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The haversine formula gives great-circle distance on a sphere of mean radius 6371 km. Flight time adds about half an hour for taxi, climb and descent to the cruise time at roughly 850 km/h, ignoring wind — which on a transatlantic route can shift it by an hour.
Haversine
a = sin2(dLat/2) + cos(lat1)cos(lat2)sin2(dLon/2); d = 2R asin(sqrt(a))
Because a great circle projects as a curve on a Mercator map. The apparently longer polar route is genuinely shorter on the globe.
The jet stream. Eastbound transatlantic flights ride a tailwind and can be an hour or more quicker than westbound.