What would sunset look like if you were on the planet HD209458b, a gas giant orbiting a star 150 light years away? According to exoplanetary scientist Frédéric Pont, it looks like this:

Isn’t that pretty? And there’s quite a bit of science in that, too.

First things first: HD209458 is a star pretty similar to our Sun. It was one of the first stars determined to have a planet orbiting it (way back in 1999) — the aforementioned HD209458b, nicknamed Osiris — and it turns out the planet’s orbit is so close to edge-on as seen from Earth that we see that planet passing directly in between us and that star once per orbit. When the planet transits that star the amount of light we see dips a little bit. From that we can get the period of the orbit and the size of the planet (a bigger planet blocks more light).

But we can get more, too. There’s a camera on board Hubble called the Space Telescope Imaging Spectrograph, or STIS. It can take the light from an object and break it up into thousands of separate narrowly sliced colors, called a spectrum. By analyzing that spectrum we can find out an astonishing amount of things about astronomical objects: their temperature, rotation, even their composition!

Shortly after HD209458b was discovered to be a transiting exoplanet, STIS was pointed at the star. The camera took hundreds of very short exposures during a transit in the hope of being able to detect the atmosphere of the planet. Osiris was known to be massive, about 70% as massive as Jupiter, so it most likely has a thick atmosphere. It also orbits so close to its parent star — 6.7 million km (4 million miles), much closer than Mercury orbits the Sun — that the heat from the star puffs the atmosphere up, making it easier to see.

In fact, the spectra did reveal the presence of an atmosphere; the first time the atmosphere of an alien planet was ever observed. Different elements and molecules absorb light at different colors, so in the spectrum there are dark spots where the planet’s air absorbs the light from the star behind it during a transit, and how dark that spot gets tells you how much light is absorbed.

It’s this information Prof. Pont used to create the image above (inspired by investigation and an animation done by Alain Lecavelier des Etangs). By knowing the color of the star itself, and using the way the planet’s atmosphere absorbs light, he created this image of the star using sophisticated computer modeling. (more…)

There’s a wonderful comedic quiz show in the UK called "QI" — for "Quite Interesting" — which is hosted by none other than Stephen Fry. The participants are comedians, and they’re asked questions ranging over just about every topic you can imagine. The BBC recently uploaded a clip about which alert BA Bloggee Brett Warburton informed me. In it, Fry shows the contestants a video of the Sun setting, and asks them to ring in when they think the Sun has completely set. Here’s the clip:

This is, in fact, correct! The Earth’s air bends the image of the Sun upward, so we can still see the Sun even though it is physically below the horizon. If we didn’t have air, daytime would be shorter. In fact, this effect works for sunrise as well, so we see the Sun rise before it’s physically cleared the horizon.

And Stephen was correct in the amount too; the light is bent upward by just about the same size as the Sun, so when the lower limb of the Sun just kisses the horizon it’s actually already set.

But it’s a bit more complicated, of course. (more…)

Every few months, like clockwork, someone sends me an email telling me about a lovely picture they’ve seen. This photo, it’s claimed, shows a sunset at the north pole with the crescent Moon looming hugely over the horizon. Perhaps you’ve seen it via email or a social network; the picture really is stunning, as you can see for yourself:

It is pretty, isn’t it? But it has one teeny tiny problem: It’s not a photograph! It’s a drawing, called “Hideaway”, created by Inga Nielsen. It’s a really, really good drawing, beautifully done, so realistic it can fool people into thinking it’s a photo. I’ve seen people thinking so on bulletin boards for years, and in fact it just popped up again, this time on Google+. And, as usual, a lot of folks thought it was real.

I can’t blame them, since it’s photo-realistic, as many digital drawings are these days. And if you see this without attribution to the artist, and don’t know the astronomy behind the scene, it’s hard to say whether it’s real or not.

So how can I tell it’s a drawing? Ah. Glad you asked.

Size does matter

Right away, the size of the Moon in the picture compared to the size of the Sun is a dead giveaway this isn’t a real photograph. In the real sky, the Moon and Sun appear to be the same size.

(more…)

[Note: ROSAT fell to Earth last night; see this post for details and links to more info.]

Time lapse videos can be breathtaking, lovely, and a joy to watch… but they can also show you something you may not have thought about before. Before I even read the caption for Murray Fredericks’ video called "IRIDIUM", I knew it was filmed in the southern hemisphere. Can you guess how?

[Make sure to watch it in HD, and make it full screen.]

If you live in the northern hemisphere — and odds are very good that you do — then you may have noticed the motion of the Sun and stars looked a bit odd. For example, as you watch the Sun set at the beginning of the video, it does so at an angle moving from the upper right to the lower left. The stars do too. When they rise, they move from the lower right to the upper left.

To me that’s backwards!

(more…)

I know this isn’t strictly astronomy, but it’s a lovely time lapse video of sunrises and sunsets in the arctic… and since the Earth is tilted, in the summer the Sun skims just below the horizon as it circles the sky. So hey, I guess it is astronomy related!

You can really see this effect well starting at 1:06 into the video; the Sun sets at a very low angle to the ground, and comes right back up. The sky never gets completely dark.

The video was shot by a man who calls himself Mr. TSO, and he has many other lovely videos on Vimeo. It’s Friday, so take some time and look. You’ll breathe just a little bit easier if you do.

If we didn’t go into space, we’d never see such beauty as this:

Sunset over Bolivia, as seen by astronauts aboard the International Space Station. Click it to get the glorious full size version.

And you know, this isn’t the reason to go into space, but it’s not a bad one. Not a bad one at all.

Wow.

I know, it may not look like much, but think about what you’re seeing: a sunset on another world. And those images were taken by a robotic probe that took years to design and build, months to travel the hundreds of millions of kilometers to get to Mars, a harrowing few minutes to descend on a breath of fire through the thin air to land on the surface, and then nearly seven years to travel the landscape long, long past its design specifications.

All that, plus all the amazing science, exploration, and discovery done by Opportunity and its sister rover Spirit… and yet, it’s sometimes the stark beauty of simple things like this that remind us that we have, at least by proxy, placed our feet on other worlds.

I know there are worries here on Earth. But when I see something like this, I remember that the good we do, the awe we feel, and the inspiration we can generate are mighty.

Tip of the pancam to Emily Lakdawalla on Twitter.