Читать книгу Primitive Time-reckoning. A study in the origins and first development of the art of counting time among the primitive and early culture peoples онлайн

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"The risings and settings of the stars. It has already been remarked that the sun in the course of a year runs through the zodiac backwards, so that one particular star culminates 3 min. 56 secs. earlier every day. Hence it is evident that if we indicate the exact interval of time between the culmination of the sun and that of one particular star, or name the star with which the sun precisely culminates, we can determine the day of the solar year. This is the principle of one method of computing time which was very common among ancient and primitive peoples, but has entirely dropped out of use in modern times owing to our paper calendar. The stars are so to speak the stationary ciphers on the clock-face and the sun is the hand. In practice we naturally have to do not with the invisible culmination of the stars but with the position of the sun and certain neighbouring stars on the edge of the horizon, whereby the matter becomes more complicated on the astronomical side. For this observation the so-called circumpolar stars are singled out, that is to say the stars situated so near the pole that they do not set (e. g. the Great Bear). If the star rises or sets simultaneously with the rising of the sun, this is called the true cosmic rising or setting. If the star rises or sets simultaneously with the setting of the sun, this is termed the true acronychal rising or setting. These risings and settings of the star are not visible, since the sun hides them by its light: the rising and setting are perceptible only when the star stands at some distance from the sun, i. e. only the so-called apparent rising and setting are practically observable. We have already seen that the sun every day drops nearly 4 minutes behind a certain star. Assuming that sun and star rise simultaneously on one day (true cosmic rising), then after a few days have passed—the period varying somewhat according to the latitude of the place of observation, the time of the year, the size and place of the star—there will come a day on which the star rises so early that it is visible in the morning twilight, immediately before the sun appears. This is the heliacal or morning rising. From this day the star will rise earlier and earlier, and will therefore remain visible for a longer and longer period. In the course of half a year, commonly a little sooner or later, the time of rising will have been pushed so far back that it will take place in the evening twilight; when it is pushed still farther back the rays of the setting sun eclipse the star and its rising is no longer visible. The last visible rising of the star in the evening twilight is the apparent acronychal or evening rising. After a few more days the star goes so far back that it rises at the very moment in which the sun sets—the true acronychal rising. The rising, which is advanced constantly further into the light of day, is no longer visible, but on the other hand we now see the setting of the star. If it is assumed that the star is situated on the western horizon, i. e. sets, when the sun is on the eastern horizon, i. e. rises—and incidentally it is to be noted that this position, when the star is not situated in the ecliptic, may be divided by an interval of a larger or smaller number of days from the opposite position, viz. star on the eastern, sun on the western horizon—this is the true cosmic setting. The star moves forward, i. e. its setting takes place earlier in the morning, and after a few days it will be noticed in the morning twilight immediately before it sets, and this is the first visible setting in the morning twilight, the apparent cosmic or morning setting. From this day the setting moves further and further forward into the night and approaches the evening twilight. At length it will be so near sunset that the star no longer sets in the night but in the evening twilight. The last visible setting of the star in the evening twilight is the heliacal or evening setting. After a few days the star has approached still nearer to the sun: both set at the same moment, the true cosmic setting. If the star stands in the ecliptic, the true cosmic setting coincides in date with the true cosmic rising, otherwise these are divided by a greater or smaller number of days (see above). As the star moves on, a heliacal rising follows again, and so on. Between the day of the heliacal setting and that of the heliacal rising the star is invisible, since it stands so near the sun that it is eclipsed by the sun’s rays. It has already been remarked that we can determine the day of the year by indicating the true rising and setting of a star at a certain spot. As far as the apparent rising and setting are concerned this indication can only be approximate, since the visibility of a star depends on several variable factors—the size of the star (because a smaller star, in order to be visible, must move farther from the sun than a brighter one), the transparency of the atmosphere, the keenness of vision of the observer, the geographical latitude of the place of observation (since the farther north or south the sun is, the more slowly, because more obliquely, will it sink below the horizon). In this latter respect, for instance, there is a perceptible difference between Rome and Egypt. Only an approximate indication of time, therefore, can be derived from the rising and setting of the stars”.