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Time zones and travel times

The earth spins through 360 degrees in 24 hours, meaning that in one hour the earth spins through ${(360 \div 24 =)}$15 degrees. This means that, technically, as you travel around the earth, each 15 degrees of longitude difference in the location of places means that they should have a time difference of one hour.

Greenwich Mean Time (GMT) was established at the Royal Observatory at Greenwich (just outside of London) in 1675 to assist mariners in calculating their longitude at sea. This was at a time when every town and city in England kept its own time, based on the local noon. These differences became a problem when railway networks were built across the country. The English railway companies were the first to adopt a standard time in the 1840's, based on GMT and managed with portable chronometers, but GMT did not become the standard time for the whole of England until 1880.

New Zealand, at the time a British colony, was the first country to adopt a standard referenced time (New Zealand Mean Time) in 1868 when it set its clocks to 11 hours and 30 minutes ahead of GMT.

Standard time zones based on hourly differences were running in most countries by 1930, but it took until 1986 for all countries to set their time zones with reference to GMT. However, a number of countries, including Australia (for South Australia and the Northern Territory), have time zones offset by half-hours or quarter-hours to better suit local noon time. Other countries, such as India and China, have a single time zone for the entire country despite the fact that the extent of their borders exceeds 15 degrees of longitude.

Calculating journey times incorporates the time taken for the journey based on given local times plus a time factor allowing for the difference in longitudes. Where the longitudes are in the same direction, use the difference in values. Where the longitudes are in different directions (one East and one West) add the values together.

Example

A direct flight from Melbourne to Perth leaves Melbourne at 5.00 pm local time and arrives in Perth at 7.10 pm local time. If Melbourne is on the ${145^{\circ}}$ E meridian and Perth is on the ${115^{\circ}}$ E meridian, calculate the total journey time.

Solution

Difference in longitude = ${145^{\circ}}$ - ${115^{\circ}}$ = ${30^{\circ}}$

Time difference due to longitude = ${30^{\circ} \div 15^{\circ}}$ = 2 hrs.

Total journey time = 7.10 pm - 5.00 pm + 2 hrs = 4 hrs 10 min.

Example

The Australian Cricket team is flying from Perth to Johannesburg (South Africa) for a series of matches. Their flight leaves Perth at 6.05 am (local time) and arrives in Johannesburg at 8.50 pm (local time). If Perth is on the ${115^{\circ}}$ E meridian and Johannesburg is on the ${28^{\circ}}$ E meridian, calculate the total journey time.

Solution

Difference in longitude = ${115^{\circ}} - {28^{\circ}} = {87^{\circ}}$

Time difference due to longitude = ${87^{\circ}} \div {15^{\circ}} = 5.8 \approx 6\text{ hrs}$.

Total journey time = 8.50 pm - 6.05 am + 6 hrs = 14 hrs 45 min + 6 hrs = 20 hours 45 min.

Example

Australian Eastern Standard Time (which includes Melbourne and Sydney) is based on the ${150^{\circ}}$ E meridian and New York (USA) time is based on the ${75^{\circ}}$ W meridian. If a flight leaves Melbourne at 10.30 am and arrives in New York at 10.15 pm on the same day, calculate the total journey time.

Solution

Difference in longitude = ${150^{\circ}}$ + ${75^{\circ}}$ = ${225^{\circ}}$

(Add values since different directions)

Time difference due to longitude = ${225^{\circ}} \div {15^{\circ}}=15\text{ hours}$

\begin{align*} \text{Total journey time} &=10.15\text{ pm} - 10.30\text{ am} + 15\text{ hours}\\ &=11\text{ hr}\; 45\text{ min} + 15\text{ hours}\\ &=26\text{ hours}\; 45\text{ min}. \end{align*}

Example

Australian Eastern Standard Time (which includes Melbourne and Sydney) is based on the ${150^{\circ}}$ E meridian and London (UK) time is based on the ${0^{\circ}}$ W meridian. If a flight leaves Melbourne at 2.20 am and arrives in London at 4.45 am on the next day, calculate the total journey time.

Solution

Difference in longitude = ${150^{\circ}}$ + ${0^{\circ}}$ = ${150^{\circ}}$

(Add values since different directions)

Time difference due to longitude = ${150^{\circ}} \div {15^{\circ}} = 10\text{ hours}$

\begin{align*} \text{Total journey time} &=4.45\text{ am} - 2.20\text{ pm} + 10\text{ hours}\\ &=14\text{ hr}\; 25\text{ min} + 10\text{ hours}\\ &= 24\text{ hours}\; 25\text{ min}. \end{align*}

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