All time is measured from
Greenwich a suburb of London
The zero degree longitude line
or the Prime Meridian passes
Greenwich Mean Time (GMT)
is the time it is in Greenwich.
Greenwich was chosen as a lot
of the world’s sea charts were
being drawn up in England at
Time zones are calculated in relation to GMT.
Greenwich Mean Time
For locations to our east we add time.
For locations to our west we subtract time.
Perth = Adelaide – 1hr 30min
= 9am - 1hr 30min
= 7:30 am
DURING DAYLIGHT SAVING TIMES IN AUSTRALIA, TIMES CHANGE SLIGHTLY
Note: Times are still calculated in relation to GMT times.
1 revolution of Earth = 365.25 days 6 hours x 4 years = Leap year
& we catch up
1 rotation of Earth = 24 hours
Circle = 360°
360° /24 = 15° = 1 hour
Earth rotates 15° each hour
At the equator Earth ≈ 24,900 miles = 1037.5 miles in an hour
UTC = GMT
UTC = Coordinated Universal Time
11.23am Gold Coast Time
Los Angeles is 18 hours behind Gold
Coast and is still the day before.
Los Angeles is
18 hours before G.C. Time
14.33-18 = 12-4 = 8.33pm
New York is
15 hours before G.C. Time
=0033 -1 = 2333hrs
By this photo, you can see more of the world is now into the
next day of the calendar. You can see that it has just
passed midnight in Bolivia. i.e. 14.33pm – 14hrs = 0033hrs
You can see here, that it is about 10pm in Queensland. In this hour, the whole
Earth is on the same day. Once it hits 11pm, (Qld time), 15 degrees west of the
International dateline is in the next day.
If it is 7:08am
hours in London
… what time is
it in Sydney?
Sydney = London + 10hr [East]
= 07:08 + 10hr
= 17:08 = 5:08 pm
If it is 10:20am
hours in Sydney …
… what time is
it in Honolulu?
Honolulu = Sydney - 20hr [West]
= 10:20 - 20hr
= -10:20 + 24:00 = 14:20 yesterday
= 2:20 pm yesterday
When crossing the International Date Line…
•going east – subtract (24 hours) one day
•going west – add (24hrs) one day
If we look at two island nations that are about 900 kms apart.