Have you ever seen a bright star glide silently across the evening sky and then fade away? There's a good chance it was the Space Station, and this is the path it weaves around our planet in just three days.

Somewhere in tonight's sky there is a star whose light set off the year someone you love was born, and it is only now reaching you.

Do you understand what a light-year is? Here's a simple explanation. Light is the fastest thing there is. It goes 300,000 km every second, and in one year it covers about 9.46 trillion km. We call that distance a light-year. So a light-year is not an amount of time. It's a way of measuring distances that are too big for kilometres. Sirius, the brightest star you can see at night, is 8.6 light-years away. That means its light needs 8.6 years to reach you. Any time you look at Sirius, the light hitting your eyes left the star 8.6 years ago. You're not seeing the star as it is now. You're seeing it as it was back then.

You've probably seen this ellipse trick before: everything that leaves one focus bounces off the curve and arrives at the other focus at the same instant. Since 1980, doctors have used it to break kidney stones without a single cut. The patient lies on a dish of water shaped like half an ellipse. A spark at one focus sends out a shock wave, the dish bounces it back, and every part of it arrives at the other focus at once, right where the stone is. For a millionth of a second the stone feels about 300 times the pressure of the air. After about 2,000 pulses, around half an hour, the stone is gravel and sand, and pieces under 2 mm can pass out of the body naturally.

Earth isn't alone in its orbit. Five real asteroids share our path around the Sun, and if you hold Earth still, each one draws its own hidden shape. Here are the next 400 years, simulated from NASA's real orbits.

Go outside before dawn this week and you may see a shooting star. It's a speck of dust from Halley's Comet. Every 76 years it sheds dust as it passes the Sun, and over thousands of years that dust has become a river around the Sun. Every October, Earth flies through it.

I let a single pebble loose in a lotus-shaped mirror and made it split in two every time it hit a glowing point. Watch it all the way to 1,024 pebbles to see the beautiful result. 🪷

At some point, our Milky Way is likely to crash into Andromeda, our nearest big neighbour, and merge with it into one giant galaxy. This is a simulation of what's likely to happen. Our Sun is the orange dot, so watch where it ends up.

Everyone's talking about #PHP2Shell and I'm completely lost. Can someone explain it like I'm five? Is it really as bad as Heartbleed, and do normal website owners need to patch right now? #PHP #cybersecurity

You can't fold an ordinary sheet of paper in half more than about 7 or 8 times, however hard you try. Every fold doubles its thickness and halves its size, so after a few folds you're trying to bend a tiny block that's thicker than it is wide. But what if you could keep going? If you could fold that same sheet 42 times, how thick do you think it would be? Make your guess, then watch to the end.

For 4.6 billion years, our Sun has been carrying us around the Milky Way at more than 800,000 km an hour. One lap takes about 220 million years, and so far it has gone around about 20 times. This one makes me feel so small. Everything that has ever happened, every moment in history, every great person who ever lived, even the dinosaurs, all of it happened on that one tiny speck. And it makes me wonder: where is heaven in all of this? 🤔

I put 640 metronomes on one platform, each playing its own note, and started them all at once in total chaos. Watch what happens when they all sync in this physics simulation. 🔊