Since time immemorial there have been predictions of Doomsday, now with instant global communication these prophecies attract international attention and have even created an end of the world industry selling trips to safe havens, bunkers, water purifiers and emergency supplies.
Apparently many end of the world events were scheduled for 2012. The most popular being on 21 December, inspired by a misinterpretation of the end of a Mayan calendar cycle. Many cataclysmic scenarios involve cosmic accidents such as collisions with rogue planets, wayward comets, reverses of Earth's rotation, polar shifts and other manifestations of cosmophobia – an irrational fear of the cosmos. Yet it is a reminder that all life on Earth is totally dependent upon the predictability of our planet's motion in relation to the universe, because this determines day, night, tides, seasons, weather, atmosphere, climate, currents, cycles etc.
All the planets in the Solar System orbit around the Sun in an easterly direction, it takes the Earth 365.25 days to complete its rotation around the Sun. The Earth rotates in an easterly direction on its own axis, in 24 hours it rotates a full circle (360 degrees).
The Earth's axis is tilted at an angle of 23.5 degrees, and it is the change in orientation of the tilt in relation to the Sun that causes the seasons by changing the angle of impact of the Sun's rays to spread or concentrate their heat. When the top half of the Earth tilts towards the Sun it is Northern Hemisphere summer/Southern Hemisphere winter, when the lower half of the Earth tilts towards the Sun it is Northern Hemisphere winter/Southern Hemisphere summer; autumn and spring are transitional seasons.
Position in relation to the Sun also determines the change in the length of day (LOD) which for a given day, at a given location, is determined by the time of year and the latitude of the location. On the equator there is very little change in position so LOD is an almost constant 12 hours. At the poles there is only one sunrise and sunset per year and these occur about the time of solstice.
The Earth's rotation on a north south axis creates the rotating dynamics which drive currents, wind, weather through the Coriolis Effect, which causes particles to move slightly to the right in the Northern Hemisphere, and slightly to the left in the Southern Hemisphere.
The Earth's tides are caused by the gravitational pull of the Moon and the Sun combined with the centrifugal force of the Earth's rotation. The Moon's gravitational force pulls the ocean towards it, so the oceans facing the Moon develop a bulge. At the same time at the opposite side of the Earth the centrifugal force pulls water in the opposite direction and forms another bulge of water. These bulges are high tides, and areas which lose water to the bulges experience low tides.
Although the dynamics of the solar system do change over millions of years for our purposes we can consider them stable – but what about collisions with meteors, comets and asteroids – more next time.