ABSTRACT
The sun is a nuclear fusion reactor located approximately 150 million kilometres away from Earth. This is not only a reassuringly long distance away at which to have a nuclear power plant but also means that the solar energy that reaches Earth is actually the result of nuclear energy. The Earth moves around the sun, performing a complete orbit in approximately 365¼ days (which is rounded to 366 days approximately every fourth or leap year). The Earth is also rotating about its own axis, taking around 24 hours to complete one rotation. This axis of rotation, through the North and South Poles, is not at right angles to the plane of orbit around the sun but is inclined at 23°27′ (~23.5°) to the normal to this plane. The importance of this tilt is that it is responsible for the seasons and the changing positions of the sun at different times of the year. For half the year (March to September) the northern hemisphere is tilted towards the sun, with the longest day occurring around 21 June (the summer solstice). From September to March it is the southern hemisphere that is tipped towards the sun, with the longest day around 21 December (the winter solstice). During spring and autumn the rotation axis is nearly at right angles to the orbital plane, giving rise to a day and night with near equal lengths each of approximately twelve hours (hence the name equinox). The spring event takes place around 21 March (the spring or vernal equinox) and for autumn it is around 21 September (the autumn or fall equinox).
