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# If someone says "Within a radius of 100km", what does he actually mean?

Is it like 100km from a fixed point in all directions (giving a diameter of 200km), or is "within a radius of 100km" basically an area with a diameter of 100km?

Thank you for the answers. This question was triggered by this article:

https://www.sciencealert.com/the-most-accurate-mea...

Which basically said "Within a radius of 129,000 light years". But would this not make our Milky Way galaxy 258,000 light years in diameter? That seems not right? It is said only to be around 170,000-200,000 light years in diameter. So, this is kind of confusing.

### 9 Answers

- Anonymous9 months ago
A radius is NOT a diameter!

100 km radius is 200 km diameter.

IF you put the center of a 200 km circle on your location, THEN "within a radius of 100 km" would mean anywhere WITHIN (inside) of the 200 km diameter circle.

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- oubaasLv 79 months ago
Usually it is like 100km from a fixed point in all directions (giving a diameter of 200km), very very seldom it could refer to a spheric volume

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- busterwasmycatLv 79 months ago
Inside a circle that is centered around a location, so everything inside that circle is within 100 km of the center. Nothing (being discussed) is further than 100 km from that location. The furthest away that any two places in that circle could be, would be 200 km, as that would be the diameter of the circle (the longest line you can fit inside the circle).

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- billrussell42Lv 79 months ago
100km from a fixed point in all directions (giving a diameter of 200km)

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- DavidLv 79 months ago
Within the bounds of a circle that has 200*pi circumference.

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- husoskiLv 79 months ago
What that means is "less than 100km away".

A mathematical way to say the same thing is "inside a circle of radius 100km". This is almost certainly the origin of the common phrase "within

a radius of".

The center of the circle is the "fixed point" that you are measuring distance away from.

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- az_lenderLv 79 months ago
Your first interpretation is the right one.

"Radius" means RADIUS.

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- Anonymous9 months ago
The former. A circle all around one point with a radius of 100km, therefore a 200km diameter.

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