DETERMINATION OF TIME IN OBSERVATORIES

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

DETERMINATION OF TIME IN OBSERVATORIES

I By “ A.G.C.”] Since we have all experienced the effects of an alteration m the system of time keeping in New Zealand might be interested to know how the time is derived in tlie first place at observatories established for that purpose. At most observatories a carefully surveyed meridian is marked down, and an important part of the building, termed the transit room, is built foursquare to the cardinal points. Tin's room is completely divided into two parts by a slit several indies wide, generally placed centrally, running up the north and south sides and across the roof from north to south, inside the room and in the plane of this slit, which marks the meridian, is erected the transit instrument, which is a telescope pivoted on two supports, one east and Qie other west of the telescope tube. Thus Ihe transit telescope is free to point only to objects lying due north or south of itsell—that is, in the meridian.

in the transit room is kept a sidereal dock which indicates the time corresponding to the right ascensions of the stars which are passing the meridian. For example, when Alclebaran is on the meridian the dock reads 4h 32min, ami that time is also the right ascension of Aldebaran. Alpha Centnuri may bo observed, similarly, when the dock reads 14.35, but us Alpha Centnuri never sets it may also be observed at 2U 35mm sidereal time, when it is to be found below the South Role. The telescope contains a series of "ires, over which the star passes in its transit, so that, each Lime observation is the result of an average of the measurements on the several wires. Generally the observer taps a key and records cadi passage of the star across a wire on a tape chronograph, alongside of which Ihe seconds of the sidereal clock are also recorded electrically. The right ascension of the star being found from a nautical almanac, and care being taken to allow for errors, in the adjustment ■of tlie instrument, the error of the sidereal dock may be found, and then, by the use of convenient tables, the corrected sidereal time may he converted into Greenwich mean time, and the error ol the mean time clock, which sends the time signals, may be found. The use of wireless rime signals enables observatories to compare their determinations of mean time, with the result that greater confidence may he placed in their determinations bv astronomers and the public generally.

The meridian of the Dominion Observatory at Kelburn, AVellingtou, is marked by a stone pillar, always illuminated by a lamp, above Connaught terrace, Brooklyn, two miles due south of the transit room and not far from the highest point on the east side of Brooklyn.