What gives rainbows their curved shape?

We see part of that ring as the curved arc of a rainbow. Here’s how it works: the sunlight that shines into a raindrop leaves that raindrop at an angle of 138 degrees from the path that the light traveled before it entered the drop. That’s the “rainbow angle,” discovered by Descartes in the year 1637.

If the light left at 180 degrees, it’d head straight back toward the sun. As it is, the light is traveling in a direction somewhat back toward the sun, which is why you always see rainbows when the sun is behind you.

The sunlight emerges from many raindrops at once. The combined effect is a mosaic of light, spread out in an arc in the sky. But that’s not the end of the story. When sky conditions and your vantage point are perfect, the rain and sun work together in this way to create a complete ring of light – a circle rainbow. You’ll never see this from Earth’s surface because your horizon gets in the way. Source: Earth and Sky

 

What would happen to an apple on the surface of Mars?

If an astronaut carried an apple to the surface of Mars, and left the apple sitting on the martian surface, what would happen to it? It’s a striking vision: a bright green or red juicy apple against the barren red rocks of Mars. But an apple on the martian surface would shrivel like a raisin in a matter of minutes.

Its juices would boil away into vapor almost immediately. With its liquid gone, the apple would essentially become mummified.

What’s more, Mars is colder than Earth. That dried-out apple on Mars would freeze. Soon, you’d have a freeze-dried mummy of an apple.

But here’s the good news, apple lovers. The apple wouldn’t become rotten. You need bacteria to enable something to rot, and there are no bacteria on Mars.

On the other hand, Mars does have a lot of wind. So the apple might be buried by blowing dust. In that case, the martian soil would corrode the apple – in about a million years.

If the apple didn’t get buried in a martian windstorm, it’d be exposed to intense ultraviolet radiation from the sun. That would turn the apple’s skin black and tarry. But, underneath its blackened skin, the freeze-dried apple would be unchanged.

So you could come back a thousand years later, brush off the dust – or scrape off the tar – and eat the apple. Yummy!

And that’s the fate of an apple left behind by a future astronaut on the surface of the planet Mars.Image Credit: USDA

 

Can I see our galaxy from Earth?

Yes. It’s a rare sight nowadays, but a trip to the country at the right time of year can bring you a beautiful view of the starlit trail of our Milky Way galaxy.

The edgewise view into the galaxy translates in our sky as a starlit trail – like a river of stars stretched from one horizon to another. The galaxy is all around us in space, but some parts of it appear brighter than others. The most dramatic place to look is toward the center of the galaxy – toward what we see as the constellation Sagittarius.

This part of the sky comes into view each year before dawn in early March – and rides high in the sky each evening in July and August – only to be behind the sun again by New Year’s, all as Earth orbits the sun. Under a dark, clear sky – when the galaxy’s center rides highest in the sky, you’ll recognize it as a broad, bright part of the starlit trail of the Milky Way. It looks almost like billowy clouds of steam.

We can only look toward the galaxy’s center. We can’t see the exact center because it’s obscured by dark curtains of interstellar dust. So when you look at this broadest part of the Milky Way – in the direction of the constellation Sagittarius – you’re not looking at the actual core of the Milky Way. Instead, you’re gazing at part of one of the prominent spiral arms of our galaxy, called the Sagittarius Arm.

In this direction, you’ll see this spiral arm of the galaxy studded with countless bright stars, clusters, and nebulae. Just remember, dust prevents us from looking directly at the galaxy’s center, which lies 27,000 light-years away. Earth and Sky

 

Why do stars seem brighter in winter?

As seen from the Northern Hemisphere, the stars seem brighter in winter. Why? It’s because – as seen from this hemisphere – we’re actually looking toward many, many more stars in summer than in winter.

In summer, our evening sky is facing toward the center of the Milky Way galaxy – some 25,000 to 28,000 light-years away. We don’t see into the exact center because it’s obscured by galactic dust. But, as we peer in summer edgewise into the galaxy’s disk, the hazy quality of the summer sky is really the combined light of billions of stars in the direction of the galaxy’s center.

