The Sun and the Moon travels around the Planet, I presume. Their world works different from ours.
Both the Sun and the Moon travels across the sky... so they're travelling around the backside of the planet too? Maybe it's just the Rise and Set that has to be done magically; the rest is travelling by inertia?
(it's a bit late for a reply... but if someone was to read the comments later)
They look so happy! It's good to see that the holders of the Highest Office of the State do have some free time to chill out. With two more princesses now I guess the Royal Sisters can enjoy the fruits of friendship that they worked so hard to provide the entirety of Equestria.
Fantastic work, dear sir! Thank you for providing this art to us for all to see!
All hail the House of Equestria! Long live the Princesses!