THE DIVISION OF TIME

Evening Star, Issue 19778, 31 January 1928, Page 9

THE DIVISION OF TIME

[By Mr Jamiss Stewart, Invercargill.]

Apparent or sun time was the time formerly kept—i.e., each town kept its own local time, but this was changed on the introduction of railways. It may be easily imagined that it was utterly incompatible with the management of public business as now conducted that such a variable timekeeper as the sun would be-used, when it is considered that disastrous conse-

quences might arise if the running of trains were not subject to greater precision than could- be obtained under such circumstances However, the chronometric measures in a given place nuay be regulated, their indications necessarily differ from those of similar chronometric measures in other places having different longitudes. The cause of this difference is the successive arrival of the mean sun at the several meridians of such places. By the apparent diurnal motions ol the heavens, the sun arrives in succession, from hour to hour, at the meridians of places situate one westward of the other, and ns the sun thus carried round the globe makes a complete revolution in twenty-four hours, it moves from meridian to meridian at the rate of 360 degrees in twenty-four hours, or 15 degrees per hour, which is one degree in four minutes. Tims at two places differing in their longitudes by one degree the local time will differ by four minutes, that which is east being earlier than that which is west. , , . Greenwich Observatory was founded in 1075, and a line drawn parallel with the axis ol the earth, and running through this observatory was fixed as tlie starting point or standard for rectifying the tables of the motions of_ the heavens and the places of the fixed stars in order to find out the so much desired longitude at sea for perfecting the art of navigation. To-dav. therefore, the whole sky may bo considered as a vast clock set to Greenwich time, the stars being the numbers on the dial lace. A sundial therefore necessarily shows apparent or sun time; 'consequently its indications will only agree with mean or clock time when the veal and the imaginary suns happen to coincide or pass the meridian at the same instant. Accordingly on these days the equation of time is nilXew Zealand mean time is calculated from longitude 172 deg 30min cast from Greenwich; therefore the sun arrives on the meridian at all places in New Zealand on that longitude 11 h 30min before it does at Greenwich. A sundial fixed anywhere on that longitude—viz., 172 deg tiOmin in New Zealand —will show exact clock lime four times only in the year—i.c., April 15. June 15, September 1, December 24. All other days it will be cither fast or slow, as previously explained. Consequently it is absolutely necessary that to bo able to obtain the mean or clock time from a sundial an equa-tm-i table must be referred to to know how much to add to or subtract from the shadow cast by the gnomon on the dial face. The simplest and the easiest way to accomplish this is to engrave on the dial face a table showing how many minutes the sun is fast or slow ior every tenth day throughout the year. The intermediate days—i.e., dates—must be calculated by the beholder, The following will show the actual local time in different towns in Now Zealand, which, as previously explained, differs owing to their being east or west of the mean longitude of this country, which is 172dcg 30min east of Greenwich. Christchurch is the nearest town to that longitude or mean line, being on longitude 172dcg 40min, which is equivalent to its getting the sun twothirds of a minute before mean time.

_ Napier is 18|min faster than mean time, Auckland and AVellingtou omin, and Now Plymouth 6min faster than the mean, while Dunedin is Bmin and Invercargill IGQnin after the mean longitude, so that there is a difference of 35min between Napier and Invercargill and I7min between Wellington and Dunedin.