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Sirius Alexius Black (littlesirius) wrote in compass_comm,
@ 2022-02-03 14:13:00

Entry tags:

tom sully jr, ~robert langdon, ~sirius black (1845)

[Radio]

Hello! This is Sirius. Can anyone help me please? I am doing my arithmancy prep and one question is why the number 3.14159 has special magic properties. I know the properties of 3, but not 3.14159. It is a strange number to pick.

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	tom_jr 2022-02-03 02:53 pm UTC (link)
	Hi, Sirius. My name's Tommy. That number's got its own name, did you know that? (Reply to this) (Thread)

	littlesirius 2022-02-03 02:56 pm UTC (link)
	Hello Mr Tommy! I didn't know that. Is that why it is special, because it has a name? (Reply to this) (Parent) (Thread)

	tom_jr 2022-02-03 03:03 pm UTC (link)
	I think it's the other way round. It's special, so it got a name. It's called Pi, which is one of the letters in the Greek alphabet, and it's really important for when you're working with circles. Tell me if you know what any of these three words are, okay? Radius, diameter, and circumference. And then I can tell you what Pi has to do with them. (Reply to this) (Parent) (Thread)

	littlesirius 2022-02-03 03:14 pm UTC (link)
	Oh! Pie is a funny name for a number. Um...the next chapter in my book is about circles! It is called Practical Geometry. But I didn't read it yet because Uncle Arcturus and I are going to read it together next time we study. Diameter is how thick something is but I don't think I know the other ones you said. (Reply to this) (Parent) (Thread)

tom_jr
2022-02-03 03:22 pm UTC (link)

That's true, it is pretty funny. I will tell you something pretty cool about Pi and Pie when we're finished here, okay?

And you're right about diameter. It's the thickness of a circle. If you were to draw a circle, and then draw a straight line through it that starts at one side of your circle, passes through the center, and then stops at the other side of the circle, then the length of that line is the diameter of that circle. The radius is the length of the line from the center out to the edge, so it's half the length of the diameter. And the circumference of the circle is the length of the line that would go round the outside of the circle. So, if you rolled your circle once along the ground like a big hoop, that distance is the circumference. Are you with me so far?

(Reply to this) (Parent) (Thread)

	littlesirius 2022-02-03 03:33 pm UTC (link)
	Um...yes sir I think so but I am not sure what the pie number is for. It isn't the...radius number or the others because you can draw circles that are all different sizes so the lines won't be the same length. (Reply to this) (Parent) (Thread)

	tom_jr 2022-02-03 03:40 pm UTC (link)
	It isn't, no. What Pi is, is the number you will always get if you divide the circumference of any circle by its diameter. Always. It doesn't matter how big or how tiny the circle is. It's what's known as a constant. (Reply to this) (Parent) (Thread)

	littlesirius 2022-02-03 03:48 pm UTC (link)
	Always? Then that's the answer! That is why it's special. If you want to know the...circumfrence do you have to make a circle out of string and measure it first? (Reply to this) (Parent) (Thread)

	tom_jr 2022-02-03 03:57 pm UTC (link)
	Correct! That's great, Sirius, well done. If you know the radius or the diameter, then you don't need to make a string circle, no. If you know the diameter, then you multiply it by Pi, and you get your circumference. And if you know the radius, how do you think you would work it out? (Reply to this) (Parent) (Thread)

	littlesirius 2022-02-03 04:08 pm UTC (link)
	Thank you sir! If you know the radius...you double it because that's what the diameter is? And then you multiply it. But if the diameter is not an easy number that is going to be a VERY long multiplication! (Reply to this) (Parent) (Thread)

	tom_jr 2022-02-03 04:14 pm UTC (link)
	You've got it! Awesome job. It is, yeah. Did you know that some people celebrate Pi day? This is the cool thing I was going to tell you about. (Reply to this) (Parent) (Thread)

	littlesirius 2022-02-03 04:20 pm UTC (link)
	Really? When is it? What do they do? (Reply to this) (Parent) (Thread)

	tom_jr 2022-02-03 04:24 pm UTC (link)
	The first 3 digits are 3.14, right? So on March 14 - since March is the third month - people pick their favorite pie, and eat the full thing. Or maybe just a slice or two. Because it's Pi day. My dad loved all kinds of pie, so he was a big fan of Pi day. One year, I got him a pecan pie in the shape of the Pi symbol. So I think we should do that this year. (Reply to this) (Parent) (Thread)

	littlesirius 2022-02-03 04:33 pm UTC (link)
	But that is backwards! You should eat a pie on the third of January at 4.15, and then it would be teatime. I have never had a pecan pie but I like apple pie, maybe we can have that too? (Reply to this) (Parent) (Thread)

	tom_jr 2022-02-03 04:35 pm UTC (link)
	...Pi day 1 and Pi day 2? Yes. Yes, we can have apple pie too. (Reply to this) (Parent) (Thread)

	littlesirius 2022-02-03 04:40 pm UTC (link)
	Yes I suppose so! Do you have an elf to make all the pies? (Reply to this) (Parent) (Thread)

	tom_jr 2022-02-03 04:42 pm UTC (link)
	No, we just make them ourselves or else ask Lacey or Bucky or Maryanne for pie. And they're not elves. (Reply to this) (Parent) (Thread)

	littlesirius 2022-02-03 04:47 pm UTC (link)
	Oh okay. Maybe I will ask somebody. (Reply to this) (Parent)

	symbolism_ 2022-02-03 04:43 pm UTC (link)
	Oh, I am so down for this. (Reply to this) (Parent) (Thread)

	tom_jr 2022-02-03 04:55 pm UTC (link)
	Great! (Reply to this) (Parent)

	symbolism_ 2022-02-03 04:07 pm UTC (link)
	Oh, pi is great fun. I once memorised the first 417 characters for a competition. (Reply to this) (Thread)

	littlesirius 2022-02-03 04:12 pm UTC (link)
	Um...I don't understand, sir. Are there lots of different pies? 3.14159 is not tricky to memorise. (Reply to this) (Parent) (Thread)

	symbolism_ 2022-02-03 04:20 pm UTC (link)
	Pi is what you call an irrational number. It presumably has a place value, because it's more than 3.1, and it's less than 3.2. It's value is finite, but it's decimal representation is infinite. So, it's irrational. (Reply to this) (Parent) (Thread)

	littlesirius 2022-02-03 04:27 pm UTC (link)
	That is interesting sir. I did not know that a number could be irrational. Maybe that means you have to be more careful with how you use it magically. (Reply to this) (Parent) (Thread)

	symbolism_ 2022-02-03 04:33 pm UTC (link)
	Yeah, there are a few. Pi, Euler's number, the golden ratio... a lot of square roots are irrational, but not all of them. I'm not sure about using it magically. Perhaps because it is difficult to be precise with it, and I assume precision is important in magic. (Reply to this) (Parent) (Thread)

	littlesirius 2022-02-03 04:44 pm UTC (link)
	Yes it is very very important to be precise, but lots of spells involve circles and the pie is in them even if you didn't put it there on purpose. But spells with an arithmantical property [he pronounces this out slowly and carefully, trying not to stumble over it] of being irrational might be harder to get right. (Reply to this) (Parent)