On 30/03/2017 20:00, Mark Carver wrote:
On 30/03/2017 19:39, Bill Wright wrote:
I'm hopeless at maths, but I'm trying to get something across to an
intelligent child. So, if the distance from here to London (250km) is
represented by the thickness of a hair (0.2mm) how far is a light year?
It doesn't have to be a hair; it could be anything familiar and small.

It's 2.5 million light years to the Andromeda Galaxy; that's what we're
discussing.

Speed of light, 299,792,458 m/s

Number of seconds in a year

60 x 60 x 24 x 365 = 31,536,000 seconds

Therefore in one year light travels 31.536 x 10^6 x 299.792458 x 10^6

= 9.4542547 x 10^15 metres

So in 2.5 years

2.3635637 x 10^16 metres

2.3635637 x 10^13 km

So 2.3635637 x 10^13 km / 250 x 10^3 = 9.4542548 x 10^7

Therefore if a human hair represents 250km, it's 94.5 million hairs to
Andromedia

It's much more than that.

A light year is 9.461*10^12 km

2.5 light years is 2.365*10^13 km.

Call it 2.5*10^13 km for ease of maths, which is 250*10^11 km.

Divide by 250 km (the distance to London) and you get:

2.5 light years is 10^11 times the distance to London. That's
100,000,000,000 or 100 billion times the distance to London.

Hairs are an irrelevant confusion. Forget them.