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Wednesday, June 20, 2012

Bmag June 19th 2012 - Shortest day

It’s not going to make you any warmer knowing this, but we are about to head back towards summer! The Winter Solstice – where the midday sun appears directly above the Tropic of Cancer – occurs at 9.09am AEST on Thursday 21 June, making that our shortest day. After 21 June, you can start looking forward to your beach holiday at Noosa, Christmas prawns, backyard cricket and what you’re doing for New Year’s Eve! Okay, enough with the summery daydreaming. Let’s get back to mid-winter.

Allow me to point out some fascinating curiosities surrounding the solstice. For instance, you would expect sunset to be getting later from this week. In fact, our earliest sunset, 5pm, has already come and gone (from 6 to12 June). At the other end of the day, sunrise continues to get later until it reaches 6.39am between 27 June and 6 July. A commonly asked question at this time of year is “why don’t our latest sunrise and earliest sunset coincide with our shortest day?”

Rebecca Jenkins, a writer with the ABC’s Health and Wellbeing website, explains: “This phenomenon is created by a combination of the Earth’s oval-shaped orbit and its tilt of 23.5 degrees. The Earth orbits the sun in an elliptical pattern, running faster when it is closest to the sun [during the winter and summer solstices]. This quirk means that the length of a solar day — the time between two solar noons (when the sun is at the highest point in the sky) — is not always the 24 hours we measure on a clock.

“At the same time, the Earth’s axial tilt means we are getting a few seconds less daylight every day in the lead up to solstice, but this has a small effect on the sunset and sunrise times compared with the much larger difference between solar time and clock time. It is this effect that leads to the staggering of the latest sunrise, the solstice and the earliest sunset.”

Another question that always arises at this time of year is “why is 1 June considered the beginning of winter when Britons, for example, have always used the winter solstice?” Well, it seems Australia has led the way here, with the UK’s Met Office moving to align its seasons with calendar months. So now the UK winter officially starts on 1 December, spring on 1 March and so on. You can imagine the uproar! Labour MP for Middlesborough Stuart Bell told BBC News at the time: “Spring starts on 20/21 March and if the Met Office is not aware of this simple fact, it reflects a casual approach to facts, which is all too inherent today!”

As for our Bureau of Meteorology, I have only two relatively minor gripes. And no, I’m not talking about its actual forecasts, which I find impressively accurate.

Firstly, it’s time the Bureau launched itself on social media so that forecasts and warnings can be delivered to you. I’m encouraged by an online survey by the Bureau, in which respondents are repeatedly asked (in subtly different ways) whether they would like to receive their weather information via social media. Here in Brisbane, a couple of individual forecasters, namely Rick Threlfall (@ RickThrelfall) and Tony Auden (@TonyAuden) have started the ball rolling via their personal Twitter accounts. It can’t be too long before we see an official Bureau presence.

My only other quibble is how the Bureau determines whether the current temperature is above or below average. I would have thought it logical to compare the temperature with the long-term average for that time of day and for that date. Instead, the Bureau uses the average for that time of day over the entire month. This means 16 degrees at 9am on 31 May is described as “2 below average” (the 9am average for May being 18 degrees), whilst the same temperature at the same time the very next day is “1 above average” (the 9am average for June being 15 degrees). Obviously the 9am temperature is lower at the end of May than at the beginning, and continues to slide during June. Therefore, at the beginning and end of each month, statements like “we’re experiencing above/below average temperatures” can be useless and misleading.

Again, minor quibbles, and none of this is going to keep you warm tonight. For that, I suggest wearing slippers around the house and taking a hot water bottle to bed!

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