Convert roman mile to micrometer

Atoms and molecules are measured in ångströms, 0.1 nm: the Bohr radius of hydrogen is 0.53 Å, a carbon–carbon bond 1.54 Å, and X-rays have wavelengths of about 1 Å, which is why crystallographers keep the unit although it is not part of the SI (chemistry tables give the same radii in picometres, 53 pm). The nanometre is the scale of the DNA double helix (2 nm across), of viruses (20–300 nm) and of visible light (380–750 nm); a chip's '3 nm' process is a marketing name, its transistors are really tens of nanometres apart. Bacteria, red blood cells (7 µm) and hair (50–100 µm) belong to the micrometre, and the femtometre, or fermi, to the atomic nucleus: a proton is 1.7 fm across.

Roman lengths were built on the foot (pes, about 29.6 cm): 16 digiti (finger widths) or 4 palmi (palm widths) made a foot, 1½ feet a cubit, 5 feet a passus (a double step), 10 feet a pertica (perch) and 120 feet an actus, the length of a furrow. Longer distances counted in stadia of 625 feet and in mille passuum, a thousand double steps or 5,000 feet, which is where the word mile comes from; the Gallic leuga was 1½ Roman miles. Milestones along the Roman roads were set one mille passuum apart.