- What is the Haversine formula?
- The Haversine formula computes the great-circle distance between two points on a sphere from their latitudes and longitudes. The short form: a = sin²(Δφ/2) + cos(φ₁)·cos(φ₂)·sin²(Δλ/2), then c = 2·atan2(√a, √(1−a)), and distance = R·c, with R the radius of the sphere (6371 km for Earth). It's the standard for online distance tools because it's simple, stable near antipodal points, and accurate enough for most uses.
- How accurate is it?
- About 0.5%. The Haversine formula assumes Earth is a perfect sphere. It isn't — the actual shape is an oblate spheroid, slightly flattened at the poles. For more precision (sub-meter at flight-test scale) you'd use the Vincenty or Karney algorithms on the WGS-84 ellipsoid. For trip planning, classroom homework, or any application that's already rounding to the nearest mile, 0.5% is fine.
- Is this driving distance?
- No. This is straight-line (great-circle) distance — the shortest path across the Earth's surface. Roads bend around mountains, lakes, borders, and traffic patterns; driving distance is always longer, sometimes by a factor of two or three. For a road trip, use a routing tool that follows roads.
- Is it the same as flight distance?
- Close, not identical. Planes try to fly great-circle routes because they're shortest, but they detour around weather, jet streams, no-fly zones, and ATC corridors. Eastbound transatlantic flights ride the jet stream, westbound flights fight it — same great-circle distance, different time and fuel. The Haversine distance is a good first approximation; actual flight tracks add a few percent.
- What does 'initial bearing' mean?
- It's the compass direction you'd start out in if you flew the shortest path. On a great-circle route the bearing changes continuously because the path curves over the globe — a flight from New York to Tokyo leaves heading roughly north-northwest, crosses the Arctic, and arrives heading south. The initial bearing is just the heading at the start.
- Why are the kilometers, miles, and nautical miles all different?
- Same distance, three units. A kilometer is 1000 meters. A statute mile is 1.609344 km. A nautical mile is 1.852 km — defined as one minute of arc along a meridian, which is why pilots and sailors use it: 60 nautical miles is one degree of latitude, anywhere on Earth.
- Where do the city coordinates come from?
- The list is roughly the top 500 cities by metropolitan population, with latitude and longitude from standard public-domain geographic datasets. Coordinates point to the city center (typically the historic or administrative center), not the airport or the metro boundary. Two cities labeled the same way in different data sources can disagree by a few kilometers; we picked one set of values and stuck with it.
- Why is my city not on the list?
- The list is capped at around 500 to keep the bundle small and the autocomplete fast. If you want a city that isn't here, you can plug its latitude and longitude into the standard distance formula yourself — or open an issue and we'll add it.
- Does this work without an internet connection?
- Once the page is loaded, yes. The cities database and the Haversine math are bundled into the JavaScript, and no network calls are made when you pick cities or read the distance. Refreshing the tab while offline will still work if the page is cached.
