Designed by Roberto Casati and Glen Lomax, Astrini is an open source software for teaching elementary astronomy. Unlike virtual observatories (Stellarium and Celestia), and unlike standard Orreries, Astrini allows you to manipulate certain parameters, such as the tilt of the Earth’s axis and of the lunar orbital plane relative to the ecliptic. Doing so creates a world without seasons and with regular, monthly eclipses. From this simplified situation, teachers/students can find their playful way out why there are seasons and why eclipses have a more complex periodicity.