Defining a city using cell-phone data

Humans are becoming more urban, with more than half of the world's population now living in cities. This rapid growth poses unique challenges to both the study and governance of cities—a challenge made harder because we ...

What's the earliest the moon could have formed?

Astronomers are pretty sure they know where the moon came from. In the early solar system, a Mars-sized object dubbed Theia smashed into Earth. This cataclysmic collision knocked a huge mass of material into orbit, which ...

page 8 from 40