230 II. TO CONVERT TIME INTO LONGITUDE (ARC), multiply the time by 15 (or by 3 and 5 successively, since 3 x 5 = 15); then the seconds of time become seconds (") of arc, the minutes of time become minutes (') of arc, and the hours become degrees. Or, if we turn the hours into minutes, and divide by 4, then the minutes of time give degrees, the seconds of time give minutes (') of arc, and the remainder (if any) multiplied by 60 and divided by 4 gives seconds (") of arc. Or the conversion may be made at sight by Table (see Norie's Tables, p. 198). Example.-Convert 8h. 40m. 34s. into longitude (or arc). H. M. S. H. M. S. 120° 26 I 42 60 40m. = ΙΟ o' 5 130° 8' 30" 130° 8′ 30′′ 130° 8′ 30′′ By these Rules we get the following results, which you can work out COMPARISON OF CIVIL TIME AND ASTRONOMICAL TIME Civil Time. For business purposes and the registry of the common transactions of daily life, it has been deemed more convenient to begin the day at midnight, that is when the sun is on the meridian at its lower transit. But the civil day-consisting of 24 hours-is divided into two periods of twelve hours each, viz., from midnight to noon, marked a.m. (ante meridiem), and from noon to midnight, marked p.m. (post meridiem). Astronomical Time.-A solar day at any place is the interval of time between two successive transits (passages) of the sun's centre over the meridian of that place and solar time at any instant is the westerly hour angle of the sun at that instant. The solar day (it may be apparent, or it may be mean time) is conceived by astronomers to commence at noon (apparent or mean), and is divided into twenty-four hours, numbered successively from 0 to 24. Astronomical time (apparent or mean) is, then, the hour angle of the sun (apparent or mean), reckoned on the equator westward throughout its circumference from oh. to 24h. a.m. and p.m. never appear in astronomical time. To convert civil time into astronomical time.-Since, from what has been said above, civil time agrees with astronomical time from noon to midnight, while from midnight to noon they differ, hence the rules— I. If the civil time is p.m., and thus referring to the hours from noon to midnight, it agrees with the astronomical time; thus Civil T., May 4th 5h. 20m. p.m.=Astronomical T., May 4d. 5h. 20m. 2. If the civil time is a.m. and thus referring to the hours from midnight to noon, add 12h. to the civil time and write the date a day back, to get astronomical time. Or, to get civil time from astronomical time, subtract 12 hours from the astronomical time and write the date a day forward. Civil T., Jan. 19th 5h. 17m. 4s. p.m. = Astron. T., 19d. 5h. 17m. 4s. Civil T., May 4th 5h. 42m. a.m. = Astron. T., May 3d. 17h. 42m. = Astron. T., July 31d. 21h. 20m. Civil T., Dec. 4th 3h. 5m. a.m. Civil T., Feb. 1st 6h. 3m. a.m. Civil T., Jan. Ist 6h 10m. a.m. TIME AT DIFFERENT MERIDIANS, AND GREENWICH DATE Solar or Sun Time is measured by the daily motion of the real or true sun; it is called the apparent time. An apparent solar day is the interval of time between two successive transits of the sun's centre over the same meridian. It is apparent noon when the real sun's hour circle coincides with the meridian of the place, that is, when the sun's meridian altitude is observed for the latitude; it is the readiest and most natural measure of time, and hence the unit of time adopted by the navigator at sea is the apparent solar day. But these intervals, though convenient for sea purposes, are not equal, and therefore cannot be taken as a standard, since the sun describes in a day an arc of 57'-2 of the ecliptic in July, and of 61'2 in December. This want of uniformity in the real sun's motion is obviated by the adoption of a fictitious, or mean sun, which is supposed to move in the equator with a uniform velocity, which is the mean velocity of the true sun in the ecliptic. Hence mean time is the westerly hour angle of the mean sun; a mean day is the interval between two successive transits of the mean sun over the meridian; and mean noon is the instant when the mean sun's hour circle coincides with the meridian. Mean time, being perfectly equable, and lapsing uniformly, is measured by the chronometer when the correct rate is applied from day to day. The difference between apparent and mean time is the equation of time. Four times in the year-in April, June, August, and December-apparent and mean time coincide; in the intervals they differ, and their greatest difference occurs in February, May, July, and November. The equation of time is used to convert apparent time into mean time, and mean time into apparent time. Thus, if the apparent time be given, the corresponding mean time will be found by adding or subtracting the equation of time according to the precept at the head of the column in which it is found on p. I. of the month