# Ask Mr Brain...all will be explained

PUBLISHED : Friday, 10 September, 1999, 12:00am
UPDATED : Friday, 10 September, 1999, 12:00am

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Why is there lightning before thunder? When there is a thunderstorm, we usually see lightning flash across the sky and do not hear thundering until a few seconds later. It is because light travels at a much higher speed than sound. Light travels at 300,000 kilometres per second while sound travels at a speed of 340 metres per second. Lightning reaches the ground almost immediately, while sound takes a few seconds to reach the ground.

Sometimes we see lightning but do not hear thunder. It is because the thunder is too far away and the sound has dispersed before it reaches our ears.

How did early European explorers work out latitude and longitude on their voyages? ALEX KWAN PLK No 1 WH Cheung College Even for early explorers, finding latitude was a relatively simple affair. By measuring the angle of the sun at noon above the horizon, one could determine one's latitude, or position north or south. By the 1520s, the Portuguese were able to accurately fix latitude, using simple tools and mathematical tables, but finding longitude was a different matter.

In those days, to sail east or west was often to take your life in your hands, since you could never be sure where you were. Early attempts to determine longitude included observing eclipses of Jupiter's satellites, which must have been tricky, especially from the deck of a ship tossed by the waves.

With thousands of lives being lost at sea because of this problem, in 1714 the English Parliament offered a prize of GBP20,000 (equivalent to about HK\$16 million today) for the first person who invented a de vice for accurately determining longitude.

Carpenter John Harrison took up the challenge. A self-taught clock- maker, Harrison believed longitude could be determined using an accurate clock. If you took an accurate clock with you on your journey, set on your home country's time, you could work out your longitude by seeing what time your clock said when it was noon where you were. Since an hour's difference equals 15 degrees of longitude, the calculations were fairly straightforward.

Others also realised that accurate timekeeping held the key and one of the more bizarre methods advocated was using 'powder of sympathy'. A wounded dog on board a ship was supposed to howl when the knife that wounded it, coated in a special powder, was touched at noon in the ship's home country, thus providing an accurate time check. This method smacked of superstition and did not even work.

While the best clocks of the day were fairly accurate, none of them could maintain this accuracy on a moving ship. In 1761, after almost 40 years, Harrison finally produced a clock accurate to within 0.2 sec onds a day, even on a ship at sea. While an accurate clock, or chronometer, is still a vital piece of equipment on board ship, the advent of the Global Positioning System using satellites has made navigation much easier and more accurate.