In Ireland many of us follow the Celtic tradition and mark February 1 as the start of spring. Our European neighbours are only just beginning the season as we are half way through. In the UK, March 1 is considered the beginning of spring. Most of the countries in the northern hemisphere wait for the spring equinox to mark the official start of the season.
Meteorologists define the first day of spring as March 1. This is for practical reasons, so that the seasons are grouped together in months. Scientists and astronomers usually refer to the spring equinox of the beginning of the season.
The word equinox literally means ‘equal night’ in Latin. In theory, it is a day when everywhere on earth experiences equal day and night (12 hours of each). In reality this is not the case, due to weather conditions, how the sun is reflected in the earth’s atmosphere and how we define sunrise and sunset.
We have two equinoxes in our calendar year, one in March (between 19th and 21st) and one in September (between 20th and 23rd). The March one is referred to as the spring (vernal) equinox in the northern hemisphere and the September one is the autumnal equinox.
These are reversed in the southern hemisphere.
To understand the equinox we need to consider three things; firstly, we need to remember that day and night happen because the earth rotates on its (polar) axis every 24 hours. Secondly, we need to consider that the Earth takes a full year to travel around the sun.
Thirdly, the Earth is tilted on its polar axis, at an angle of 23.5 degrees; this means, for half the year, the northern hemisphere is pointing slightly away from the sun. For the other half of the year it points slightly towards the sun.
Except for two times a year, the equinoxes, when the earth is not tilted towards or away from the sun. The sun instead sits directly overhead the equator.
It takes the Earth 365.25 days (on average) to complete a full orbit of the sun. This is slightly longer than the 365 days of the Gregorian calendar so we add an extra day every four years (a leap year). This pushes the equinox timing out by ca six hours every year and means it takes 400 years for the exact time and date of an equinox to repeat.
If you are standing on the equator at the exact time of the equinox and the weather conditions are just right you might find that you cast no shadow. This is because the centre of the sun will be directly overhead.
If you are standing at the North Pole on the spring equinox you will see the sun start to peep over the horizon, signalling the end of six months or darkness and marking the start of six months of daylight.
The opposite is true when standing at the South Pole on the spring equinox. This year the spring equinox will happen at exactly 10.28am today, March 20.