Observations

The start of winter and the shortest day – winter solstice 2010 from Australia

A winter sunset from Sydney Observatory on 28 July 2006, picture Nick Lomb

A winter sunset from Sydney Observatory on 28 July 2006, picture Nick Lomb

Brrrr! It is starting to be cold. Winter is almost upon us. In Australia the seasons are taken to start at the beginning of the month so that winter begins on 1 June and spans June, July and August. Supposedly, this is because in the early days of the Colony the NSW Corps changed from their summer to their winter uniforms at the beginning of the month. It makes sense though as experience suggests that the coldest days are in the middle of July and hence in the middle of the winter season.

There is often some confusion as there is also an astronomical winter that is from the winter solstice, 21 June in 2010, until the spring equinox, 23 September. This astronomical definition comes about as the winter solstice is the day when the Sun is at its furthest north, it is at its lowest in the sky and the length of daylight is the shortest in the year.

The variation in the length of daylight - sunrise to sunset - during the year 2010, calculated for Sydney

The variation in the length of daylight – sunrise to sunset – during the year 2010, calculated for Sydney. Note the small change in day length from day-to-day at the time of the winter solstice in June and the summer solstice in December. Plot by Nick Lomb

The winter solstice is the day when the Sun provides the least amount of heating so why are the coldest days a few weeks later? This is because it takes time for the ground and the atmosphere to cool down and to come into balance with the heating provided by the Sun.

In some countries the astronomical definition of winter is more prominent than in Australia. As indicated in this press release from the US National Oceanic and Atmospheric Administration, meteorologists prefer to use the meteorological definition of the seasons from the beginning of the appropriate month. In the US, which is of course in the northern hemisphere, meteorological winter is December, January and February.

So feel free to bring out those winter woollies from Tuesday 1 June 2010 as winter has arrived. You do not need to wait until the instant of winter solstice on Monday 21 June 2010 at 9:28 pm AEST, though at least you know that after that time the days will start becoming longer, imperceptibly at first, but steadily.

16 responses to “The start of winter and the shortest day – winter solstice 2010 from Australia

  • The UK uses the first of the month. Try convincing the average pom that winter doesn’t start until the winter solstice on about 22 December. It  is already jolly cold and the leaves have well and truly fallen off the deciduous trees by the official start of winter on 1 December!

  • Get in line with the rest of the planet Australia! Use solstices and equinoxes as beginning of each of the 4seasons. 1st of the month is totally random and confusing, because it does not make sense!

    • Why would we? Our weather is different. It is NOT like the Nth Hemisphere; and in fact we only have three seasons, not four ~ Hot, Cool and Wet; and up north we have only two ~ Dry and Wet. It’s far more logical and accurate for our seasons to reflect our weather.

  • Why do the Australians have to be difficult and not make use of the astronomical definitions of the seasons/ Rick J

  • Can I ask, what on average are the hottest and coldest dates of the year in Sydney, say like 28 January and 27 July (just guessing!)

    • Hello Oliver. Interesting question.

      According to the Bureau of Meteorology’s website at Observatory Hill, Sydney, the hottest average maximum temperature is 25.9°C, which occurs from 10 to 13 January. This suggest 10 & 11 January as the hottest days. At the same location the coldest minimum temperature is 7.9°C, occurring from 8 to 21 July. This suggests that the coldest day is around 14 July.

      Note how well the hottest days fit in the middle of the designated summer period of December, January and February. Similarly the coldest days occur in the middle of the designated winter period of June, July and August.

  • So if the winter days daylight hours are shorter… & we eat our meals by the sun’s position… then the meal times are closer together… now I know why I get FAT during winter.

  • Hi, 

    Does the Spring Equinox occur exactly in between the Winter Solstice and the Summer Solstice? If so, does that mean that the length of the day is the midpoint between solstices?  

    • Hello Adrian. Yes, the length of daylight at the equinoxes is halfway between the length of daylight at the two solstices. At Sydney, for example, in 2011 the longest day is 14 hours 25 minutes while the shortest day is 9 hours 25 minutes. Allowing for rounding error as all calculations are to the nearest minute, their mean matches the listed daylight length at the spring equinox of 12 hours 8 minutes.

  • The Observatory has a winter solstice viewing night which you can book. lets hope the weather is better by then

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