Time, Part Two (U.S. Navy)

as we have shown before different  civil times are occurring at the same instant all over the world this  would mean that a ship sailing East or West would have to be setting its clocks  continuously which would not be at all practical for the convenience of Navigators  the Earth has been divided into zones all places within a Zone keep the time of the central  Meridian of the Zone not their own local several times time zones are based on the fact that there are 24 hours in a day and the  distance around the world is 360° therefore the circumference of the earth  has been divided into 24 bands 15° in WID these 15° bands are the zones the central Meridian  of the various zones are those meridians East or West which are integral multiples of 15°  these zones have been designated Plus in west longitude and minus in east longitude each Zone  being also named by a number which is called The Zone description and is equal in value to the  longitude of the central Meridian of the Zone converted into ours each Zone extends 7  and 1 12° east and west of the central Meridian for example the longitude of  San Francisco is about 122° West 122 ided 15 gives us 8 with 2° left over  this remainder is less than 72° from the 120th Meridian and therefore  San Francisco is in Zone plus 8 to obtain grenwich Cil time then we have only to  add a plus Zone description to or subtract a minus Zone description from The Zone time remember  Zone time is the time kept in an area 7 and 1 12° on either side of the central Meridian of the  zone and is the local civil time of that Central Meridian if we project this Zone time diagram on  a Mercator Projection of the world we can show the relationship of Zone time to the times  at various positions on the Earth's surface this Zone time is similar to our Standard Time  such as Eastern Standard Time Central Standard Time Etc this system has been extended to  the ocean areas of the world World in this fashion watch the little ship as it  sails Eastward across the Pacific Ocean now it reaches the border of  a new Zone see how the clock jumps ahead 1 hour the clock face shows the  time the ship is keeping as it steams along now our ship reverses course and  steams Westward see as it crosses this time zone Bo order its clock jumps backward 1 hour so remember that zone time plus or minus  Zone description will give grenwich civil time now let us consider the matter  of a new day in relation to the date as one day ends and another commences at midnight or at the lower Transit of the mean  Sun for any position on the Earth it is evident that a new day is always commencing  somewhere in the world consequently there can be two different dates occurring in different  parts of the world at the same instant of time to help understand this matter of different  dates occurring at the same time let us consider the Sun moving Westward across the surface of  the world represented by this Mercator chart the sun is now transiting the upper branch of the  grenwich meridian and therefore it is noon at any place on the grenwich Meridian and the date is  the same all over the world for example let us assume that the date is the 24th the gray area  represents that portion of the world which is in the dark and the heavy black line represents  the Meridian where a new day is beginning that is to say it is midnight at the black line and  a new date is occurring there the Eastern edge of the gray area represents morning where the  sun is rising and the Western Edge represents evening and the Sun is setting the figure  in the box is the date for that particular Meridian as the sun moves to noon at  New York note that the sun has just transited the lower branch of the meridian of  Singapore and a new day has commenced there therefore it has become the 25th at Singapore when the Sun reaches  Honolulu it is noon there and a new day is beginning at Athens  Greece the date at Athens is now the 25th as the sun transits the 180th Meridian we  know that the date at grenwich has changed as the 180th Meridian is the lower branch of the  grenwich meridian and therefore grenwich will have the same date as places in east longitude  while places in west longitude will still be a day behind when the Sun reaches Manila the date  at Buenos Aries becomes the 25th at Bombay noon a new day starts at Denver and it is the 25th  the sun continues on its way until it again reaches the the upper branch of the grenwich  meridian and it is now the 25th all around the world to illustrate this same matter of dates in  another way let us view the Earth from the South Pole commencing with the sun on the grenwich  Meridian we know that it is noon there and the date is the same all around the earth  we will assume that the date is the 7th as the sun moves around the earth it transits  the lower branch of the meridian capital M Small M as it transits at small M the date  changes and a new day begins at capital M if we allow the black area to indicate the  positions where the date has changed to the eth we can see that the date of all places in east  longitude becomes the eighth before Greenwich as the sun transits the lower branch of  the grwi meridian it becomes the eth at grenwich and all places in west  longitude are behind Greenwich in date as the sun again reaches grenwich  we can see that the date is the eight and once more it is the same all around the world in the matter of date there is one important  point which should be considered this is the change of date as a ship crosses the  International Date Line note that the date line follows the 180° Meridian for the  greater part of its length let us suppose that it is500 at grenwich and the date is August  88 observe that as we move Eastward in longitude the time becomes later and later because  the sun is farther past the local [Music] Meridian until here in 176° east longitude The  Zone time is 12 hours later than at grenwich and therefore it is 0300 and the date is August 9th  however if we move Westward from Greenwich the time becomes early earlier and earlier until here  the sun is on the local Meridian and it is [Music] noon while here in 176° west longitude  the time is 12 hours earlier than at grenwich and therefore it is 0300 and  the date is August 8th the same as at