It's about six-and-a-half times as long as The Mississippi River
In other words, 21,000,000 ells is 6.3740 times the length of The Mississippi River, and the length of The Mississippi River is 0.15690 times that amount.(from Lake Itasca, Minnesota through Pilottown, Plaquemines Parish, Louisiana)
The total length of the Mississippi River is 3,295,000 ells. Prior to the development of soil erosion programs, the Mississippi River would carry approximately 400 million metric tons of sediment from its banks to the Gulf of Mexico every year.
It's about seven times as long as The Volga
In other words, 21,000,000 ells is 6.80 times the length of The Volga, and the length of The Volga is 0.1470 times that amount.(a.k.a. Волга, a.k.a. İdel (Идел), a.k.a. Атăл (Atăl), a.k.a. Edil, a.k.a. İdil, a.k.a. Рав (Rav)) (from the Valdai Hills, Tver Oblast, Russia through the Caspian Sea)
The largest river in Europe, the Volga river runs 3,090,000 ells in total length. World War II's Battle of Stalingrad between the Nazi Germany and the Soviet Union, which resulted in nearly two million casualties, was fought in part for control of the river.
It's about seven times as long as The Oregon Trail
In other words, 21,000,000 ells is 6.880 times the length of The Oregon Trail, and the length of The Oregon Trail is 0.1450 times that amount.(from Independence, Missouri to Oregon City, Oregon)
The historic Oregon Trail stretched 3,050,000 ells across five (sometimes six) states in the western United States. In the early years of the Trail's existence, the trip was estimated to have taken an average of 160 days.
It's about eight-and-a-half times as long as The Danube
In other words, 21,000,000 ells is 8.420 times the length of The Danube, and the length of The Danube is 0.1190 times that amount.(a.k.a. Donau, a.k.a. Dunaj, a.k.a. Dunav, a.k.a. Duna, a.k.a. Дунав (Dunav), a.k.a. Dunărea, a.k.a. Дунáй (Dunay), a.k.a. Tuna) (from Breg Martinskapelle, Black Forest, Germany through the Danube Delta, Romania)
The Danube river runs 2,490,000 ells in total length. Intersecting Vienna, Austria; Bratislava, Slovakia; Budapest, Hungary; and Belgrade, Serbia, the Danube touches more capital cities than any other river.
It's about ten-and-a-half times as long as The Colorado River
In other words, the length of The Colorado River is 0.0971 times 21,000,000 ells.(a.k.a. Aha Kwahwat, a.k.a. Grand River, a.k.a. Tizon River, a.k.a. Tecon River, a.k.a. Tison River) (from Grand Lake, Colorado through the Gulf of California)
The Colorado River runs 2,040,000 ells in length. Along this length, the river descends from its source, approximately 2,410 ells above sea level in the Rocky Mountains, to its mouth in the Gulf of California.
It's about one-fifteenth as long as The Distance from Earth to the Moon
In other words, 21,000,000 ells is 0.06243684 times the length of The Distance from Earth to the Moon, and the length of The Distance from Earth to the Moon is 16.01619 times that amount.(a.k.a. Luna) (median of semi-major axis, or apigee, and semi-minor axes, or perigee)
The orbit of the Moon around the Earth is elliptical, with an average distance of 336,339,900 ells from center-to-center. About 428.376 kg of moon rock have been cataloged on the Earth, from the American Apollo and Soviet Luna Programs, and from samples that have been extracted from meteorites.
It's about eighteen times as long as The Rhine
In other words, 21,000,000 ells is 18.20 times the length of The Rhine, and the length of The Rhine is 0.0549 times that amount.(a.k.a. Rhein, a.k.a. Rijn, a.k.a. Rhin, a.k.a. Rain, a.k.a. Reno, a.k.a. Rhenus, a.k.a. Ryn) (from near Reichenau, Switzerland through Hoek van Holland, Rotterdam, Netherlands)
The Rhine river runs 1,150,000 ells in total length. The World War II Battle of Arnhem, as memorialized in the 1974 book and 1977 movie A Bridge Too Far, was the first of several attempts to capture key bridges across the Rhine, beginning with the Arnhem Bridge in the Netherlands.