Equinox v equilux – what's the difference?


A dawn in mid-March at Island Hill, Strangford Lough

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Ronnie Mills


A dawn in mid-March at Island Hill, Strangford Lough

Most of us know the term equinox and on Wednesday, March 20th, it was the spring equinox that marked the beginning of astronomical spring.

It is at that time that the sun was passing over the equator or that the equator was the part of the Earth closest to the sun, the exact moment being 9:59 pm GMT.

The term equinox means "equal night" when day and night are supposed to be the same length of 12 hours.

However, it is only an impression, and it is not quite true – "almost" of equal length is more accurate.

At the equinox, the measurement is taken from the moment when the center of the sun is at the horizon and corresponds to 12 hours from sunrise to sunset, and vice versa.

However, it is the upper edge of the sun that is visible first at sunrise and finally at sundown.

Moreover, because of the refraction of light, there is still light once the sun is no longer visible. This adds extra time to daylight hours.

In the spring, this means that the moment when the day and the night are actually equal takes place before the equinox.

This is what is called equilux which means "equal light".

This year, it happened March 17, St. Patrick's Day, three days before the equinox.

Copyright of the image
Marty DishYouWereHere


The sun begins to climb on Strangford Lough last week

There are of course two equinoxes each year, with the fall equinox falling in September while the sun is heading south again.

In autumn, the equilux falls after the equinox which is this year on September 23 at 8:50 BST.

The autumn equilibrium, where day and night are equal, will follow two or three days later.

Technically, this means that for a year we should have a little more light than at night.

However, what we are getting depends actually on another factor – the weather – so sky clearer than the clouds, please!


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