Why is the sky dark at night? Olbers’ Paradox

Light at different wavelengths. Explore the appearance of galaxies

Understanding astronomical redshifts. Velocity and the Doppler effect

Understanding cosmological redshifts. Doppler effect from changes in path

Light responds to gravity. A gravitational lensing explorer

The expansion of the universe. Play with Hubble’s law

The fate of the universe. Play with the cosmological parameters and see their effects

Gravity, orbits, and Black Holes. How do astronomers measure mass?