• Written by Jonti Horner, Professor (Astrophysics), University of Southern Queensland
shutterstock

Happy February 29! It doesn’t come round very often, so make sure you enjoy it.

But why do we have these extra days? Well, if we didn’t, the seasons would gradually move around the calendar. Rather than midsummer in the southern hemisphere falling around December 21, it would arrive in January, then February, and so on. After a few centuries, the Australian summer would end up be in July!

But what exactly is a year? The simplest answer is the amount of time it takes for Earth to orbit the Sun. At school, we all learn that that is 365 days. Nice and simple, right?

There’s just one problem: there are several different definitions of a year.


Read more: It's going to be a long summer's day today, seriously


The sidereal year

The “sidereal year” is the classic classroom definition. It’s the time it takes Earth to complete one lap of the Sun and return to exactly the same place in its orbit, judged by the position of the Sun relative to the background stars.

But one sidereal year doesn’t take 365 days. Rather, it takes 365.256 days.

Then there’s another problem. In addition to spinning on its axis (which gives us day and night), our planet also wobbles. More accurately, Earth’s axis “precesses”, spinning around once every 26,000 years or so, like a wobbling spinning top.

The precession of Earth’s equinoxes causes the direction of the pole in the sky to shift, and the ‘first point of Aries’, the location of the vernal equinox, to move through the zodiacal constellations.

This is important because the direction in which Earth’s axis points is what controls the seasons. When the southern hemisphere points away from the Sun, we experience our southern winter while the northern hemisphere sees summer, and then vice versa.

But the precession (wobbling) of Earth’s axis means that in 13,000 years’ time, the directions would be the opposite of today. Today, the South Pole is angled towards the Sun during the southern midsummer, but in 13,000 years it would be tilted away (midwinter) at the same place in Earth’s orbit.

This means that, over thousands of years, the location at which we would experience midwinter or midsummer in Earth’s orbit would change. In other words, if we tied our calendar to the sidereal year, the seasons would still shift through the calendar!

The seasons on Earth are the result of the tilt of Earth’s axis. When we tilt towards the Sun we get summer, and when we tilt away we get winter. Wikimedia Commons

The tropical year

Fortunately, we have another way to define a year that can fix this problem. Instead of measuring the exact time it takes to orbit the Sun, we can instead measure the time between the vernal equinox of one year and the next.

The vernal equinox is the point in Earth’s orbit where the Sun moves from the southern hemisphere of our sky to the northern one. Each year it falls on or around March 21.

The time between one equinox and the next is called the “tropical year”, and is slightly shorter than the sidereal year. It comes in at 365.24219 days.

This difference is pretty small (about 20 minutes), but it equates to the amount that Earth’s axis has precessed in that time - just under 1/26,000 of a full lap.

Leaping into the future

But what does all this have to do with leap years? Well, because the tropical year is not exactly 365 days long, the date of the vernal equinox (and midsummer, midwinter, and any other seasonal event you care to name) will gradually drift through the calendar. If every year had 365 days, those events would gradually fall later and later in the calendar - by 0.24219 days per year.

That doesn’t sound like much, but it would mount up. After 100 years, the dates of those events would be 24 days later. The calendar would fall out of alignment with the seasons.

To remedy this, we have leap years, in which we add a single day to the length of the year. If we take a single four-year period, and work out the average length of the year, we get 365.25 days, which is pretty close to the real thing. But it still isn’t close enough.

The Julian calendar

This approximation worked well enough for a long time. In 45BC, the predecessor to the modern calendar began. Known as the Julian calendar, it was introduced by Julius Caesar.

Julius Caesar introduced his new calendar in 45BC, a year before his death for presumably unrelated reasons. Wikimedia Commons

The Julian calendar implemented a process of leap years: every fourth year, without fail, there would be an extra day at the end of February.

There were some problems implementing this new calendar - and for a few decades, leap years were incorrectly added every three years. Things were sorted out by 12AD, and from then on, every fourth year had a leap year.

But by the mid-1500s, errors were again beginning to mount. Remember that this approach gives an average year length of 365.25 days, whereas the true tropical year is 365.24219 days.

After one and a half millennia, this small difference had resulted in the dates for the solstices drifting by ten days through the calendar.

The Gregorian calendar

To fix this very slow drift, a new calendar was devised in the second half of the 16th century. Named after Pope Gregory XIII, the Gregorian calendar was released in 1582.

Pope Gregory XIII launched the Gregorian calendar in 1582 to fix a ten-day error that had built up in the 1627 years of the Julian calendar’s primacy. Wikimedia Commons

It shifted the dates of the year, moving the solstices back to their intended place. It then tweaked the way leap years were handled, to ensure those dates didn’t drift again in the future.

The small change was that leap years that were ‘“century years” (years ending in 00) had to be divisible by both 100 and 400. If the year can be divided by 100, but not by 400, it’s not a leap year.

Let’s take the century years 1900 and 2000 as examples. 1900 is divisible by 100, but not by 400. So 1900 wasn’t a leap year.

