Why do we declare constant lat and lon value
Static variables are allocated storage only once in a program lifetime
The const keyword specifies that a variable's value is constant and tells the compiler to prevent the programmer from modifying it and you can't change const variable's value
and i guess you know double is a data type that can save floating point numbers
that line declare two variables with names LONDON_LAT and LONDON_LON and both of them are in one line
and because london's latitiude and longtitude doesn't change the const keyword is used
also i guess because those variables are constants you can use #define visit https://www.arduino.cc/reference/en/language/structure/further-syntax/define/