The world is a much different place than it was a thousand years ago. Pegasi played a huge role in the world and way of life around the country, as well as formed of the most popular past times. Unicorns were still as noble and deceiving as ever depending on their up bringing, but the amount of scholars in there midst have dwindled alarmingly. And Earth Ponies....they were no longer dirt on the hooves of those higher than them. They were nobles, busniess ponies, preformers, or critically acclaimed engineers that helped run today's transportation.
Being intrested in the Earth Pony's rise from poverty and into an equal light, Luna removes herself from duty temporarly and places herself in the care the one of the most trustworthy earth ponies she knows for the length of a year to help her learn the ways of the earth pony to help better understand them. Poor Applejack isn't quite sure what she is getting herself into when she agrees...
Thanks to MasterFrasca99 for editing