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Is it Spring yet? Explaining the Equinox

This year, the Spring Equinox for the UK will be at 10:28am on Monday 20 March 2017. But what does it all mean? And why do some people say that Spring already started on 1 March?

In meteorological terms, spring did indeed start on 1 March but on the astronomical calendar, it’s not until 20 March. Meteorological seasons help with the comparison of statistics, but astronomical seasons are more accurate in terms of when the weather will change. It can be confusing because astronomical seasons start on different dates each year and vary in length. This is due to the Earth’s tilt on its rotational axis as well as its orbit around the sun.

The Equinox also affects the timing of Easter which is apparently always the first Sunday after the first full moon after the Spring Equinox. So, that’s why Easter is also on a different date each year!

 

What is the Equinox?

The equinox is an astronomical event when the Sun crosses the celestial equator – the imaginary line in the sky above the Earth’s equator. The Sun can be seen for half the year above the equator and half the year below the equator. It therefore appears to cross the equator twice a year. At the times when the Sun is crossing the celestial equator, day and night are of equal lengths all around the globe. The word equinox actually means ‘equal night’.

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Why does it happen?

The Earth spends a whole year travelling around the Sun. This period of time can be divided into four – each of which is marked by either an equinox or solstice. Equinoxes and solstices mark key points in the astronomical cycle of the Earth. So, we have the Spring Equinox, the Summer Solstice, the Autumn Equinox and the Winter Solstice.

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Equinoxes occur because of the tilt of the Earth (23.5°) in relation to the sun – this is what causes the seasons. During summer, the Northern Hemisphere is tilted towards the sun so we get lots of light and longer days; but in winter, the Southern Hemisphere is tilted towards the sun so they get most of the light and the longer days.

The Spring Equinox, also called the Vernal Equinox, is of course only for the Northern Hemisphere – in the Southern Hemisphere, it is called the Autumn Equinox.

 

Why is it not on the same day each year?

The reason why the equinox is not always on the same day each year is because it takes the Earth 365 and a quarter days to go around the Sun. That’s also the reason why we add an extra day to the calendar every four years making a leap year. Equinoxes therefore occur about six hours later each year (a quarter of a day) and a full day earlier when it’s a leap year.

 

What’s the Equilux then?

Interestingly, on the date of the equinox, the length of the day and night may only be nearly equal, not exactly. This is due to the angular size of the sun and atmospheric refraction. The Equilux, meaning ‘equal light’ is when they are exactly equal and that usually occurs just a few days before the Spring equinox and a few days after the Autumn equinox. Confused yet? Basically, it’s almost spring!

 

Fun Facts

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  • The Spring and Autumn equinoxes are the only two times during the year when the sun rises due east and sets due west!
  • On the first day of Spring, somebody at the North Pole would see the start of the sun skimming the horizon beginning six months of constant daylight; while somebody at the South Pole would see the start of six months of constant darkness!
  • If the Earth rotated on an axis perpendicular to the plane of its orbit around the sun rather than on a tilt, every day would be the same length and there would be no change of seasons!
  • There is a myth that the first day of Spring is the day when you can balance a raw egg on its end! Go on – try it!

 

If you enjoy learning about astronomical events and how they affect our Earth, you could choose to study A Level Physics and find out more!