Calculating Time Using Longitude

Section 1Why longitude affects time

Earth rotates once every 24 hours. Since a full circle is 360 degrees, Earth turns 15 degrees each hour. That is why places at different longitudes have different local times.

Section 2East and west rule

Places east of you experience sunrise earlier, so they are ahead in time. Places west of you experience sunrise later, so they are behind. Here's the trick: east add, west subtract.

Section 3The calculation steps

First find the difference in longitude. Then divide by 15 to convert degrees into hours. If the place is east, add the time difference; if it is west, subtract it.

Section 4When minutes appear

Sometimes the longitude difference is not a neat multiple of 15. Since 1 degree equals 4 minutes, you can convert the leftover degrees into minutes. For Grade 9, many questions use neat hour differences, but know the 1 degree = 4 minutes shortcut.