Equinox marks start of 'astronomical Spring'

Equinox marks start of 'astronomical Spring'

Astronomical spring begins in New Zealand at 2.21am on Friday, marked by the Southern Hemisphere’s Vernal Equinox.

NIWA forecaster Ben Noll says the weather is expected to generally cooperate, with pockets of clouds and sunshine along with near or above average maximum temperatures for much of the country.

"The best weather will be found across the south and west of the South Island while a few showers will ride a gusty southerly into Wellington and the east of the North Island," Mr Noll said.

What is the Vernal Equinox?

The Vernal, or September, Equinox is the start of astronomical spring in the Southern Hemisphere and start of astronomical autumn in the Northern Hemisphere.

It occurs the moment the Sun crosses the celestial equator (the imaginary line above earth’s equator) from north to south. This happens on September 22, 23, or 24 every year.

Mr Noll said that while the Vernal Equinox represents astronomical spring in the Southern Hemisphere, several countries use a slightly different convention to mark the official start of seasons.

New Zealand, Australia, and South Africa call September 1 the start of spring, which is the start of “meteorological spring” opting to combine the months with the most similar temperatures together. Spring ends on November 30.

Meanwhile, at the South Pole, the equinox represents the first day that the sun rises above the horizon since the autumn equinox. The sun rises higher in the sky with each passing day and reaches a maximum height at the Summer Solstice on December 21.

What the Vernal Equinox is not

Contrary to popular belief, day and night are not of precisely equal duration in New Zealand – or anywhere on the globe – on the Vernal Equinox. While day and night are of approximately equal duration, the angular size of the sun and atmospheric refraction prevent exactness. Because of this effect, the days on which the length of day and night are exactly equal are called the equilux and occur a few days before spring equinox (September 19 in Auckland and Wellington, September 20 in Christchurch).

Daylight rapidly increasing

Mr Noll said Auckland will have 12 hours, 9 minutes, and 5 seconds of daylight on Friday. This is about two hours and 32 minutes longer than the shortest day on June 21, but two hours and 32 minutes shorter than the longest day on December 21. Elsewhere, Wellington will have 12 hours, 9 minutes, and 47 seconds of daylight and Christchurch 12 hours, 10 minutes, and 13 seconds on Friday.

Day length is currently increasing between two minutes and 14 seconds in Whangarei and an impressive three minutes and 16 seconds in Invercargill with each passing day. This represents the period of maximum daylight gain during the whole year and lasts until October 4.

Day Length Increases across New Zealand

Daylight Saving starts on Sunday at 2am – put your clocks forward an hour. .

Spring tidbits

Mr Noll said the first few days of September were much warmer than usual for the time of year, with afternoon (daily max) temperatures reaching over 20°C in numerous places across the east of both islands.

Christchurch (Riccarton) reached 25°C on 2 September, giving New Zealand its first 25°C maximum since Napier reached the mark on 10 June.

On 8 September, the average maximum temperature at New Zealand’s main centres (Auckland, Hamilton, Tauranga, Wellington, Christchurch, and Dunedin) was 9.97°C, making it the 3rd coldest spring day in at least two decades. Only 5 October 2009 (9.35°C) and 6 September 2007 (9.68°C) were colder in the last 20 years.

Wet Wellington: Between September 17-18, Wellington received 69 mm of rain. This was the wettest two-day period in almost 2.5 years or since April 17-18 2014 when 79.6 mm fell.

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