Where tomorrow starts, and why two islands 4 km apart are almost a day apart.
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In the sea between Russia and Alaska lie two rocky islands, only about 4 km apart. For most of the day, it’s already tomorrow on the Russian one! People call them Tomorrow Island and Yesterday Island. How can that be?
The Earth spins once a day, so noon, when the Sun is highest, moves west round the world. It goes 360° in 24 hours: that’s 15° of longitude every hour. So the world is split into time zones, each about 15° wide, with clocks an hour apart. (The clocks here show the Sun’s time.)
Go all the way round the world and you’d end up a whole day out. So there’s a line where the date changes: the date line, in the middle of the Pacific Ocean. Cross it going west and you jump to tomorrow; cross it going east and you get today again!
A dead straight line would cut some countries in two, with each half on a different day! So the real date line bends round them. Tap the button to see.
Countries can move the line! In 1995, Kiribati bent it round its far eastern islands, so the whole country had the same day. In 2011, Samoa hopped over to be on the same day as Australia and New Zealand: there, Thursday 29 December was followed by Saturday 31 December!
Every new day starts at the date line, then midnight sweeps west round the world. So who gets New Year first? Drag the slider through New Year’s Eve and follow midnight (the dotted line).
In 1522, the ship Victoria came home to Spain after sailing right round the world, the first ship ever to do it. On the way, Antonio Pigafetta wrote down every single day. But when they stopped at the Cape Verde islands, his diary said Wednesday, and the people there said Thursday! (The route is drawn simply.)
Time for a trip! The dotted line is a plane’s path from Tokyo, in Japan, to Honolulu, in Hawaii.
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The globe pictures are simplified to show the idea. Earth images: NASA Blue Marble (public domain).