grenwich thus we see that while the  time of day here at 176° East and at 176° West are both 0300 the date here  is August 8th while here it is August 9th this is always the situation adjacent to  the International Date Line West of the date line in zone minus 12 the date is one day  later than it is east of the date line in zone + 12 this change of date can be very well  illustrated by the use of this ship observe as it steams Eastward the time is 0300 and the  date is the 9th as it crosses the line the time of day Remains the Same but the date becomes  one day earlier and it is 0300 on August 8th since data in the almanac are tabulated for  Greenwich date as well as time establishing the date at Greenwich is most important  suppose that we assume our longitude to be 55° West and the Zone time 1900 on August  6th as any place in longitude 55° West is in Zone plus 4 we add the Zone time time and the  Zone description and the sum is Greenwich civil time since the total is less than 24 hours we  know the Greenwich date is the same as the local date it can be seen that the sun has not  transited the lower branches of either the local or Greenwich meridians since Grenich noon  therefore the dates are the same 3 hours pass by note that the sun has transited the  lower branch of the Greenwich Meridian but it has not reached the lower branch  of the local Meridian The Zone time is now 2200 August 6th to this add Zone  description plus 4 and we find the sum is 26 hours therefore at Greenwich it is  the next day Day and The Time is 0200 August 7th the sun has transited the lower branch of  Greenwich but has not reached the lower branch of the local Meridian therefore the date at  Greenwich is one day later than the local date let us now examine the problem  for an observer in east longitude our longitude is 118° East The Zone time  is 2200 May 4th note that we are in zone minus 8 so therefore we subtract  the Zone description from The Zone time the result is the grenwich Civil  time 14400 and the date is still the fourth 5 hours pass by the The Zone time  increases 5 hours and is now 0300 the local date is now May 5th as our Zone description is  minus 8 it cannot be subtracted from The Zone time of 3 hours so we add 24 hours to the Zone  time and take away one day from the local date therefore the Greenwich date must be earlier than  the local date by one day and the Greenwich civil time will equal 27 hours minus 8 or 1900 May  4th note on the diagram that the sun has Tred the lower branch of the local Meridian but has  not reached the lower branch of the grenwich Meridian in this situation the date at Greenwich  is always one day earlier than the local date in the foregoing discussion of Zone and civil time  we have been considering the mean Sun however the sun which we see transiting our Meridian and the  one on which our Navigator takes his observations is the apparent or true Sun the hour angle of  the apparent sun is measured by Apparent time how to convert civil time to apparent time  can be shown on this diagram the white sun represents the true or apparent sun and the gray  Sun represents the mean Sun as they apparently revolve about the world in the course of  a year note that the space between them varies as first the true sun is ahead of the  mean sun and then behind it this angle is a measure of the time that the true sun is ahead  of or behind the mean Sun this varying amount of time is called the equation of time  and is tabulated in the nautical Almanac against Greenwich civil time as the note on the  bottom of each page States the equation of time is added to or subtracted from Granite civil  time according to the sign which precedes it as we can see the grenwich hour angles of the sun  which locate for us the positions of the true Sun are also tabulated against Granite civil time  the tabulated values are determined by taking the grenwich Civil time and to it applying the  equation equ of time this gives us the grwi apparent time or the location of the true sun with  reference to the lower branch of the Greenwich Meridian as our angles are measured to  the westward from the upper branch of a Meridian we can see that it is necessary to  add 12 hours to the grenwich apparent time of the sun to obtain the grenwich hour angle  of the sun if the Sun is east of the meridian if the Sun is west of the meridian we must  subtract 12 hours to obtain the grenwich hour Angle now the position of the sun  has been located in units of time as it is more convenient in solution of the  astronomical triangle to use units of Arc degrees minutes and T of minute the grenwich  hour angle is converted into Arc and this is the value in degrees which is found in the  almanacs tabulated with respect to Greenwich civil time to convert grwi civil time to grenwich  apparent time add or subtract the equation of time to convert grwi apparent time to grwi  hour angle of the true Sun add or subtract 12 hours that is how grenwich hour angles of the  sun are determined and tabulated in the Alan a a different method is used to determine the  granite hour angle of the Stars to illustrate this we return to consideration of  siderial time which is measured from the upper branch of a Meridian Westward to  the hour Circle through the March Equinox if we measure from the upper branch of  the local Meridian to the March Equinox the time is local siderial time if  we measure from the upper branch of the granich meridian to the March  Equinox the time is granich siderial time note on the diagram that siderial  time is the same as the hour angle of the March Equinox as our angles are also  measured Westward from the upper branch of a Meridian due to this relationship the grenwich  hour angle in units of Arc can be tabulated instead of grenwich siderial time in units of  time this is the basis for one of our systems of tabulation of granich hour angle in the Alman  X therefore the granich hour angle of the March Equinox plus the siderial hour angle of a star  gives the grenwich hour angle of the star star which is required in navigation to determine one  of the coordinates of the star the local hour angle born in the vastness of space and  measured by the perfect regularity of the rotation of the earth upon its axis time  is the essence of celestial navigation