Viktor T. Toth:
You can look up the numbers easily enough: The astronomical unit (the average distance between Earth and Sun) is defined as exactly 149,597,870,700 meters, while the light is defined year as the distance the light travels in a Julian year (365.25 days), which means exactly 9,460,730,472,580,800 meters. The rest is a simple subdivision.
But instead of typing 149597870700/9460730472580800 into Google for you (yes, Google can serve as a calculator), let me show you how to estimate the answer to this question fairly accurately, without looking up anything or with any machine, as long as you remember two useful numbers.
It is not difficult to notice that the Earth-Sun distance is about 1
It is not too hard to remember that one light year is about 10,000 trillion meters; That is, the number 1 follow 16 zeros.
So 15, followed by 10 zeros divided by 1, followed by 16 zeros … this is the same as 15, followed by no zero divided by 1 followed by 6 zeros. or 15 parts in a million. And this answer is correct within 5.5 percent.
[The error is due mainly to the fact that the light year is overestimated by more than 5 percent, so we underestimated the ratio. “Boosting” that 15 parts in a million by an extra 5 percent gives 15.8 parts in a million, which is actually quite accurate. But at this point, you’re probably better off using a calculator after all.]
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