Aviation & Time, I
Time. Usually taken for granted but it is of the utmost essence in aviation. For time and speed relates to distance and we can calculate how long a flight will take and estimate our time of arrival and know our fuel consumption.
Do not forget that someone will start looking for us if we do not arrive on time, hopefully.
Precise time is measured with sophisticated atomic clocks with an accuracy of a couple of seconds in a million years or so. Which is just about good enough for us pilots.
Aviation law also requires us to have a time indicator either as wrist watch or as a clock in the aircraft.
Time is also used in navigation, think GPS, which relies on precise time to calculate speed and position. Some countries only allow VFR during daylight so the pilot must know when the end of daylight is to make sure he lands before that. Flightplans use UTC for time measurement.
Determining Time
To measure time we need a certain reference, the sun is a good candidate as it crosses our skies pretty much every day. And the days will change into weeks, months and seasons until years have gone by.
Earth rotation
In 365 days, the earth rotates around the sun and this period is called a year. At the same time it also rotates around its own axis in a certain time period called a day. The daily rotation is to the east and this makes the sun rise in the east and setting in the west.
Leap year
The orbit of the earth around the sun is not exactly 365 days, it is 5 hours, 48 minutes and 45 seconds longer. A correction is made every four years to enter a leap year of 366 days (end of February). With a final correction of removing 3 days every 400 years, the next one coming up in 2100.
Seasons
The axis of the earth is tilted about 23°27'. Resulting in seasons when the earth moves around the sun. The tilt is also not quite stable, it wobbles a bit thus causing slight variations throughout the seasons. Even the distance from the earth to the sun is not the same.
Aphelion & Perihelion
The greatest distance is during the beginning of July, called aphelion. The shortest distance is during the first days of January, perihelion. And this varies too causing all kinds of seasonal variations to the weather. Don't worry, its normal.
Days and hours
As said before, the earth rotates around its axis and does that in 24 hours, that is 15 ° per hour. Each hour is divided into 60 minutes from 00 to 59 and hours are numbered from 00 to 23. Worldwide aviation uses an 24 hour clock and this is based on UTC, Coordinated Universal Time, and this used to be GMT, Greenwich Mean Time.
Time zones
The earth is also divided into time zones and each zone is 15° apart, equal to an hour. This way each zone has its own local time, although local mean time (see below) is different for each of the 15° meridians of longitude in that zone. Greenwich (UTC) is in the Z (Zulu) time zone, Z is sometimes used in meteorological forecast to indicate UTC.
Noon
At midday (noon), the sun is at its highest point in the sky and this occurs over each meridian, and again 24 hours later. A day starts at 00 hours at the anti-meridian of the sun position.