In winter, we’re looking the opposite way – into the spiral arm of the galaxy in which our sun resides. The winter stars tend to be closer to us – and there really are some gigantic stars located in this direction. We’re looking edgewise into the disk of the galaxy in winter, too. But we’re looking toward the outskirts of the galaxy – so we’re seeing far fewer stars, and we’re looking more deeply into the space beyond our galaxy’s boundaries. That’s why the winter sky has a clearer, sharper quality than the summer sky.Earth and Sky

 

Stars twinkle. Planets shine steadily. Why?

Stars always twinkle because they’re so far away from Earth that, even through large telescopes, they appear only as pinpoints. And it’s easy for Earth’s atmosphere to disturb the pinpoint light of a star.

As a star’s light pierces our atmosphere, each single stream of starlight is forced by the atmosphere to zig and zag this way and that. . . . and so stars appear to twinkle. On the other hand, planets don’t twinkle (usually) simply because they’re closer to Earth. You’d know they’re closer if you looked through a telescope. Through telescopes, planets don’t look like pinpoints. Instead, they look like tiny disks. And while the light from one edge of a planet’s disk might be forced to “zig” by Earth’s atmosphere, light from the opposite edge of the disk might “zag” in an opposite way. The zigs and zags cancel each other out . . . and that’s why planets appear to shine steadily.

It’s pretty tough to figure out which objects are stars and which are planets just by looking for the twinklers vs the non-twinklers. But if you can recognize a planet in some other way, you might notice the steadiness of its light by contrasting it to a nearby star.

By the way, if you could see them both stars and planets from outer space, both would shine steadily. There’d be no atmosphere to disturb the steady streaming of their light.

What’s more – while it’s true that, for the most part, planets don’t twinkle – you might see them twinkling a little if you spot them low in the sky. That’s because, in the direction of any horizon, you’re looking through more atmosphere than when you look overhead. Even planets can’t withstand too much atmosphere, because it’s the atmosphere that makes them twinkle! Earth and Sky

Why can’t we feel Earth’s spin?

You don’t feel the Earth spin because you, the atmosphere, skyscrapers, and everything else are spinning along with the Earth at the same constant speed.

It’s the same sensation as when you’re riding in a car or flying in a plane – as long as the ride is going smoothly. A jumbo jet flies at about 500 miles per hour – that’s about 800 kilometers an hour – about half as fast as the Earth spins at its equator.

But if you close your eyes, you don’t feel like you’re moving at all. And when the flight attendant comes by and pours coffee into your cup, the coffee doesn’t fly to the back of the plane. That’s because the coffee, the cup and you are all moving at the same rate as the plane.

Likewise, Earth is moving at a fixed rate – and we’re all moving along with it. Now imagine being on the jumbo jet again – think about what happens when the pilot suddenly speeds up or slows down the plane. You sometimes sense this change as a feeling of being pushed into your seat. In the same way, if the Earth were suddenly to speed up or slow down, you would definitely feel it.

But as long as Earth spins steadily – and moves at a constant rate in orbit around the sun – you as an earthly passenger move right along with it.

If the earth suddenly started to speed up we’d fall over backwards, and we’d have to lean into the direction of the motion to stand. If the earth were speed up enough that it spun once every hour and twenty minutes, we would fly off its surface. The next time you’re in a car or a plane traveling at a constant speed, close your eyes and try to feel that you’re moving. You won’t be able to tell.

Ancient humans noticed that the stars, and the sun and the moon all appeared to move above the earth. Because these people couldn’t feel the earth move, they logically interpreted this observation to mean that the earth was stationary and the “heavens” actually physically moved above us.

With the exception of the beliefs of ancient Greek scientist Aristarchus, who first proposed a heliocentric (“Sun centered) model of the universe, this geocentric (“Earth-centered”) idea was upheld for a long time. It would be Copernicus’s 16th Century heliocentric model that, although it was not without errors, would eventually convince the world that the earth spun on its own axis and that it moved around the Sun. Earth and Sky