John Pratt

In today’s story, which originally aired in 2014, we meet a very special cylinder. It's the gold standard (or in this case, the platinum-iridium standard) for measuring mass.

The Mayan calendar ends on December 21, 2012, prompting some to believe the world will end on that date. For this week’s Please Explain, we thought we’d find out about that calendar and the many others used through history by different cultures to account for the days and months that make up the year. Joining us are John Pratt and Ken Seidelmann, Research Professor at the University of Virginia’s Department of Astronomy.