Main Page
Welcome to the explain xkcd wiki!
We have an explanation for all 2205 xkcd comics,
and only 18
(1%) are incomplete. Help us finish them!
Latest comic
Types of Approximation 
Title text: It's not my fault I haven't had a chance to measure the curvature of this particular universe. 
Explanation
This explanation may be incomplete or incorrect: Created by an APPROXIMATOR. Please mention here why this explanation isn't complete. Do NOT delete this tag too soon. 
In physics and engineering, problem solving typically requires approximations, as physical properties of the universe can be difficult to model. For example, in introductory physics classes, theories are introduced in frictionless environments.
In the comic, Cueball, the physicist, generally dealing with straight math, is introducing a problem with the assumption that the particular curve is a (perfectly) circular arc with a radius represented by R. Megan, the engineer, also assumes that the curve is similar to a circle, with a deviation factor of 1/1000.
The joke arises when Ponytail, the cosmologist, uses the ridiculous approximation of pi equal to 1. In actuality, pi is an irrational number, usually truncated to 3.14. Choosing the value of pi as 1, or 10, as later suggested, completely defeats the purpose of pi for describing a circle. This is a parody of the tendency of cosmology to change the units of measure such that important constants (such as the speed of light or the gravitational constant) are equal to 1, which highly simplifies the formulas without compromising the math. In this case, the number pi is a dimensionless factor, not a directly measured quantity, which means the math will not work.
Transcript
This transcript is incomplete. Please help editing it! Thanks. 
[There are three panels, each labeled on top]
[Physicist Approximations]
 Cueball: We'll assume the curve of this rail is a circular arc with radius R.
[Engineer Approximations]
 Megan: Let's assume this curve deviates from a circle by no more than 1 part in 1,000.
[Cosmologist Approximations]
 Ponytail: Assume pi is one.
 Offpanel voice: Pretty sure it's bigger than that.
 Ponytail: OK, we can make it ten. Whatever.
Is this out of date?
.New here?
Last 7 days (Top 10) 


You can read a brief introduction about this wiki at explain xkcd. Feel free to sign up for an account and contribute to the wiki! We need explanations for comics, characters, themes and everything in between. If it is referenced in an xkcd web comic, it should be here.
 If you're new to wiki editing, see the explain xkcd:Editor FAQ for a specific guidance to this Wiki and the more general help on how to edit wiki pages. There's also a handy wikicode cheatsheet.
 Discussion about the wiki itself happens at the Community portal.
 You can browse the comics from List of all comics or by navigating the category tree at Category:Comics.
 There are incomplete explanations listed here. Feel free to help out by expanding them!
 We sell advertising space to pay for our server costs. To learn more, go here.
Rules
Don't be a jerk.
There are a lot of comics that don't have setinstone explanations; feel free to put multiple interpretations in the wiki page for each comic.
If you want to talk about a specific comic, use its discussion page.
Please only submit material directly related to (and helping everyone better understand) xkcd... and of course only submit material that can legally be posted (and freely edited). Offtopic or other inappropriate content is subject to removal or modification at admin discretion, and users who repeatedly post such content will be blocked.
If you need assistance from an admin, post a message to the Admin requests board.