Cathy Powell, age 13, of Prattville, Alabama, for her question:

Who decided on the Prime Meridian?

The earth has no natural boundaries to mark the 24 global time zones in the calendar day. So it was up to mankind to make its own. Several centuries ago, this job of accurate timekeeping was undertaken by English astronomers in their special observatory at Greenwich, London. When their instruments checked the high noon position of the sun overhead, they announced the time to the world. People called it Greenwich Time. Global time, of course, is related to the 360 meridians that run from pole to pole, and the sun crosses 15 meridians    15 degrees of longitude    each hour. People located 15 degrees east of the Greenwich meridian kept their clocks one hour ahead of Greenwich time. People located 15 degrees to the west of this line were one hour behind.

Through the years, it became the custom to base time and longitude on the meridian that passed through the Greenwich observatory. Then, in 1885, an international conference of astronomers met in Washington D.C. and made the Prime Meridian official.