle.

Over a long period of time the Earth’s eccentric orbit around the Sun changes orientation due in part to the Apsidal precession of the Earth. In a normal orbit, the Earth is closest to the Sun during January but over time this will change. When the Earth is closer to the Sun, it receives a larger amount solar radiation. This small difference in solar radiation the Earth receives at the closest and furthest points creates a slight change in overall climate patterns. In a single year, this change is not terribly noticeable, but over the scale of 100,000 years, the changes in climate become more significant.

So what does solar radiation have to do with the Milankovitch cycle? As Earth’s eccentric orbit changes its orientation, different parts of the Earth receive the more direct solar radiation. This matters because it has a huge long term effect on global climate processes such as glaciation. Understanding that the position of Earth’s tilt will change over time can lead scientists to study periods of Earth’s history (such as the Ice Ages) in a different light, or make predictions about the long term climate future of the planet.

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