Aircraft Navigation, Time
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.
Most countries now implement some form of daylight saving, make sure to read to AIP to see when the period starts or expires, important when flying VFR and the airport closes at of just after sunset.
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.
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. 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.
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.
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.
Local Mean Time, LMT
This is the time on a meridian of longitude and each meridian has its own local time. For example: if the LMT on 30°W is 1200 then on 15°W it is one hour later, 1300 LMT. It helps to draw a horizontal line and set 0° in the middle and divide the line from the middle to the left and right in equal parts each indicating 15° until you get to 180°E or W.
To easily calculate the time on a given meridian of longitude, draw the said line above and remember that the earth rotates 360° in 24 hours, or 15° per hour, 1° in 4 minutes, 15' in 1 minute and 1' in 4 seconds. This helps in visualizing how to calculate LMT.
If you cross the international date line (located on 180°E/W) going west then enter the next day, and with going east come back a day.
Coordinated Universal Time, UTC and LMT
UTC is the LMT on the 0° meridian. It is the time reference in aviation. To calculate the LMT on a certain meridian take a look at this: what is the LMT at 90°45'E at 0915 UTC?
Calculate this as follows: 90° times 4 minutes equals 360 minutes (6 hours) and 45' times 4 seconds is 180 seconds (3 minutes). Thus 0915 UTC plus 0603 hours is 1518LMT on 90°45'E.
Daylight and saving
The sun rises in the east and sets in the west. Anywhere during this period we have sunlight, daylight and twilight. As you can imagine, this will be different on each latitude. During winter on 90°N there will be not much daylight at all and the days between 30°N and 30°S will be about the same length.
Sunlight
Sunlight is the period between sunrise and sunset. Sunrise is the exact time where the top part of the sun is visible on the horizon and sunset is where the top part of the sun disappears under the horizon.
Twilight
After the sun sets below the horizon here is still a bit of daylight left and the same happens in the morning just before the sun is visible. These short periods are called twilight and is measured when the center of the sun is 6° below the horizon. The time the sun spends in this 6° windows varies considerably as in the tropics evening darkness sets in abruptly, a short period of twilight in the lower latitudes.
Daylight
Daylight starts when the center of disc of the sun is 6° below the horizon in the east and ends when the center of the sun is more than 6° below the horizon in the west. Thus: daylight = morning twilight + sunlight + evening twilight.
The duration of daylight depends on the season (inclination of the earth) and the latitude the observer is on. Atmospheric conditions and altitude also have a pronounced effect on the duration of daylight, which can influence your VFR flight near the end of the day.
Daylight saving
Most countries have during the period of March to October some form of daylight saving where the clock is advanced one hour.
