After a winter of epic weather in the northern hemisphere, people’s thoughts are, this month, turning gratefully towards Spring. March is the month of the vernal equinox, recognised as the first day of Spring in the north; in the southern hemisphere it is the autumnal equinox, signalling the transition from summer into autumn.
An equinox occurs twice a year (around March 20 and September 22). This year, on March 20, 2013, at precisely 11.02 Universal Time (the modern equivalent of Greenwich Mean Time) or 07.02 Eastern Daylight Time, the sun will cross directly over the Earth’s equator, marking the vernal or spring equinox in the north and the autumnal equinox in the southern hemisphere.
Understanding the reasons for the equinoxes is important not just because they have been seasonal markers for millennia – for instance, the ancient Egyptians built the Great Sphinx so that it points directly toward the rising Sun on the day of the vernal equinox – but also because it helps us to understand the meteorology and physics of our planet. At the same time, we recognise that Earth’s weather systems are part of a wider astronomical system – the solar system.
The most important factor driving the planet’s seasons is the fact that the Earth’s axis is tilted at an angle of 23.4 degrees to the Sun: if the axis of Earth’s rotation were exactly perpendicular to the plane of our orbit around the Sun, there would be no variation in day length or seasonal temperatures throughout the year.
Because of this tilt, each hemisphere of the Earth receives the Sun’s rays most directly during its summer months and the corresponding polar region experiences continuous daylight at this time of year. Conversely, during the winter months, the relevant hemisphere is tilted away from the Sun; solar rays have less of a warming effect because of the angle at which they strike the atmosphere. In winter, the polar regions are in perpetual darkness.
The name ‘equinox’ is derived from two Latin words: aequus meaning ‘equal’ and nox meaning ‘night’. For the vast majority of the year, days and nights are of unequal length. At the time of the equinox, they are approximately equal, with the centre of the Sun spending roughly the same amount of time above and below the horizon at every location on Earth.
However in reality, even at the time of the equinox, days are slightly longer than nights. This is because of the way that the Earth’s atmosphere is able to bend the light from the rising and setting Sun.
If we define ‘day’ as the period when sunlight reaches the ground in the absence of local obstacles like hills, then this remains true even when the upper part of the Sun is actually just below the horizon. This effect makes the day about 14 minutes longer than the night at the equator and longer still towards the poles. Truly equal night and day length is sometimes referred to as ‘equilux’ (equal light) to avoid confusion and only happens in places far enough from the equator to have a seasonal difference in day length of at least seven minutes, and actually occurs a few days towards the winter side of each equinox.
This refraction phenomenon is also the reason that we frequently experience orange skies at sunrise and sunset, as it causes the sun’s rays to travel further through the atmosphere to reach the observer; because shorter wavelengths of light tend to be scattered by atmospheric particles more than the longer end of the visible spectrum, the effect accentuates yellow, orange and red colours.