By contrast, 2000 can be divided by both 100 and 400, so it remained a leap year, and is called a “century leap year”.

So at the end of the 19th century, the sequence of leap years went: 1892, 1896, 1904, 1908. But at the end of the 20th century, leap years continued without a break (1992, 1996, 2000, 2004).


Read more: Wait a moment: 2016 goes a little longer thanks to a leap second


What does this mean? In the old Julian calendar, there were 100 leap years in every 400 years. But in the Gregorian calendar, we have just 97 for every 400 years.

This gives a remarkably good fit to the length of the tropical year. The average length of one year in the 400-year Gregorian cycle is 365.2425 days. That is almost (but not quite) exactly one tropical year - in fact, the two differ by just 26.8 seconds.

In the distant future, we may live in places that require a very different calendar - but on Earth, the Gregorian calendar should be accurate for thousands of years to come! Donald Davis

That’s close enough that we don’t have to worry about our seasons shifting in the calendar for thousands of years to come.

Jonti Horner does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.

Authors: Jonti Horner, Professor (Astrophysics), University of Southern Queensland

Read more https://theconversation.com/how-a-seasonal-snarl-up-in-the-mid-1500s-gave-us-our-strange-rules-for-leap-years-132659

Carpet Cleaning Tips

After work, the best feeling is to get home, your haven, where you can find peace and calm. But when you take a glance at your carpet, your heart shatters into pieces. The grime, dirt, and is it all...

News Company - avatar News Company

All Australians will be able to access telehealth under new $1.1 billion coronavirus program

Scott Morrison will unveil on Sunday a $1.1 billion set of measures to make Medicare telehealth services generally available during the coronavirus pandemic and to support mental health, domestic viol...

Michelle Grattan, Professorial Fellow, University of Canberra - avatar Michelle Grattan, Professorial Fellow, University of Canberra

How much more expensive is a two storey home?

The idea of adding a second storey to your home is exhilarating. Imagine, more living space, more bedrooms, more bathrooms, and a significant bump on the resale value of your home! For families, ha...

Digital 360 - avatar Digital 360

Parents, These FAQs Will Help You Prep for Childcare in Australia

If you’re a new immigrant in Australia, perhaps you have plenty of questions about childhood education. What school should you pick, or what’s the best age for them to attend school? To help you...

Darren Cunningham - avatar Darren Cunningham

Hotel quarantine for returning Aussies and 'hibernation' assistance for businesses

All Australians arriving from overseas will be quarantined in hotels or other facilities under strict supervision for a fortnight, under the latest crackdown in the battle against the coronavirus.Anno...

Michelle Grattan, Professorial Fellow, University of Canberra - avatar Michelle Grattan, Professorial Fellow, University of Canberra

how sharing your data could help in New Zealand's level 4 lockdown

New Zealand and much of the world is now under an unprecedented lockdown. Public health experts say this is the best way to suppress the spread of the virus. But how long will such a lockdown be socia...

Jon MacKay, Lecturer, Business Analytics, University of Auckland - avatar Jon MacKay, Lecturer, Business Analytics, University of Auckland

What is orthohantavirus? The virus many are Googling (but you really don't need to worry about)

ShutterstockAccording to Google Trends, the top globally trending topic this week is “orthohantavirus”, as spurious sites claim it’s the next pandemic on the horizon.Take it from me:...

Allen Cheng, Professor in Infectious Diseases Epidemiology, Monash University - avatar Allen Cheng, Professor in Infectious Diseases Epidemiology, Monash University

MyGov's ill-timed meltdown could have been avoided with 'elastic computing'

DAN PELED/AAPThese past few weeks have shown the brittleness of Australia’s online systems. It’s not surprising the federal government’s traditionally slow-moving IT systems are buck...

Erica Mealy, Lecturer in Computer Science, University of the Sunshine Coast - avatar Erica Mealy, Lecturer in Computer Science, University of the Sunshine Coast

Why New Zealand’s coronavirus cases will keep rising for weeks, even in level 4 lockdown

ShutterstockThe number of New Zealanders testing positive for COVID-19 will continue to rise despite the strict conditions of the four-week lockdown that began this week. As of today, there are 368 ca...

Arindam Basu, Associate Professor, Epidemiology and Environmental Health, University of Canterbury - avatar Arindam Basu, Associate Professor, Epidemiology and Environmental Health, University of Canterbury

Sick and Tired of Your Dead End Job? Try Teaching!

Tired of the same old grind at the office? Want an opportunity to impact lives both in your community and around the world? Do you love to travel and have new experiences? Teaching English is the perfect job for you! All you need is a willingness to ...

News Company - avatar News Company

The Impact of an Aging Population in Australia

There’s an issue on the horizon that Australia needs to prepare for. The portion of elderly citizens that make up the country’s overall population is increasing, and we might not have the infrastructure in place to support this. Australians h...

News Company - avatar News Company

CBD



News Company Media Core

Content & Technology Connecting Global Audiences

More Information - Less Opinion