Science, live

So why don't we feel it?

Because the speed is perfectly steady, and everything around you (the air, your room, the oceans) is moving with you at the same speed. Your body only feels changes in speed, like when a bus suddenly brakes. Close your eyes on a smooth flight and it feels like you're standing still. Same thing.

The four speeds don't simply add up, because they point in different directions. Each one answers a different question: how fast, compared to what?

Why does the Moon change shape?

Half of the Moon is always lit by the Sun. But as the Moon circles Earth, how much of that sunlit half faces us changes every night. See all of it: full moon. See none of it: new moon. One full cycle takes about29.5 days.

The first thin crescent after a new moon shows up low in the west just after sunset, usually one or two evenings later if the sky is clear.

Why does the distance change?

The Moon's orbit isn't a perfect circle; it's slightly stretched. So every month the Moon swings about31,000 miles (50,000 km) closer, then drifts away again. A full moon at the close end is what people call a "supermoon".

Why is the line curved, and why does it swing?

Earth is tilted 23.4° on its axis. In June the Northern Hemisphere leans toward the Sun: long days, more heat. In December it's the other way around. Press "Play a year" and watch the line rock back and forth like a pendulum. In March and September it stands straight up, and on those two days, day and night are almost equal everywhere on Earth.

How fast does the line move?

About 1,040 mph (1,670 km/h) at the equator, and about 790 mph (1,270 km/h) at New York's latitude. That's faster than a passenger jet: sunrise races across the planet from east to west. It reaches Maine about three and a half hours before it reaches California.

Looking up means looking back in time

The Sun you see is the Sun from 8 minutes ago: that's how long its light takes to get here. Even the light reaching your eyes tonight from the nearest star left it more than four years ago.

Why can't NASA drive a Mars rover with a joystick?

A command takes … to reach Mars today, and the reply takes just as long to come back. So NASA's rovers aren't driven live. Engineers send a whole day's to-do list at once, and the rover finds its own way.

Why doesn't the ISS fall down?

It does! It's falling toward Earth every single moment. But it's also flying sideways so fast (about5 miles, or 7.7 km, every second) that Earth's curved surface drops away just as fast as it falls. So it keeps falling around the planet instead of into it. That's what an orbit is.

Can you see it with your own eyes?

Yes: it looks like one of the brightest "stars" in the sky, gliding across without blinking. But only just after sunset or before sunrise, when your sky is dark but the Sun still shines on the ISS high above. The astronauts see about 16 sunrises every day.

Why is your age different on other planets?

"One year" just means one trip around the Sun. Mercury is so close to the Sun that it makes a lap in only 88 days, so there you'd be more than four times older. Saturn is so far out that one lap takes about 29½ years: you might not have had your first birthday there yet.

What's a light-year?

The distance light travels in one year: about 5.9 trillion miles (9.5 trillion km). So however many years old you are, the light from your birthday is that many light-years away. At 25, it passes the star Vega.

Are these real counts?

Not one by one: they're yearly totals divided into seconds. About 130 million babies are born each year, so on average that's about 4 every second. Any single moment can run a little higher or lower, but over a whole day the total lands very close.

Where does lightning strike most?

About 90% of lightning happens over land, mostly in hot, humid places: Central Africa, South America and Southeast Asia. In the US, Florida gets the most. When thunder roars, go indoors and stay away from open fields and water.