The last of the summer days

Beachgoers days are numbered: Photo: Supplied.

Today is the equinox, half way between the longest day and the shortest day of the year.

From now on the nights will grow longer and the days shorter until the winter solstice on June 21, when planet earth turns the other ‘corner' and southern hemisphere days begin getting longer again.

The equinox is when the day and the night are both supposed to be 12 hours long, but according to MetService, it rose today at 7.18am and is expected to set at 7.28pm – a 10 minute difference.

MetService explains the difference is because sunrise is defined at the time the top limb of the sun is just visible on the horizon, and similarly sunset is defined when the top limb of the sun disappears below the horizon.

Due to refraction of the atmosphere, and the fact the sun is not just a point, the sun appears and disappears when its centre is slightly below the horizon, explaining the ‘extra' daylight.

Astronomically the Autumn Equinox occurs at 6.14pm this evening, which possibly marks the minute the sun returns to the northern hemisphere when the sun's declination goes from negative to positive.

While the negative/positive description of the declination is Northern Hemisphere prejudice – the term declination describes the latitude where the sun appears to be directly overhead. Positive declination is used for latitudes in the northern hemisphere, and negative declination for latitudes in the southern hemisphere.

Declination varies in a regular way throughout the year and can be taken as a marker of the seasons.

Astronomers time the seasons using the solstices and equinoxes of earth's orbit around the sun.

By their reckoning, spring in the southern hemisphere starts with the vernal equinox in September.

This makes the New Zealand winter last 93 days and summer 89 days because the earth's orbit is slightly elliptical and the earth speeds up a bit during the southern summer.

For practical purposes climatologists measure climate data averaged over each calendar month, and by this reckoning the months of September to November are taken as the southern hemisphere spring. Note that by this reckoning summer lasts 90 days (91 days in a leap year).

In New Zealand this method of marking the seasons is more popular than using the equinox and solstice, and it adds one or two days to summertime.

There are less than two weeks to go until daylight savings ends on April 2, and our clocks go back one hour - giving us an extra hour's sleep in.

The 3 months with the least amount of sunlight begins in six weeks' time, sometimes referred to as the solar winter. The shortest day is exactly 12 weeks away today.


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