Cambio de Fecha en el meridiano de los 180º y la Linea Internacional de Cambio de Fecha

In this new video from the Sirocodiez nautical school, included in the online CY course, I'm going to explain a concept that often raises many questions in nautical astronomy: the date change at the 180° meridian and the International Date Line. Then, I'll answer this question from the Valencia exam for the last session in December 2017. Here you can see true noon on November 21, 2017, in the town of Játiva. The sundial marks noon, 12 solar hours, and the true sun is at its maximum junction today. It is located over the upper meridian of the location, that is, its highest altitude above the observer's horizon. But currently, the official time is 12:46 in Játiva, which is the same official time in Madrid and Valencia. At this very moment, places farther east will have more hours, like Bangkok, which has 6 hours more, and places farther west will have fewer hours, like Nassau in the Bahamas, which has 6 hours less, and Los Angeles, which has 9 hours less, all with the same date. But we can also see that at this moment, Auckland, New Zealand, has 12 hours more, but with one more date. The Earth moves around the Sun in an ellipse, therefore with a non-uniform speed. If we consider the apparent motion of the Sun across the celestial sphere, the Sun travels along the Ecliptic at a variable speed. To define the day, we cannot use the different passages of the Sun across our meridian because we would have days of different lengths. Since the true Sun cannot be used to measure time, an imaginary Sun has been devised, which does not exist, and we will call it the mean Sun, which will be useful for measuring time. The mean Sun travels around the celestial Equator in the same amount of time that the true Sun travels along the Ecliptic, but with a uniform speed. The mean sun takes one year to travel around the celestial equator, and it is useful for measuring time because every day will have the same length: 24 hours. The calendar day is regulated by the mean sun and is defined as the interval between two consecutive passages of the mean sun along the same lower meridian of a given location. The calendar day is of great importance because it is the one adopted in our daily lives, and therefore each day will begin at midnight wherever we are. The equation of time measures the difference at noon between the true sun, which moves at a non-uniform speed, and the mean sun, that imaginary sun, which moves at a constant speed. Thus, true noon does not usually occur at 12 o'clock, but rather a few minutes earlier or later. In this figure, you can see that the time difference measured by the equation of time varies throughout the year. The greatest difference occurs in early November, when measured solar time is more than 16 minutes behind apparent solar time, and in mid-February, when mean solar time is more than 14 minutes ahead of apparent solar time. The NA provides the GPM for each day, which is the civil time of true noon as the Sun passes the meridian. The equation of time is equal to 12:00 hours – the value of the GPM. Thus, for today, the value of the equation of time is… 12 hours minus 11:45.9 minutes equals 14.1 minutes ahead, as you can see today. There are four times of the year when the equation of time is zero, meaning solar time coincides with civil time. Here you can see one of these moments: April 15th, the PMG is exactly at 12 solar hours, which coincides with 12 hours UT or HCG. In this interesting figure I made, notice what happens on the 180º meridian………………….. There is always a day's difference; on one side of this meridian, the date is different than on the other. Later, we'll look at this figure in detail, step by step, to better understand when the date change occurs. Logbook Time (LBT) is the time that governs the life and activities onboard a ship's crew. Each ship chooses the time it wishes to keep on board, although LBT is usually set to Official Time (O) if sailing within the same country. And when sailing long distances across the ocean, LBT must be set to the legal time of the zone in which the ship is sailing. If the ship changes time zones while sailing, heading east, the log clock will be advanced one hour, and heading west, it will be delayed one hour. When sailing east, the ship will advance the log clock one hour as it passes through each time zone, and if sailing west, it will be delayed one hour as it passes through each time zone. If the ship crosses the 180° meridian, the time is not changed, but it must advance one day if it is sailing east, that is, from east to west. It must be delayed one day if it crosses from west to east.