Longitude difference to Meters

Question

I have been trying to find an effective method (be it an equation or table filled with values) that would be useful for converting differences in longitude values into a distance in kilometers.

It was easy to find for Latitude since 1 degree is more/less equivalent to 111.19km, so any difference between 2 Latitude values would be as simple as:

distance = 111.19*(latitude1 - latitude2)

However, Longitude varies when going to a different Latitude line. At the highest and lowest latitude values (90 and -90 respectfully), each degree of Longitude is 0km in distance (obviously since all Longitude lines meet up and are right next to each other).

At the Equator, 1 degree of Longitude equates to 111.32 km (obtained from a random online source).

Is there an equation that could be used to calculate the distance between two longitude points? I have been searching everywhere but have yet to find a source. And if someone believes a table would be better than an equation, would you happen to have a link to such a table?

Answer 1

Just in case you want the distance along the parallel of latitude (and not the shortest distance) between two points at a given latitude,

distance = r * (longitude 2 - longitude 1)

with longitude in radians and where r, the radius of the parallel of latitude, is

r = R * cosine latitude

where R is Earth radius.