L05_IS_Student PDF - Time Zones & Definitions

Reason to have time zones • The Earth is a sphere which means that sunlight does not fall equally on every country at any one time. • Having time zones is to ensure the local time correspond to the amount of daylight that is available. E.g. 12 pm when the sun is overhead. GMT, UTC and Zulu time •...

Reason to have time zones • The Earth is a sphere which means that sunlight does not fall equally on every country at any one time. • Having time zones is to ensure the local time correspond to the amount of daylight that is available. E.g. 12 pm when the sun is overhead. GMT, UTC and Zulu time • Greenwich Mean Time (GMT) is the mean solar time at 0° longitude. Solar time is derived from Earth’s rotation on its axis. • Coordinated Universal Time (UTC) is derived from atomic time and follows closely to GMT. UTC is the international basis for civil and scientific time. • Zulu time is the military name for UTC. • For everyday use, GMT, UTC and Zulu time are the same. Local time • Also known as standard time • Clocks within a geographical region are synchronized into a single time standard. • Local time is expressed in terms of difference with GMT. E.g. Singapore’s local time is GMT+8 • Countries get to decide the local time they want to be in. E.g. the whole of China is on GMT+8 local time, although the country spans 5 time zones (GMT+5 to GMT+9). East Leads, West Lags • For locations with longitudes that are East of the Prime Meridian: • Local time leads GMT i.e. local time is ahead of GMT • E.g. Singapore (longitude 103º48´E)’s local time is 8 hours ahead of GMT i.e. 0800hr local time in Singapore is 0000hr GMT. • What is the local time in Singapore when it is 1000hr GMT? 1800 East Leads, West Lags • For locations with longitudes that are West of the Prime Meridian: • Local time lags behind GMT • E.g. Los Angeles (longitude 118°22'W)’s local time is 8 hours behind GMT i.e. 0800hr local time in Los Angeles is 1600hr GMT International Date Line (IDL) • The International Date Line is an imaginary line on the earth’s surface. BA • It roughly follows the 180° longitude, with diversions to avoid cutting through a country. • Two locations separated by the IDL may have the same time of day. However, the location west of the IDL will be one full day ahead of the location east of the IDL. E.g. 14th July at A, 15th July at B. International Date Line (IDL) • Countries near the line can choose to either be on the East or West side of the line. E.g. Samoa moved from west to east of the IDL on 29 Dec 2011. • Howland Island (0°48′24″N 176°36′59″W, UTC-12:00hr) is the last place on Earth for any date to exist. BA Local Mean Time (LMT) Vs Local Time • Local Mean Time (LMT) is the mean solar time of a location, i.e. the time is calculated according to the position of the sun over that location. • Used in the 19th century before time zones were implemented. • LMT is different from local time, which is based on time zones. Example • Singapore’s local time is ahead of GMT by 8 hr. • However, Singapore’s LMT is ahead of GMT by 103.8 x 4 = 415.2 mins ≈ 6 hrs 55 mins Calculate LMT Using Longitude Difference Given the LMT of one location, the LMT of another location can be calculated if the longitudes of both locations are known. The Earth takes 24 hours or 1440 mins to rotate 360°. This means that it takes 4 mins to rotate 1°. Therefore, 1° difference in longitude is equivalent to 4 mins of time difference. Example • Longitude difference between Los Angeles (34°03′08″ N 118°14′37″ W) and Greenwich (51°28’ 41'' N 0° W) is 118.37°. • LMT difference between LA and Greenwich is 118.37° x 4 mins = 473.48 mins ≈ 7 hrs 54 mins Daylight Saving Time (DST) • Many countries advance clock time in summer, usually by an hour, to take advantage of longer daylight hours and better weather. This new time is known as Daylight Saving Time in the USA or Summer Time in the UK. • In the UK, clocks are advanced one hour on the last Sunday in March. It is then put backward one hour, to align with standard time (UTC), on the last Sunday in October. • E.g in the month of April, 1200 UTC becomes 1300 British Summer Time (BST) in the UK. New York City (in the UTC –5 hours time zone) will be 0800 LMT (1200 – 4 hours instead of 1200 – 5 hours). Removing of DST • Daylight saving time is not universally adopted. The dates when clocks are adjusted are also not uniform across countries adopting daylight saving time. • More and more governments are voting to removing the practice of adjusting the clock twice a year. • 2020, Ontario, Canada, Time Amendment Act https://www.blogto.com/city/2021/11/daylight-savings-timeontario/ • 2022, USA Sunshine Protection Act https://news.yahoo.com/u-house-stops-clock-making155400726.html Discussion: When is it nighttime? In flight operations, it is important to know when nighttime starts and ends everyday. E.g. a light aircraft flying at night needs to display the following lights: In your team, list as many regulations as you can find from the Air Navigation Order and Aeronautical Information Publication that governs flight operations during nighttime. Post your findings on Padlet. Singapore’s definition of night Civil Aviation Authority of Singapore defines “night” to be the time between 20 minutes after sunset and 20 minutes before sunrise. Question: Based on the table, what time does the sun set today? USA’s definition of night • Federal Aviation Authority defines night to be a) The time between evening civil twilight and morning civil twilight, as published in the American Air Almanac. (FAR 1.1) b) The period beginning one hour after sunset and ending one hour before sunrise. (FAR 61.57b) • Civil twilight is the period of time when the centre of the sun is between 0° and 6° below the horizon