Good Questions: All the time in the world

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DATES, times and ostriches provide our weekly diet of instant erudition.

What does the 'Mean' in 'Greenwich Mean Time' refer to?

(E Pugh, Bolton)

Even without bringing Stephen Hawking into it, the measurement of time is more complex than it seems. The definition of Greenwich Mean Time depends on the difference between Apparent Solar Time and Mean Solar Time. Apparent Solar Time is the time shown by a (properly calibrated and aligned) sundial. It is a measure of the apparent progress of the sun through the sky and was perfectly adequate - during the daytime - until clocks became widespread in the 17th century.

Time measured by the sun, however, is not uniform. The elliptical orbit of the earth means that its angular speed is not constant throughout the year. In fact it is greatest at the beginning of January when the earth is closest to the sun. The obliquity of the earth's axis (it is inclined at an angle of 231 2 degrees to its plane of motion round the sun) also affects the length of days. These combined effects make the dawn-to-dawn day-length vary.

For the definition of Mean Solar Time, we must envisage an imaginary sun, moving steadily through the sky at a constant rate that averages out the vagaries of the real sun's path.

The difference between Mean Solar Time and Apparent Solar Time can be as much as 16 minutes, calculated from a formula known as the Equation of Time. A consequence is that sundials only tell the right time on four days each year, around 15 April, 14 June, 1 September and 25 December.

Once Mean Solar Time had been defined, the creation of time-zones followed naturally enough. First proposed by Charles Dowd of Saratoga Springs, New York state, in 1869, they were adopted by an international conference in 1884, with the Royal Observatory, Greenwich, selected as defining the zero meridian.

So, back to the original question, the Mean in Greenwich Mean Time is the average of the sun's motion as seen from Greenwich.

Which leads neatly into the next question:

21 December is the shortest day, ie the day on which the time that elapses between sunrise and sunset is a minimum. The day, in other words, when the sun rises at its latest hour and sets at its earliest? Not so. According to my diary, the sun rises at its earliest a week later on the 28th and has already set at its earliest on the 14th. What is the explanation?

(T Seville, Iffley, Oxford)

Now that we understand Apparent Solar Time and Mean Solar Time, we need to bring in Sidereal Time, the time the earth takes to rotate on its axis. As everyone knows, the earth rotates once a day, or to be more precise once every 23 hours and 56 minutes. The other four minutes is accounted for my the progress of the earth in its orbit round the sun. When you combine the difference between Sidereal Time, Apparent Solar Time, Mean Solar Time and the effects of small oscillations known as nutation in the earth's rotation, you begin to realise that 8 o'clock one morning is not totally comparable with 8 o'clock the following day.

Perhaps the simplest demonstration of the shortest day problem is to stretch your hands in front of you, imagining them to represent the times of sunrise and sunset. As you move through a period around 21 December, they come closer then move apart again. Now repeat the movement while sitting on a wobbly swivel chair, slowly turning from side to side. The combination of the two motions now means that each hand may reach its furthest point at a moment other than when the two are closest together.

Sources: Encyclopaedia Britannica, Whitaker's Almanac.

Do ostriches really bury their heads in the sand because they think it makes them invisible? (Ms A Strauss, Slough)

The short answer is no, though the myth has been around at least since the first century. It is mentioned in Plutarch's Lives and Pliny also comments on their stupidity, stating that they imagine that when they have thrust their heads into a bush the whole body is concealed.

Diodorus of Sicily, a Greek historian of the first century BC, cited the same behaviour in praising the ostrich for its intelligence. Since its head is the weakest part, he thought it very sensible to hide it in a bush. Diodorus, incidentally, believed the ostrich to be the missing link between a bird and a camel.

The modern view on head concealment inclines towards Diodorus rather than Pliny. They are probably not so much trying to bury their heads as lie on the ground with necks stretched out to escape detection.

The emperor Heliogabalus, incidentally (reigned 218-222) once served 600 ostrich heads at a banquet. The Romans were generally fond of roast ostrich, and the usurper Firmus, who rebelled in Egypt against Aurelianus, performed the tour de force of eating an entire ostrich in a day.

Primary source: Ostrich Egg-shell cups of Mesopotamia and the Ostrich in Ancient and Modern Times (Chicago Field Museum of Natural History, 1926).

(Photograph omitted)