Friday 23 April 1999
Yes. Because of how we measure time, we sometimes need to add leap seconds to match up our clocks with the actual position of the Earth around the Sun. The rotation of the Earth about its axis isn't exactly even, which makes the days vary in length. These slight changes didn't matter until 1967, when atomic clocks were invented and the second was defined as a certain number of oscillations of a caesium atom. This definition is what gives us International Atomic Time (TAI).
But the TAI isn't based on the earth's rotation, so a calender based on TAI gradually becomes out of step with one based on GMT. We could use TAI as our official time (and in fact we do, with slight alterations of leap seconds) but then the time would get out of step with day and night and the seasons. So for convenience we use GMT and adjust TAI accordingly. In 1972 a new Coordinated Universal Time scale (UTC) was adopted for international use. It combines all the regularity of atomic time with most of the convenience of GMT. The seconds of UTC are of the same length as those of TAI, and then UTC is kept within one second of GMT by the insertion of extra leap seconds.
Q Which way do the Earth's magnetic field lines go?
On a bar magnet, the field lines go from north to south, but the direction of the Earth's magnetic field goes from south to north.
The problem arose because maps originally used the top of the Earth as north, and the bottom as south. Lodestones (naturally occurring magnets) were used as direction pointers, and it was a long time before physicists came up with bar magnets and theories for magnetism. Unfortunately this mistake was not realised until it was too late to rectify, so the problem has stuck with us.
You can visit the Technoquest World Wide Web site at http://www. sciencenet.org.uk
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