Longitude and time

Topic: Latitude, Longitude and Time · NCERT: Class 6, Ch 1 "Locating Places on the Earth"; Class 11, Ch 3 "Latitude, Longitude and Time"

Meaning

Longitude and time is the rule that links a place's longitude (how far east or west it is from the Prime Meridian) to its local time. The earth turns through 360° in 24 hours from west to east, so the sun seems to cross 15° of longitude every hour, or 1° every 4 minutes [3]. This means places east of Greenwich are ahead in time and places to its west are behind.

This rule lets us work out the local time of any place from its longitude. It is also the reason countries that stretch far from east to west need a standard time.

Explanation

How the rule works: rotation → longitude → time

• Cause: the earth's rotation
• The earth turns from west to east.

• It turns through 360° in 24 hours.

• Process: turning angle into time

• 360° ÷ 24 h = 15° of longitude per hour [3].

• 60 minutes ÷ 15° = 1° every 4 minutes.

• Result: the sun seems to move from east to west

• It crosses one meridian (line of longitude) after another.

• So each meridian reaches noon at a different moment.

• Longitude is the angular distance (distance measured as an angle) of a place east or west of the Prime Meridian. The Prime Meridian is the 0° line through Greenwich, England. The antimeridian is the 180° line on the opposite side of the earth [2].

Arc-to-time table (for exam sums):

Longitude (arc)	Time
360°	24 hours
15°	1 hour
1°	4 minutes
15' (minutes of arc)	1 minute
1'	4 seconds

East is ahead, west is behind

• Places to the east turn towards the sun first
• So they see sunrise, noon and sunset earlier.

• So time increases eastward and decreases westward.

• Example: when it is noon at your place, a place 15° east has 1 p.m., because its meridian faced the sun an hour earlier. A place 15° west has 11 a.m. [3].

Local time and local noon

• Local time: the time of a place set by the sun over that place's own meridian.
• Local noon: the moment the sun is highest in the sky over that meridian. In the northern hemisphere the sun is then due south [3].
• Same meridian = same local time. Latitude does not matter. Every place on one meridian has local noon at the same moment [3].
• Different meridians = different local times. So a wide country has many local times. India uses one: Indian Standard Time (IST), the local time of the 82°30'E meridian.

Method: working out local time

• Step 1: find the difference in longitude.
• Same side of Greenwich (both E or both W): subtract the smaller longitude from the larger one.

• Opposite sides (one E, one W): add the two longitudes.

• Step 2: multiply the difference by 4 minutes. Change the answer into hours and minutes.

• Step 3: if the place you want is east, add this time. If it is west, subtract it.

Worked examples (noon at Greenwich):

Place	Longitude	Working	Local time
Thimphu (Bhutan)	90°E	90 × 4 = 360 min = 6 h; east, so add	6 p.m.
New Orleans (USA)	90°W	90 × 4 = 360 min = 6 h; west, so subtract	6 a.m.

• Check: the two cities are on opposite sides, so add: 90° + 90° = 180°. Then 180 × 4 = 720 min = 12 hours. That matches the gap from 6 a.m. to 6 p.m.

Where it occurs

• Greenwich, England: the Prime Meridian (0°), the base for all time calculations. The antimeridian (180°) lies on the opposite side of the earth [2].
• Delhi (north India) and Bengaluru (Karnataka, south India): both lie near 77°E, so their local time difference is zero, even though they are far apart from north to south.
• Porbandar (west coast of Gujarat) and Tinsukia (eastern Assam):
• The longitude difference is about 30°. 30 × 4 = 120 minutes, so about 2 hours.

• Tinsukia is about 2 hours ahead. The sun has already set in Assam while Gujarat still has daylight.

• India as a whole: IST is set at 82°30'E. A question in Parliament noted that the sun rises nearly two hours earlier in Arunachal Pradesh than at the western tip in Gujarat [4].

• Thimphu (Bhutan, 90°E) and New Orleans (USA, 90°W): the standard NCERT examples. They are 12 hours apart.
• At sea: sailors found their longitude by comparing local noon with a clock kept on Greenwich time. Each hour of difference = 15° of longitude [2].

Don't confuse with

• Latitude: latitude does not change local time. Only longitude does. Places on the same meridian share local noon whatever their latitude, which is why Delhi and Bengaluru have the same local time.
• Standard time (IST): local time changes from meridian to meridian. Standard time is one meridian's local time used for the whole country. For India that is 82°30'E.
• Subtracting vs adding longitudes: subtract when both places are on the same side of Greenwich. Add when they are on opposite sides. A common slip is subtracting 90°W from 90°E and getting 0 instead of 180°.
• East vs west direction: going east, add time (ahead). Going west, subtract time (behind). The earth turns west to east, so the east gets the sun first.

Prelims Hooks

• 15° = 1 hour; 1° = 4 minutes; 15' of arc = 1 minute; 1' of arc = 4 seconds.
• Time is ahead in the east and behind in the west, because the earth rotates west to east.
• Trap: Delhi and Bengaluru (both near 77°E) have a local time difference of zero. Their north–south distance does not matter.
• Noon at Greenwich → Thimphu (90°E) = 6 p.m.; New Orleans (90°W) = 6 a.m. They are 12 hours apart (180°).
• Porbandar (Gujarat) – Tinsukia (Assam): about 30° → about 2 hours. Tinsukia is ahead.
• Prime Meridian = 0° through Greenwich. Antimeridian = 180° [2]. IST = local time of 82°30'E.

Mains Points

• Local time comes from rotation and longitude, not latitude.
• The earth turns 15° every hour, so each meridian reaches noon at a different moment [3].

• So countries that stretch far from east to west need a standard time and time zones.

• India's east–west spread (about 30°, roughly 2 hours) causes problems with a single IST.

• The sun rises and sets early in the north-east, but offices open by IST.
• So daylight hours are lost, and energy use and work routines suffer.

• This led to proposals for two time zones: an Eastern Indian Time (Arunachal Pradesh to Bihar), one hour ahead, and a Western Indian Time (Uttar Pradesh to Gujarat) [4].

• The longitude–time link in navigation and mapping.

• Comparing local noon with Greenwich time gives a place's longitude, at 15° for each hour of difference [2].
• This method is the basis of the global system of meridians and time zones.

Related concepts

• Local time

Read more

• Longitude and time: local time