astronomy, uranology - the branch of physics that studies celestial bodies and the universe as a whole

disappearance - the event of passing out of sight

eclipse, occultation - one celestial body obscures another

arrival - the act of arriving at a certain place; "they awaited her arrival"

incursion - the act of entering some territory or domain (often in large numbers); "the incursion of television into the American livingroom"

intrusion - entrance by force or without permission or welcome

irruption - a sudden violent entrance; a bursting in; "the recent irruption of bad manners"

entree - the act of entering; "she made a graceful entree into the ballroom"

enrollment, enrolment, registration - the act of enrolling

penetration - the act of entering into or through something; "the penetration of upper management by women"

admission, admittance - the act of admitting someone to enter; "the surgery was performed on his second admission to the clinic"