OBSERVATION OF STARS

Taranaki Daily News, 25 July 1931, Page 11

OBSERVATION OF STARS

ACTIVITY AT NEW PLYMOUTH DESCRIPTION OP THE METHODS STUDY OF SPOTS ON THE SUN. During a discuseion by the executive council of details connected with the observations made at the observatory, it was suggested that a detailed explanatory review of the methods of observation, reduction and recording would be of general interest, and the director was asked to include this in his annual report, states Mr. I. J. Morshead in his annual report to the New Plymouth Astronomical Society. The observation of lunar occultations undertaken at the request of Dr. L. J. Uomrie, now director of the Nautical Almanac office, is one of the most important of our studios. As the moon moves across the sky, passing through the constellation of stars, it occasionally occults, or passes in front of certain stars as seen from the earth. As the disappearance of any star behind the moon occurs very suddenly, if a good reliable timepiece is available, a very accurate record of the time of disappearance is obtainable. These time observations are ultimately used to check the tables from which the moon s movements are calculated, and must be “accurate to the nearest second.” Our chronometer has a rate (or error) which varies from —-2.0 to —2.4 seconds each 24 hours. A very careful comparison with the radio time-signals is made continuously, and the desired accuracy of observation is secured by the use of a stop-watch in conjunction with the cheeked chronometer. A record of the observed times of disappearance is kept at the observatory, a copy of them being sent, to the Dominion Observatory, whence they are sent to Dr. Comrie in London, who arranges for their reduction. It has been stated that an experienced computer can reduce a single occultation observation in one and a-half. to two hours of diligent computation. After reduction all results are sent to Professor Ernest W. Brown, of Yale University Observatory, who is the recognised international authority on matters connected with the moons movements in its orbit round the _ earth. They are also used in an investigation of the earth’s axial rotation, some evidence of' a slight periodic variation of the time of rotation having been put forward. PROPERTIES OF THE SUN. Perhaps the most 'interesting inquiry in the realm of astronomy is the investigation of the physical properties ot the sun, which is really more or less typical of all the stars. The elements of which it is composed, the physical state in which it has in the past exist* ed and does now exist, and any reactions registered by the earth as a result of the enormous amount of energy continually being emitted, arc of very intimate interest to mankind. We do not possess the necessary equipment, nor are we qualified to participate in the elucidation of these abetrusq problems, but one very useful and valuable investigation is open to us because the observation, as often as possible, of the amount of “spottedness” of the sun, the number, positions and types of •sunspots and spots-groups visible, and the keeping of an accurate record of these details provide material which is useful to those who are undertaking these investigations. A telescope will reveal almost always one or more dark spots standing out boldly against the luminous background of the sun’s surface. Sometimes they are extremely email and few in number; at other times they are numerous and of enormous size. They all show a uniform motion from east to west which, discarding a very slight relative motion amongst the spots themselves, reveals that the sun uh seen from the earth rotates once in 274 days. As the axefl of rotation of the sun and the earth are at different angles, it follows that, as seen from the earth, the angle from the vertical of the apparent motion of the spots across the sun’s disc changes progressively, having two maxima of greatest inclination and a zero value twice during each year. As there are no visible permanent features on its surface from which to take measures, positions on the surface of the sun’s sphere are measured in angle in latitude north and south of its equator and in longitude from an arbitrarily fixed meridian. The angles of inclination of the apparent spot paths are calculated some years ahead and are published in the Nautical Almanac, being given for each day at the time of Greenwich midnight. PROJECTION DRAWINGS. When making a projection drawing of the solar surface, a specially printed paper sheet is used, having inscribed thereon a circle six inches in diameter, bisected by a more or less horizontal line. This sheet is pinned on a drawing card attached to a wooden extension at° the eye-end of the telescope. On pointing the •telescope, properly focussed, to the sun, a picture of its surface features is depicted on the paper, just filling the six-inch circle. By rotating the frame until a spot moves parallel 0 with the horizontal line, the latter is made to provide a base from which measures may be taken, which, after correction, give exact positions in latitude and longitude. Actually the line shows where a projection of the earth s equator would cross the sun. Now all is ready to make the drawing, and the spots are carefully drawn exactly in the positione indicated on the sheet of paper, which concludes the observational part of the operation. Afterwards enlarged drawings with higher magnification are made in the case of the larger spot-groupfl. These show in a vivid manner the interesting structure and continual development of the spots. Next It is necessary to find the angle of inclination of the axes of rotation of the sun and the earth, which will give us the inclination of the sun’s equator from the horizontal line which, as explained in the previous paragraph, represents the earth’s equator projected. This we find in the Nautical Almanac, and proceed to correct the angle there given for the time which has elapecd since Greenwich midnight. *

When measuring positions of spots specially prepared transparent discs, called Stonyhurst Discs, are used, these having meridians of longitude and parallels of latitude ruled on them at intervals of ten degrees. Eight of these are provided, each one being used when it conforms most nearly to the condition of inclination prevailing when the drafting was made. The disc chosen ifl inclined at the correct angle and the positionfl in latitude and longitude are read as indicated by the ruled disc, the reading in longitude being in angle east or west of the central meridian. As the sun rotates continuously the longitude of the central meridian is constantly changing, eo that the latter reading is corrected to its fundamental

meridian by another reference to the Nautical Almanac. As the disc does not always conform exactly to the day it is used, a slight adjustment in longitude is necessary, obtained by adding to or subtracting from the corrected position in longitude an amount obtained by multiplying the cosine of the latitude by the residual value of the disc. Having explained the process of drawing the spots and measuring their positions, it remains to indicate how these are recorded for permanent reference. The values are entered on prepared forms; in the case of spot-groups the mean positions are given. At the conclusion of each solar rotation these particulars, with a descriptive report of any special features noted, such, as relative motion of the spotfl, interesting changes in the spots themselves, or other details of value, are included in a report which is forwarded to the director of the solar section of the New Zealand Astronomical Society at the Dominion Observatory, where they are combined with other New Zealand observations and finally forwarded to the 6olar eection of the British Astronomical Association, while extracts are included in the issues published monthly by the New Zealand Astronomical Society. METEOR OBSERVATION. Meteors, popularly known as “shooting stars,” are to be seen on any clear night, and briefly the observationfl arc made in the following manner. The apparent path of the meteor is plotted on a special gnomonic star-chart provided for the purpose.’At the conclusion of the night’s observations, during which from 10 to 70 paths may have been recorded, the maps are studied, and the points from which the meteors appear to have radiated are deduced. . The positions of these points of radiation are then measured in angle in two coordinates in the same way that star positions are given. We are then able to state definitely the shower or family to which any of them belong. Several other particulars, Such as the time of occurrence, duration of visibility, colour and brightness are recorded for statistical purposes on special forms, which are sent, together with the star maps, to the director of the meteor section of the New Zealand Astronomical Society, who, after examining them, for hia own information, forwards them to the director of the American Meteor Society. A close observer of the stars cannot fail to notice that certain ones vary in brightness, some irregularly, and others in definite regular periods. Records of this variation provide material which is used in studies of the physical properties of the stars, and has been used for other investigations’ also, notably to obtain an approximate idea of the distance of some of the very remote celestial bodies. The variable star observer is provided with charts of the star fields surrounding the variables, showing nonvariable stars of known brightness, which are used for comparison purposes. Estimates of the star’s brightness are recorded on each occasion and a series of observations over a period will show how it has fluctuated. These records are forwarded to the director of the variable star section of the New Zealand Astronomical Society, who ultimately sends them to a recognised authority abroad who can make the fullest use of them.