Explain xkcd: It's 'cause you're dumb.
While searching for extrasolar planets this gullible astronomer is looking at a reflection of the Earth itself. He's very excited because he found a planet in a star's habitable zone, with visible oceans and weather. It is presumably quite likely to have life on it, which would be the first discovery many astronomers are looking for.
The title text goes on and says that the planet under observation has a mirror where the telescope is pointing on, so it's just a hint that the astronomer is actually only viewing the Earth. But even pointing to a mirror at a distance of the moon would require a real huge one, probably more than one hundred kilometer in diameter.
- [Cueball stands in front of a huge telescope, looking through the eyepiece.]
- Cueball: I've discovered an Earth-sized planet in a star's habitable zone! It even has oceans! And visible weather!
- To mess with an astronomer, put a mirror in the path of their telescope.
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- Searching for extrasolar planets is still a hard job, so even the results from the Kepler mission are only classified as candidates. The findings still have to be confirmed by other (mostly earth based) telescopes.
- As in July 2013 there are no earth-sized planets confirmed, habitable or not.
Even if you placed the mirror in Space, it would be incredibly obvious what is going on. I don't think this would work. 184.108.40.206 06:56, 28 June 2013 (UTC)
- For this trick to work, the mirror would need to be placed AT LEAST two light years away and be at least 1AU big. Somehow I don't think this is worth it. Alternatively, you need more complicated optical system which would not only mirror Earth, but also create illusion it's further away. I still think such system would be more costly to build that ISS. Or ... well ... you could put an LCD display directly over the telescope. That's doable, cheap and as a bonus you can display planets from sci-fi there. -- Hkmaly (talk) 08:44, 28 June 2013 (UTC)
- The mirror could be much smaller and closer if it's convex. PS. I don't know the 'rules' for posting, so apologies if I'm doing it wrong.220.127.116.11 13:46, 1 July 2013 (UTC)
- Under Hkmaly's initial proposal, the astronomer would have to make two observations, 4 years apart, in order to see the "other" telescope. Elsbree (talk) 07:13, 29 June 2013 (UTC)
Since when do we have terrestrial telescopes that can directly resolve exoplanets? I think we're still at the stage where we get excited by troughs in light curves EDIT: TIL that there are specific techniques for exactly that: Nulling interferometry and Vortex coronagraphs. Still, they may work for hot Jupiters, but don't think we can detect Goldilocks exoplanets from the ground yet; much less see oceans and visible weather. 18.104.22.168 09:14, 28 June 2013 (UTC)
- You can't detect them from the ground, but you are invited use your pattern-recognition skills to detect planets by examining the images sent back by the Kepler telescope. It's part of the citizen-scientist project. Go to http://www.PlanetHunters.org .CoderLass (talk) 21:14, 10 July 2013 (UTC)
My first thought was that you need to point the mirror so that it's aimed perfectly at the Earth. Then, I realized that you can use a corner reflector so that the aim doesn't have to be precise at all. Then, I came to the following realization: what if a significant portion of the stars we see are simply reflections of our own solar system due to a massive prank done by aliens? 22.214.171.124 15:22, 28 June 2013 (UTC)
- Or all of them? Of course including additional variable features like red shift. So they were right! Forever alone... --Kronf (talk) 16:31, 28 June 2013 (UTC)
- Where would these aliens reside? Either we're pranking ourselves, or there are other stars. 126.96.36.199 17:57, 28 June 2013 (UTC)
- What if there's a mirage-like effect in space, that causes light rays to mirror back to us with some variability, maybe different sizes, shapes, colors, and the universe is actuallly quite small? I mean, other than light, do we seriously detect gravity and other stuff out there (other than the visible effects of those properties on other stuff we see)? 188.8.131.52 06:17, 29 June 2013 (UTC)
- If the universe is curved spherically, something similar to this would actually happen. http://en.wikipedia.org/wiki/Shape_of_the_Universe 184.108.40.206 16:28, 29 June 2013 (UTC)
Wouldn't reflected light make the mirror extremely bright and impossible to view directly? 220.127.116.11 (talk) (please sign your comments with ~~~~)
- Depends on how far the aliens decide to put the mirror. Light gets weaker with distance, which is the same reason that distant stars (many of which are brighter than our sun) don't overwhelm us with light. Also...what if the sun is merely a reflection of something? 18.104.22.168 16:28, 29 June 2013 (UTC)
A German (satirical) newspaper page had an article once, how NASA discovered a habitable planet zero light years away from Earth after they rotated the Hubble space telescope . --Chtz (talk) 22:18, 10 July 2013 (UTC)
- Please translate this. It's a great joke, but most people here will not understand. And: It's not a newspaper.--Dgbrt (talk) 22:31, 10 July 2013 (UTC)
- That was already the main part of the joke. If someone is interested in the rest, please use your favorite online translator. (Claiming to be a "newspaper" is also a joke, the same as claiming to exist since 1845) --Chtz (talk) 10:35, 11 July 2013 (UTC)
- I've translated it. --Kronf (talk) 12:55, 2 August 2013 (UTC)