sentence: equivalence to other units of length - rowspan = '' 8 '' style = '' vertical-align : top ; '' 1 kilometre ≡ align = right metre - ≈ align = right feet - ≈ align = right yard - ≈ align = right 0.621 mile - ≈ align = right 0.540 nautical mile - ≈ align = right astronomical unit - ≈ align = right light-year - ≈ align = right parsec -rcb- history although the metre was formally defined in 1799 , the myriametre ( ) was preferred to the '' kilometre '' for everyday use .