Why is my const value changing its value? (C)

Question

this is my first question here :)

I want a program that converts lighyears into kms. I defined a constant value for LIGHTYEARSINKM, but even if I just want to convert one lightyear in km, the compiler gives me a different value from the defined one.

For example if give the input 1, the output is 9460500135936.000000 (different from 9460500000000.000000)

What am i doing wrong?

Heres the code I wrote:

#include <stdio.h>
#define LIGHTYEARSINKM 9460500000000.000000

int main( void )
{
float lightyear;
float kilometer;

printf("Lightyears you want to convert into km:\n");
scanf("%f", &lightyear);

kilometer= lightyear * LIGHTYEARSINKM;

printf("%f lightyears are %f kilometer",lightyear,kilometer);

getchar(); 
return 0;
}