I found this wonderful device in the Technical Museum in Berlin. You move it across the globe and it tells you the time in a particular place as compared to Greenwich. As you move the arrow, the Greenwich time doesn't change. The other label says "Ortszeit". Ort means place and Zeit means time. Ortszeit is ahead of Greenwich which means the place must be east of Greenwich. The Earth rotates so that the Sun rises in the east, making noon arrive sooner there. If you can keep a Greenwich clock, you can work out your longitude. This was the famous problem in the 1700s and it was solved by the clock builder John Harrison. Do look up his story.
