An EC needs to be able to be evaluated by whomever is calculating your worth. To that end, the activity needs to have organization, stated goals, substantiation by a third party, etc.
If you want to do math and astronomy "stuff", do it within the bounds of some greater organization in order for it to be counted as an EC.
You might join the Science Bowl at school, or Math Bowl, or some other academic competition organization. Or tutor other students through a tutoring club, or use your skills to complete a project for your community. But just doing the things that interest you in your off time does not count as an official extracurricular activity.