The Great Star Map—I
of Astronomy in the University of Oxford THE simpler name "star map" is here applied to the chart generally knDwn as the "Astrographic Chart," because this latter · CDnveys a suggestion of technicality which is absent frDm the project. What astrDnomers in different parts of the world are really abDut is the making of a large and much more de tailed map Df the stars than has hithertO' been prD duced. The map is being made by phDtography; but though the wDrd "astrographic" has been coined fDr use
... been coined fDr use when photDgraphy is applied to the stars, the work does nDt involve much technicality that is not familiar to' the users Df an Drdinary Kodak. In three details Dnly does the wDrk Df the astronDmer differ from that of the amateur phDtDgrapher: he uses a much longer · camera; he drives the camera by clock work so that it may follow the stars; and he takes pIctures at night instead Df in the daytime. It may perhaps be added that he uses the light emitted by stars, instead Df phDtDgraphing Dbjects by reflected light of the sun. But Df these details more presently. Let us first consider what is the nature of a map of the st1\rs, as this differs somewha:t in · character from the maps Df the earth's surface with which we are familiar. There is no question Df finding Dur way, nO' questiDn of delimiting prDperty, nO' questiDn of shDwing hiIls and valleys. A map of the stars is of a more monDtDnous character, being practically limited to showriIg the exact positions and the bright nesses Df individual points of light. Maps Df the stars may ·differ ' frDm Dne another in scale, in accuracy, and in cDmpleteness: In scale bec, ause we may show two given stars separated on the map either by a fDOt or by an inch, according to requirements; accuracy wiII have a tendency to be greater on the larger scale; and they may in dicate either a few bright stars or many faint ones. We are familiar with the fact that there are only a few very bright stars, mDre Df a degree less bright, more stiIl fainter stars; and the increase cDntinues as the luminDsity diminishes, long after they have ceased to' be visible to' Dur eyes, nO' limit being reached even by the longest exposures given with our largest telescopes. Completeness then can Dnly be a relative term. It is at present impos sible to think Df giving all the st ars in the sky; we can Dnly settle to give all those brighter than a cer tain fixed standard. The earliest maps of the stars were prDbably made fDr astrolDgical purposes; later they were required fDr the use of sailors. But through all the aenturies so little ha d been done towards making accurate maps that in 1674, when there arose · a questiDn of finding the longitude at sea by DbservatiDns Df the moon and stars, it was pDinted out by Flamsteed that no suf ficiently accurate maps or catalogues of the stars were available. King Charles II., to' whom this information was brought, was thDroughly alarmed at the state of affairs and immediately said that ,he must have the omissiDn rectified. Thus was Green wich ObservatDry established. When asked who was to take charge Df the observatory, the king immediately replied that Flamsteed, whO' had pointed 0ut the need ·of.su�h, an institution, was the man to put in charge. Modern observation of the positiDns Df the stars may be said to have · begun at this periDd. Greenwich tODk a great step fDrward half a century later, when Bradley was made the thi rd AstronDmer Royal and increased the accuracy of observatiDn very considerably, so that his results have fDrmed the basis Df our knDWledge of the positiDns of the stars to' the present time. But Brad ley and his successors for the most part confined their attention to the brighter stars, not cDncerning them selves with those much fainter than can 'be seen with the naked eye. There are two gODd reasons fDr this. In the first place, the number of stars required for the use of sailDrs is not large; indeed, sailors them selves use remarkably few, for only the brightest are suitable for Dbservation by the small telescopes of their sextants. Indirectly, however, sailDrs depen d upon the keeping of accurate time-Greenwich time is in use all the world over fDr determining IDngitude: and f.or keeping accurate time a much larger number of stars, called "clock stars," is required. These have had the first claim upon the attentiDn of astronomers at our great observatDries during a cDuple Df cen turies. A se cDnd reaSDn for cDnfining atten tion to these brighter stars arises frDm the limitatiDns Df instruments. The observations were generally made by watching the star cross . .the field Df view, in which were certain spider lines for reference. Now these lines cannDt be seen unless the field of view is iIlu-• Reprinted frolll Soience Progress. minated, and a faint star is then lDSt in the illumina tion. In these days of electric light it is comparative ly easy to adDpt a new instrumental method, whereby the wires themselves (and nDt the background) are illuminated; they then appear as bright lines but are not sufficiently daz:ding to Dbscure even a faint star, which can' thus be observed as well' as a bright Dne. But in former times this methDd had not been suffi ciently developed and in any case the brig, hter stars were easier to' Dbserve. For these reaSDns therefore the fainter stars have not attracted attentiDn until cDmparatively recen t1y. One motive fDr studying them came with the discovery of the minDr planets, which dates frDm the first day of the nineteenth century. It had been realized that there was a gap in the se quence Df planets (as arranged in Drder Df distance from the sun) between Mars and Jupiter. It was cl�ar that there could not be any large planet in this positihn, fDr it wDuld have been noticed; but there might be a small Dne, so search was made for it. The methDd of search was somewhat labDriDus. It was necessary to identify all the stars within a certain region in order that any strange body might be de tected. It is now easy to accomplish this by taking a photDgraph Df the region; but at the end Df the eighteenth century no such cDmpendiDus process was available; then the positions Df individual stars were either patiently and laboriously measured Dne by one, or learned by the astronDmer SO' that he could carry a picture Df the region in , his memDry. In default of an actual material phDtDgraph he practically phOtD graphed the image on his own retina. It is astonish ing to think how much was accomplished by this toilsDme process. Not one only bu t hundreds Df minor planets were discDvered in this way, thO' ugh not with out difficulty and delay. Four were found at first in rapid succession and then came a long blank during nearly half a century, so that it seemed as though the number were cDmplete: but thDUgh this view proved quite erroneDus, it was only after a search of fifteen years that Hencke, an ·ex-pDstmaster of Driessen, was at last rewarded by another discDvery.