Learning items
Explore shared ideas across books.
Astronomy
ConceptCosmic scale
Cosmic scale concerns the very different sizes, distances and times encountered in astronomy.
ConceptElliptical orbit
In an ideal bound two-body Kepler orbit, the path is an ellipse with the attracting body at one focus in the usual relative-coordinate description.
VocabularyLight-year
A light-year is the distance light travels in a vacuum in one year.
All items includes definitions still awaiting topic classification.