Why use enum when #define is just as efficient?

Although your question is tagged as C, there is a big advantage when writing in C++, you can place enum:s inside classes or namespaces.

This way you could refer to your constants like SpaceshipClass::galaxy.

I find it useful for debugging in an environment such as gdb since enum values are handled at compile time (where #define is a preprocessor macro) and thus available for introspection.

The advantages of enum show up when you have a long list of things you want to map into numbers, and you want to be able to insert something in the middle of that list. For example, you have:

pears 0
apples 1
oranges 2
grapes 3
peaches 4
apricots 5

Now you want to put tangerines after oranges. With #defines, you'd have to redefine the numbers of grapes, peaches, and apricots. Using enum, it would happen automatically. Yes, this is a contrived example, but hopefully it gives you the idea.