Local Mean Time (LMT) (061.05.01)
Mean solar day (061.05.01.01)
Explain the concepts of a mean solar day and LMT.
An apparent solar day is the time interval between two successive transits of the Sun across the local meridian (that is, from one apparent noon to the next apparent noon), as measured by the actual Sun’s position in the sky.
In simpler terms:
- It is the length of a day based on when the real Sun reaches its highest point in the sky on consecutive days.
- This length varies slightly throughout the year, mainly due to:
- The elliptical shape of Earth’s orbit
- The tilt of Earth’s axis
Because of these variations, apparent solar days are not all exactly 24 hours long, and have varying lengths (longest in Februari, shortest in November). This makes it kind of impractical for day to day use, that is why we use mean solar time(and the mean solar day) for clocks.
The mean time is based on a fictitious body: “the mean sun”. Thus, the mean day is the time between two successive transits of the mean Sun, and is a constant.
Local mean time (LMT) is the average solar time at a given longitude, based on the motion of the fictitious mean Sun that moves uniformly along the celestial equator. Local noon occurs when the Sun crosses the local meridian and reaches its highest point in the sky (12:00 hrs local time).
The LMT at Greenwich (GMT), is used for global reference. The modern term UTC, refers to a more accurate form of GMT, regulated against the international atomic time, but is, in practice, the same as GMT.
To calculate the differences between LMTs and/or between LMT and UTC/GMT, the following arc to time conversion is needed:
1° = 4 minutes
1′ = 4 seconds
Off topic:
the civil year is the time it takes for the Earth to orbit the mean sun, which is a contant of 365 days, 5 hours, 48 minutes and 45 seconds. Not being exactly 365 days, means we need a leap year (an extra day) every four years.
Note: every 4 years except whole century years, unless it is a whole century year divisible by 400.
Standard time (061.05.02)
Standard time and daylight saving time (061.05.02.01)
Explain and apply the concept of standard time and daylight saving time, and perform standard time and daylight saving time calculations.
The problem with local times is that every longitude in a country will have a different time according to local noon. A practical solution to this problem is a standard time (ST)for each 15° of longitude, which equals 1 hour (15° x 4 min/° = 60min). Standard Times are set by the government or ruling authority of a country, with common sense adjustments for political and economic zones.
Conversion of UTC to Standard Time and vice-versa is performed using information in the Air Almanac. The Air Almanac provides a conversion table, and at the top of each table there are clear instructions on how to apply the corrections.
Some states apply a hour of Daylight Saving Time (DST) during the Summer, a.k.a. “Summer Time”. The main purpose of DST is to make better use of daylight.
Living near the equator, DST is not very helpful, as day and night are nearly the same length (12 hours). But elsewhere on Earth, there is more daylight in summer than in winter. Adding one hour to ST will shift daylight towards later in the day.
Important: in summer and if daylight saving time (DST) applies, when given a ST in an exam-question, they (might) already added the additional hour (DST = ST + 1) for example:
An aircraft leaves New York, USA, for Hawaii, USA, on the 21st July, 2018, at 21:20 Standard Time (ST). Assume daylight saving time applies except for Hawall. The flight time is 12 hours 50 minutes. What will be the Standard Time and date of arrival?
Then for calculation purposes they want you to calculate with 20:20 Standard Time (ST).
International Date Line (061.05.02.02)
Since the Earth rotates from West to East, the Sun rises in the East before it does in the West. Therefore, time to the East is ahead (later) of time to the West.
When crossing the International Date Line, there is a shift in your calendar day. Traveling in a westerly direction, from the western hemisphere (e.g., Hawaii) to the eastern hemisphere (e.g., Japan), results in the acquisition of an additional day. For example, if the date is January 1st, you will find yourself on January 2nd upon completion of the crossing. Conversely, when journeying eastward, the inverse occurs, causing a loss of a day in your calendar.
Circumnavigating the globe
Traveling westwards around the world, you should set your clock:
- Back by one hour for every 15° of longitude crossed, and
- Forward by 24 hours upon crossing the International Date Line.
Traveling eastward around the world, you should set your clock:
- Forward by one hour for every 15° of longitude crossed, and
- Back by 24 hours upon crossing the International Date Line.