Maritime History of the Great Lakes

Marine Review (Cleveland, OH), 18 Jun 1908, p. 31

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A clock regulated to keep mean time is made to run uniformly, and keeps paces as it were, precisely with the mean sun, so that when the mean sun is on the meridian of any place, the clock set to the mean time of that meridian, indicates 12 o'clock; and 24 hours thereafter, the sun is again on the meridian, and so on throughout every day in the year. A clock regulated to mean time, and ac- curately run, really represents the mean sun, for any time that it be re- quired to know over which meridian the sun is crossing all that is neces- sary is to turn the number of hours and minutes that have elapsed from your noon into longitude of arc and apply it to your meridian of longi- tude according to the rules already laid down. Remember that this micans mean Stin; tO true sun you would have to apply the equation or time. The sidereal day, like the mean and apparent day, is divided into 24 equal parts or hours, but in this case the time is absolute and not relative as in the other cases. As the earth rotates on its axis at a uniform rate of speed throughout the year, a clock can easily be regulated to keep time. to its movements, but its measure must not be taken from the sun, be- cause of the wun-uniformity of the eatth's revolution in its orbit. This, as we have seen, is. called _ sidereal time, or star time; so-called from the fact that it depends on the stars for measurement, the same as solar time depends on the sun for its measure- ment. ' The stars are at such inconceiva- ble distances from the earth that the obliquity of the ecliptic and the ec- centricity of the earth's orbit be- come small and negligible quantities. This being the case, the time that it takes any star to return to the same meridian on successive nights must measure one complete rotation of the earth on its axis. A clock regulated to sidereal time would show the same star on the same meridian at precisely the same time every night (or day). But to time this star when it comes to your meridian by the mean solar clock you would find that the star culmin- ated each night about 4 minutes ear- lier. "Why is this? The mean solar clock imerely measures the time of the tnean 'sun and has' no connection whatever 'with any scale of nature, but is made to coincide with the speed of this imaginary sun, which is the mean of its true rate of speed, but this sun is not made to mark the time of a complete rotation of the earth TAE Marine ReEvIEw on its axis. In fact the earth has completed one absolute rotation, and has a start of nearly 4 minutes of time on the next rotation in the 24 hours as shown by a mean time clock, and that is the reason that the sid- ereal day is nearly 4 minutes shorter than the mean day, or when the mean time clock has reeled off 24 hours the sidereal clock will have run 24h. 4m. This likewise accounts for the stars coming to the meridian about 4 minutes earlier each night according to mean time. The beginning of the sidereal day does not, therefore, correspond to a definite fixed hour of solar mean time, but'°in the coursé of a "year 'runs through all the hours of the ordinary day by which the affairs of life are regulated. A sidereal clock, therefore, would not run with a mean time clock; supposing both to be started at the same instant, the sidereal clock would soon outstrip, or lead the other. They would agree only once in the year. -To test this roughly, multiply 365 by 4, and the product will be ap- proximately 24 hours. Hence, 366 sider- eal days are equal to 365 mean solar days. For confirmation of this, look in the nautical almanac for March, and you will find that the sun's right as- cension is Oh. 0m. Os. when on the equator (spring equinox; or at the time the sun crosses the line bound north). "the "sidereal' time, mm the last column of page 2 for the month would also be Oh. Om. Os. were it not given' for mean time. Thence on- ward, month by month, it steadily in- creases, until in September, when the sun 1S again crossing. the line, but this time in the opposite direction, the . "csidereal time,' has grown to 12 hours, and continues' to grow till it has at- tained the maximum of 24 hours in the following March. The consequence is that those stars which are now rising in the east, at any given hour of solar mean time, will be found setting in the west at the same hour six months hence; while those which at any hour are now setting, will, at the same hour six months hence, be found rising. This, of course, applies to any part of the year, and six months before or after it. It so happens that the right ascen-_ sion of the meridian and the local sidereal time are the same thing. Right ascension means celestial or astronomi- cal longitude, as opposed to terrestrial or geographical longitude. As we have seen, the sidereal . day con- tains 24 hours, but it; do¢és not begin at midnight as the legal attention called to them for the first time. 31 day does, nor at noon like the astronomical day. It begins when the prime celestial meridian (that at which celestial longitude commences). is right over the meridian on which you stand. It is, then, what you might call sidereal noon at your place, just as it is solar noon when the stn is on the meridian. In the thoughtful mind the question may be raised as to whether longi- tude when measured by an interval of sidereal time, is the same as when measured (numerically speaking) by a similar interval of mean time, seeing that sidereal and mean time have dif- ferent absolute values. Raper disposes of this question thus: "The difference of longitude is found as well by means of the mo- tion of a star.as of the sun; that is, by means of a clock regulated to sider- eal time, as well as one regulated to mean time. For although the abso- lute interval of time employed bya star in moving from one meridian to the other is less than that employed by the sun, yet it is divided into the same number of hours, minutes and seconds, but which are of smaller magnitude, and thus the difference of time re- sults, in numbers, the same." In connection with time, there is one point which deserves more than a passing notice; and which one who is not familiar with the subject finds it difficult to realize, and that is, that at the same moment that there should be a difference of time at various parts of the earth's surface--nor is this really the case so far as absolute time is concerned. The present moment here in the. United Statés is equally the present moment in the Philippine Islands, although the clock there marks some 10 hours later than it does with us. This is accounted for by the fact that the sun, which is the divider of day and night, and all over the world the recognized marker of time, crosses the meridian of the Phil- ippines some 10.hours'. before it reaches ours. In the daily course of the sun, his advent at each meridian on the earth's surface marks the hour of noon for all places ton, that meridian, It i, thus that the sailor, more. especially, reckons this time... No matter what seas he may be navigating, he consid-, ers it noon the moment the sun is meridian, high. In turning these various definitions over in the mind, one must try not to get "mixed", as will, however, very likely be the case with those whose has only been seriously Do

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