in the Nautical Almanac. If the mean time be given, the apparent time is found by applying the equation of time as directed by the precept on p. II. of the month in the Nautical Almanac. The westerly hour angle of the sun at any meridian is the local (solar) time at that meridian; i.e., the time at place which may be apparent or mean. The westerly hour angle of the sun at the Greenwich meridian at the same instant is the corresponding Greenwich time. The difference between the local time at any meridian and the Greenwich time is equal to the longitude of that meridian from Greenwich, expressed in time, on the basis of 1h. 15°. = The difference between the local time of any two meridians is equal to the difference of longitude of those meridians. In comparing the corresponding times at two different meridians, the most easterly meridian may (owing to the earth rotating on its axis from west to east) be distinguished as that at which the time is greatest, or most advanced; inasmuch as the sun will rise, culminate, and set at any given meridian earlier than at any meridian to the westward of it: for example, at a given instant of absolute time on March 20th, the sun (by local time) will be setting at 6h. p.m. in the middle of the Bay of Bengal, it will be on the meridian (noon) at Greenwich, and be rising at 6h. a.m. in the middle of the Gulf of Mexico. N.B.-The local time, at sea, is usually called the time at ship. The Greenwich date, either exact or approximate, is essential for the correction of the data from the Nautical Almanac ; and the first step in any computation is as follows Given the Time at Ship and Longitude, to find Greenwich Date 1. Express the ship date as astronomical time. 2. Convert the longitude into time, and write it under the ship date. 3. If the longitude is west, add it to the ship time. N.B.-If the sum of the hours is more than 24h., reject 24h., and carry one day to the date. 4. If the longitude is east, subtract it from the ship time. N.B.-If the longitude exceed the hours at ship, add (mentally) 24h. to the ship time, and put the date one day back. Find the Greenwich date in each of the following examples Example 1.-May 10th, time at ship being 4h. 42m. 20s. p.m., in long. 48° 32′ W. Example 2.-August 4th, time at ship 9h. 50m. 12s. a.m., 134° 12' W. Long. 134° 12′ W. D. Ship date, August 3 Green. date, August 4 H. M. S. 20 50 12 8 56 48 W 5 47 o in long. Example 3.-August 1st, time at ship 9h. 40m. p.m., in long. 49° 15′ E. Example 4.-December 5th, time at ship 4h. 32m. p.m., in long. 127° 30' E. These examples show you how, through an approximate time at ship, and longitude by dead reckoning, you can get an approximate Greenwich date. The method of proceeding is essential, inasmuch as all observations at sea are referred to chronometers regulated to Greenwich mean time, and as these instruments are only marked on the face or dial from oh. to 12h., it becomes necessary to distinguish whether it is a.m. or p.m. at Greenwich, and hence when it is necessary to increase the chronometer time by 12h. To find the correct Greenwich Date by Chronometer Proceed as by previous rule to get the approximate Greenwich date, then apply the error to the chronometer time, and give the result a date corresponding to the approximate date, adding 12 hours to the time by chrononeter when necessary to make them agree. Example 1.-May 10th, at about 4h. 42m. p.m. at ship, in long. 48° W., the chronometer read 7h. 56m. 22s., which was slow on Greenwich mean time 3m. 25s. Find the correct Greenwich date. Example 2.-July 10th, at about 8h. 50m. a.m. at ship, in long. 134° W., the chronometer read 6h. om. 10s., which was fast on Greenwich mean time 6m. IIS. Find the correct Greenwich date, mean time. Example 3.-August 6th, at about 4h. 32m. p.m. at ship, in long. 127° E., the chronometer read 8h. om. 35s., which was fast on Greenwich mean time Im. 40s. Find the correct Greenwich date, mean time. Example 4.-April 1st, at about 8h. 7m. a.m. time at ship, long. 45° W., the chronometer read 11h. 5m. 49s., which was 6m. 50s. fast on Greenwich mean time. Required the correct Greenwich date, mean time. Verify the following examples for getting the correct Greenwich date 1. N.B.-When it is noon at ship in west longitude, the Greenwich date is the longitude in time. Example.-May 15th, noon at ship in long. 95° W. (= 6h. 20m.), the Greenwich date is May 15d. 6h. 20m. 2. N.B. When it is noon at ship, in longitude east, subtract the longi tude in time from 24h., and the remainder will be the Greenwich date of the preceding day. Example.-May 15th, noon at ship in long. 152° E. (=10h. 8m.), the Greenwich date is May 14d. 13h. 52m. NOTE. This method clears up the ambiguity that sometimes exists in connection with the indicated hour by chronometer and its relation to the correct Greenwich date. |