Today is the spring equinox, but what actually is an equinox?
It's one of only two times in the year when day and night are (almost) exactly equal all over the world...
The Earth's axis is tilted.
That means, as it orbits the sun, the amount of sunlight received by each part of the world varies throughout the year.
This is where seasons come from. Winter is when that part of the world is tilted furthest from the sun; Summer when it's closest.
But, during the Earth's orbit, there are two moments when its tilt is aligned perpendicularly with the sun.
One is in March and the other in September, six months apart. These are the two equinoxes.
Day and night are (almost) exactly 12 hours on the equinox, all over the world.
If the Earth wasn't titled on its axis then the amount of light on any given day would be fixed all throughout the year, and the equinox is what *every day* would be like if that was the case.
There might still be seasons, but not as we know them.
In the Northern Hemisphere the March equinox means the end of Winter and the beginning of Spring: thereafter it is tilted closer to the sun, bringing more light and warmer weather.
In the Southern Hemisphere it's the opposite: the March equinox means the *end* of Summer.
The opposite of the equinox is the solstice.
They come between the equinoxes, when the sun is either furthest away or closest. One is in June and the other in December.
The Summer Solstice is the longest day of the year; the Winter Solstice the shortest.
For a visual representation of all this you can just look at an "analemma" - the shape you get if you take a photo of the sun at the same time every day throughout the year.
The solstices are at the top and bottom, and the equinoxes are somewhere toward the middle.
Or, for a more dramatic rendition, here's a timelapse photograph taken over the course of six months.
The highest part is the sun's course during Summer and the lowest during Winter.
Seasons aren't quite as important as they once were. When most people lived on the land, whether farming crops, rearing animals, or otherwise, the passing of the seasons was vitally important.
It's even true for animals - why do bears hibernate during winter?
The Spring equinox didn't just mean warmer weather and more walks outdoors.
It meant Winter had been survived, that time for planting certain crops had arrived, that your resources had lasted the cold months, and that quality of life would improve again.
That's partially why so many of the world's oldest civilisations built great temples and other structures in relation to the sun and the stars, like Stonehenge.
The different points of the year were of such importance that they also became religiously significant.
The Pyramids at Giza are perfectly aligned with the four points of the compass. How?
On the day of the equinox the sun rises precisely in the east (or west, depending on which hemisphere you're in).
That may have been how they calculated its orientation with such precision.
So that's the equinox, once one of the most important days in the calendar.
It comes around twice a year, marking the change from one season to another. Winter to Spring if you're in the Northern Hemisphere and Summer to Autumn if you're in the Southern Hemisphere.
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It sounds like a boring topic, but concrete is one of the most revolutionary inventions in history.
For example, concrete now weighs more than everything else humans have ever made combined.
So here's the story of how it has changed the world, for better and for worse...
What is concrete?
Put simply, it's cement (the magic ingredient) mixed with aggregates, like gravel and sand, plus water — the cement acts as a binder which reacts and sticks the aggregate together.
Your pour it into a mould and what was a liquid soon becomes hard as rock.
And it has been around for thousands of years.
The Egyptians, Mycenaeans, and Nabataeans all used some form of concrete.
But it was the Romans who were the first real masters of concrete — the things they built with it, like the Aqua Alexandrina, are astonishing even now.
You can learn a lot about history just by looking at the words we use.
Like algorithm, which is descended from the name of a 9th century Persian polymath called al-Khwarizmi.
So, from romantic to cynical, here are the strange stories behind 12 incredibly normal words...
1. Left Wing and Right Wing
In the French Assembly, after the revolution of 1789, supporters of the monarchy (i.e. conservatives) sat to the right of President and supporters of the new regime (i.e. progressives) sat to his left.
A literal meaning that became ideological.
2. Romance/Romantic
During the Middle Ages Latin was the language of the church, but not of regular people.
In France they spoke Old French, which originated in Latin but had changed a lot.
So it was called a "Romance" language because it wad descended from the Romans.
When it was finished 334 years ago they called it the Eighth Wonder of the World.
See, people usually associate Baroque Architecture with Europe, but some of the best Baroque is in Latin America...
Baroque is one of the most distinctive styles of architecture — you know it when you see it.
Extravagant, opulent, maximalist, and full of movement.
It is defined by curved rather than straight lines, lavish decorations, and large, open spaces.
If one motif sums up the Baroque it is probably the "Solomonic Column", a type of twisted column that became popular during the rise of Baroque Architecture.
They were used by Gianlorenzo Bernini for his colossal bronze baldachin at St Peter's Basilica, Rome: