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| Planet Database |
 Title text: Atmospheric transmission spectroscopy has revealed that the sunsets from the observatory are really pretty. |
Explanation
Cueball and Megan are browsing through a list of potentially habitable planets. Planetary habitability depends on many factors, including having a temperature range in which water is a liquid, having a solid surface, and having an atmosphere.
The punch line comes when Megan finds a planet with the correct habitable factors. Alas, she is looking at our Earth, which orbits our Sun (which is also called Sol). This is similar to 1231: Habitable Zone, where an astronomer fails to realize that he is looking at (a reflection of) Earth before he gets excited about its habitability. (Megan could also be looking at Venus, which is within the Sun's habitable zone, has a mass 81% that of Earth—which rounds to 1 Earth mass—and has an atmosphere. However, Venus is decidedly not habitable.)
The title text expands on this joke by saying that the observatory is looking at sunsets instead of planets. It's not clear if this is being done with the observatory's instruments or just with people's eyes, which act spectroscopically since we can see different colors. Light at sunset (or sunrise) travels through more of the atmosphere before reaching the ground, so more of its blue color gets scattered and we see more red and orange colors — making it colorful, and pretty.
Exoplanets are a recurring theme in xkcd, and the habitability or environment of other planets was also mentioned in 1298: Exoplanet Neighborhood, 1456: On the Moon, 1517: Spectroscopy, 1555: Exoplanet Names 2, 1752: Interplanetary Experience, 1991: Research Areas by Size and Countedness, 2202: Earth-Like Exoplanet, 2762: Diffraction Spikes, and 3103: Exoplanet System.
Transcript
- [Megan is sitting at her desk looking at a laptop. Cueball stands behind her.]
- Megan: I'm browsing the list of potentially habitable planets.
- Megan: This one is promising!
- Megan: Wait, no, the surface is thousands of degrees.
- [Similar panel.]
- Megan: This one could be a water planet!
- Megan: ... But it's not dense enough to be solid.
- Cueball: Yeah, that's a mini-Neptune.
- [Megan points at the laptop screen.]
- Megan: This one could be habitable if it has a greenhouse effect!
- Megan: Sadly, spectroscopy shows no atmosphere.
- Cueball: Aww.
- Megan: Here's one in the habitable zone, 1 Earth mass, with an atmosphere!
- Cueball: Ooh, what star does it orbit?
- Megan: ...Sol.
- Cueball: Dammit.
This Week's Explanation for Improvement
As part of the Explanation Improvement Project, each week the community selects one older, stagnant, or otherwise neglected explanation to expand and polish together. This week's pick is comic 430: Every Damn Morning, you can help us by editing its explanation!
To get involved with the project, you can also nominate an explanation that you think should be improved, or vote on current nominations!
| Every Damn Morning |
 Title text: There was something about a cup and a sword and a tree and a green hill... |
Explanation
|
Welcome to This Week's Explanation for Improvement! Anyone is encouraged to help explain comic 430: Every Damn Morning; click here to get started! If you are new to wiki editing, you can read our editing FAQ for guidance. Here are some suggestions for improving this explanation: .svg){kind=icon} Explain the various occurrences in the dream! Could they be references to other comics?
.svg){kind=icon} Why do humans forget their dreams so quickly?
.svg){kind=icon} Improve the #Transcript, it's not descriptive enough.
|
Cueball has had a wondrous and striking dream (seemingly involving various action-packed escapades with Megan and/or on board a Zeppelin), including, among other things, a swordfight and a nuclear detonation, but is unable to recount it to Megan, as he has forgotten it by the time he reaches her at the table downstairs, indicated by the dream fading and losing detail.
The title of the strip suggests that this is a regular occurrence: he wakes up with vivid memories of elaborate and fascinating dreamscapes, but is unable to tell anyone about it, because the memories fade so quickly, he's lost almost everything before he gets the chance. The title text is a reference to C. S. Lewis's novel The Voyage of the Dawn Treader, in which Lucy Pevensie reads a story that “refreshes her spirit” – “That is the loveliest story I’ve ever read or ever shall read in my whole life. Oh, I wish I could have gone on reading it for ten years.” – but afterwards can only remember that it had something to do with "a cup and a sword and a tree and a green hill." A cup and a sword are also present near the center of the dream, with a large mushroom cloud in the background that could be misinterpreted upon recollection as a tree; these items possibly serving as another reference and/or providing further context to the title text.
Most people have trouble remembering the majority of their dreams, or only remember the most significant or recent parts. A contributor to this phenomenon is that the ability to store long-term memories is largely switched off during REM sleep, meaning that experiences within dreams typically only last from a few minutes to seconds in memory. Furthermore, levels of norepinephrine, a neurotransmitter critical for attention and memory formation, also drop to near zero during REM sleep. Without this chemical signal, the brain cannot encode experiences normally. There are also theories that the brain naturally discards memories from dreams so that people do not confuse those mental fabrications with real-life events.
The various parts of the dream are common in Randall's other comics. Zeppelins were seen before in 73: Zeppelin and 288: Elevator, and continue to be a recurring theme in xkcd.
Another effort to remember dreams was made in 269: TCMP.
Transcript
- [In the background, a vivid dream scene is apparent, including mountains, a city with a mushroom cloud, a Zeppelin where one person has knocked another off, another person climbing on or off the blimp on a rope, another blimp anchoring to a city in the background, one Cueball and Megan holding hands, one Cueball looking at the person on the rope, one Megan using a sword to pin down a Cueball over a cup of spilled liquid, another set looking at a laptop, another set hugging, another set sliding down a separate rope from the blimp and a last set falling. In the far right there is a third blimp flying over a mountain range. All in all, this seems very much like an apocalypse and not something one would forget easily. In the inset, Cueball awakens, very surprised.]
- Cueball: !!!
- [Dream's edges are fading, mountains, city, and Zeppelin less clear. In the inset, Cueball is seen running down stairs.]
- [Zeppelin has faded out of view; city, and mountains are very faded and unclear. The people can still be seen. In the inset, Cueball gets the attention of Megan, who is sitting at a breakfast table.]
- [The dream has almost completely faded; only the outlines of three people can still be seen. In the inset, Cueball looks confused while Megan picks up the cup in front of her.]
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