Northern Ireland

Equinox - why more light than dark?

Mussenden Temple near Castlerock in County Londonderry snapped by Glenn Miles Image copyright GLENN MILES
Image caption Mussenden Temple near Castlerock in County Londonderry snapped by Glenn Miles

Most of us are familiar with the term equinox, indicating the start of spring in March and the start of autumn, which this year fell on 22 September.

Image copyright Calvin Harlow
Image caption At sunrise and sunset we don't observe the centre of the Sun first

The equinox occurs twice a year when the centre of the Sun passes directly over the equator.

Image copyright David Brazier
Image caption Light is refracted by the Earth's atmosphere when the sun is below the horizon

But while many assume that day and night are equal in length on that day, this is not exactly the case.

At sunrise and sunset we clearly see the edge of the Sun for longer than its centre - so we are really seeing the star before the official sunrise.

Image copyright Paul McFarland
Image caption How much light we see depends partly on the weather

Similarly, at sunset it is the edge of the Sun that we see last.

During these times and because of light being refracted by the earth's atmosphere when the Sun is below the horizon, we actually have light in the sky before sunrise and after sunset.

Equilux factor

That gives us more light than dark during a 24-hour period.

Image copyright Ellen Thompson
Image caption This golden sky in Bangor, County Down, was captured by Ellen Thompson

So taking into account this twilight effect, the date on which day and night actually are equal in length, falls two or three days after the equinox in autumn and similar before the equinox in spring.

This date is the lesser known Equilux which fell on 25 September.

After the equilux, nights get longer in the autumn and days get longer in the spring.

Technically or astronomically, it means that during the period of a year, we should get a bit more light than dark.

However, what we actually get depends on another factor: The weather!

It's a big ask here in Northern Ireland, but could we have more clear skies than cloud please?

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