Daily Bulletin

The Conversation

  • Written by Tanya Hill, Honorary Fellow of the University of Melbourne and Senior Curator (Astronomy), Museums Victoria

Australia is already baking in the long hot days of summer, and today brings the longest day of them all.

On December 22 we reach the summer solstice in the southern hemisphere, which means the Sun hits its maximum limit. The Sun will be at its highest and travel its longest path across the sky.

image Throughout Australia, we’ll experience the longest day of the year. But it’s not just the longest day in terms of daylight hours. It’s also the longest day, counting from noon one day until noon the next. It just so happens that not all days are created equal – and of all the months, December is when we get the most value out of our days. Read more: Curious Kids: Does space go on forever? Daily fluctuations From a timing point of view, if you go by your clock or watch then every day is a standard 24 hours long (give or take the odd leap second). But a solar day is the time it takes for the Sun to return to due north (or local noon) each day and it varies across the year. image The extra minutes of the solar day: Position 1 - the Earth is pointing towards the Sun; Position 2 - the Earth’s completes one rotation; Position 3 - the Earth must rotate a little further to face the Sun again. Wikimedia/Chris828, CC BY-SA Imagine the Earth rotating on its axis, but as it does so, it also moves a little further along its orbit around the Sun. On average, the Earth needs to rotate an extra four minutes each day to realign itself to the noonday Sun. It so happens, that at the solstices (both summer and winter), the Earth has to rotate even more to bring the Sun back to due north. This makes the solar day stretch to more than 24 hours, as I’ve explained previously. image The solstices are best explained by imagining an Earth-centric view. Museums Victoria To summarise the point here, the solstices represent that moment in time when the Sun reaches its most northern or southern point. It’s easiest to imagine when you consider an Earth-centric view – instead of having a tilted Earth orbiting the Sun, we imagine that the Sun follows a tilted orbit around the Earth. Today’s solstice officially occurs at 3:28am (AEDT), which is the moment when the Sun reaches its southern-most point and starts moving northward. image The Sun will be directly overhead today if you’re lucky enough to be anywhere on the Tropic of Capricorn such as the spire in Rockhampton, Queensland. Flickr/Philip N Young, CC BY-NC-ND As such, the solstices occur at the extremes of the Sun’s orbit. The Sun appears to stand still as it changes its north-south direction. Most of its movement is carried in the east-west direction or parallel to the Earth’s equator. In reality, it means that the Earth must rotate even further than average to realign itself with the noonday Sun and so the solar day is longer. Speeding up and slowing down So far we have only considered the tilt of the Earth. But the Earth’s orbit around the Sun is also slightly elliptical, which means the Earth isn’t always orbiting at the same rate. Kepler’s second law of planetary motion, tells us that the Earth will be moving faster when it’s closest to the Sun (at perihelion) and slower when it’s further away from the Sun (at aphelion). By coincidence, the Earth reaches perihelion on January 3, just after the summer solstice. That means right now, the Earth is moving at its fastest. It covers more ground during one rotation and therefore must keep rotating for longer to complete a solar day. Aphelion, or when the Earth is furthest from the Sun, occurs on July 4, just after the winter solstice. At that time of the year, the Sun is moving at its slowest and so the solar day doesn’t need to be quite as long because there’s less ground to make up. image How the length of the solar day varies from 24 hours - it’s longer than average around the solstices and shortest at the equinoxes. Museums Victoria Admittedly the difference in a solar day is fairly small. At this time of year the solar day is 30 seconds longer than the standard 24-hour day. But that’s almost a minute longer than the shortest solar day, which occurs in mid-September around the time of the equinox. Read more: Why the sunrise is still later after the winter solstice shortest day It’s interesting to consider, especially in today’s world where we have become so detached from the natural rhythm of the days. Also for us in the southern hemisphere, it’s not so surprising that the longest day of the year happens in December. We already expect to have days bathed in daylight at this time of year. But spare a thought for the northern hemisphere where it’s the winter solstice and the tables are turned. Northerners spend most of the longest solar day of the year facing away from the Sun in the cold of a winter’s night.

Authors: Tanya Hill, Honorary Fellow of the University of Melbourne and Senior Curator (Astronomy), Museums Victoria

Read more http://theconversation.com/its-going-to-be-a-long-summers-day-today-seriously-89491

Writers Wanted

I studied 5,000 phone images: objects were more popular than people, but women took way more selfies


Bad reactions to the COVID vaccine will be rare, but Australians deserve a proper compensation scheme


Pacific tourism is desperate for a vaccine and travel freedoms, but the industry must learn from this crisis


The Conversation


Prime Minister Interview with Ben Fordham, 2GB

BEN FORDHAM: Scott Morrison, good morning to you.    PRIME MINISTER: Good morning, Ben. How are you?    FORDHAM: Good. How many days have you got to go?   PRIME MINISTER: I've got another we...

Scott Morrison - avatar Scott Morrison

Prime Minister Interview with Kieran Gilbert, Sky News

KIERAN GILBERT: Kieran Gilbert here with you and the Prime Minister joins me. Prime Minister, thanks so much for your time.  PRIME MINISTER: G'day Kieran.  GILBERT: An assumption a vaccine is ...

Daily Bulletin - avatar Daily Bulletin

Did BLM Really Change the US Police Work?

The Black Lives Matter (BLM) movement has proven that the power of the state rests in the hands of the people it governs. Following the death of 46-year-old black American George Floyd in a case of ...

a Guest Writer - avatar a Guest Writer

Business News

Cybersecurity data means nothing to business leaders without context

Top business leaders are starting to realise the widespread impact a cyberattack can have on a business. Unfortunately, according to a study by Forrester Consulting commissioned by Tenable, some...

Scott McKinnel, ANZ Country Manager, Tenable - avatar Scott McKinnel, ANZ Country Manager, Tenable

InteliCare triple winner at prestigious national technology awards

InteliCare triple winner at prestigious national technology awards Intelicare wins each nominated category and takes out overall category at national technology 2020 iAwards. Company wins overal...

Media Release - avatar Media Release

Arriba Group Founder, Marcella Romero, wins CEO Magazine’s Managing Director of the Year

Founder and Managing Director of the Arriba Group, Marcella Romero, has won Managing Director of the Year at last night’s The CEO Magazine’s Executive of the Year Awards. The CEO Magazine's Ex...

Lanham Media - avatar Lanham Media

News Co Media Group

Content & Technology Connecting Global Audiences

More Information - Less Opinion