Local time

Also called: Local mean 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

Local time is the time of a place set by the sun's position over that place's own meridian (line of longitude). It is 12 noon when the sun is highest in the sky over that meridian. Every meridian has its own local time. Because the earth turns 15° every hour, local time changes by 4 minutes for each degree of longitude. This is why countries that stretch far from east to west need a standard time, and why sailors could use time to find their longitude.

Explanation

How local time works: rotation, then longitude, then time

• Longitude is how far a place is east or west of the Prime Meridian, measured as an angle. The Prime Meridian is the 0° line through Greenwich, England. The antimeridian is the 180° line on the opposite side of the earth [2].
• The earth's rotation sets the rule:
• The earth turns from west to east. It turns through 360° in 24 hours.
• 360° ÷ 24 h = 15° of longitude per hour [3].

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

• What we see: the sun seems to move across the sky from east to west. It crosses one meridian after another, so each meridian has noon at a different moment.

• Local noon is the moment the sun is highest in the sky over a meridian. In the northern hemisphere the sun is then due south, at its greatest height above the horizon [3].

Arc-to-time conversion (for exam sums)

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

• Each hour of difference between local noon and the time at Greenwich equals 15° of longitude [2].

Two basic rules

• 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.

• When it is noon at your place, a place 15° east has 1 p.m. and a place 15° west has 11 a.m. [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 have different local times.

Method: working out local time

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

• Places on 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 to the east, add this time. If it is to the west, subtract it.
• Worked examples (NCERT Class 11), with noon (12:00) 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 with the opposite-sides rule: 90° + 90° = 180°. 180 × 4 = 720 min = 12 hours. That is exactly the gap from 6 a.m. to 6 p.m.

Where it occurs

• Greenwich, England (0°): the starting line for all longitude and time sums [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) (NCERT Class 6):
• Take the longitude difference as 30°. 30 × 4 = 120 minutes, which is about 2 hours.
• Tinsukia is to the east, so its local time is about 2 hours ahead of Porbandar.

• On the ground, the sun has already set in Assam while Gujarat still has daylight.

• India as a whole: the country has many local times from east to west. It uses one time, Indian Standard Time (IST), which is the local time of the 82°30'E meridian (NCERT).

• Arunachal Pradesh to Gujarat: a question in Parliament noted that the sun rises nearly two hours earlier in Arunachal Pradesh than at the western tip in Gujarat [4].
• World examples: Thimphu (Bhutan, 90°E) and New Orleans (USA, 90°W) lie on opposite sides of Greenwich, so their local times are 12 hours apart.
• At sea: sailors found their longitude by comparing local noon with a clock kept on Greenwich time. 1 hour of difference = 15° [2].

Don't confuse with

• Standard Time (for example IST): one agreed time for a whole country or zone, taken from one chosen meridian (82°30'E for India). Local time is different on every meridian.
• Latitude: it does not change local time. Delhi and Bengaluru have the same local time because they are on the same meridian. Only longitude decides local time.
• Greenwich time (Prime Meridian time): this is the local time of 0° only. It is the reference clock, not the time everywhere. Places east are ahead of it and places west are behind it.
• Local noon vs 12:00 on the clock: local noon is when the sun is highest over your own meridian. In India, 12:00 IST is local noon only on the 82°30'E meridian.

Prelims Hooks

• 360° in 24 h → 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 big north–south distance does not matter.
• If it is noon at Greenwich → Thimphu (90°E) = 6 p.m. and New Orleans (90°W) = 6 a.m.. Places on opposite sides: add the longitudes (90° + 90° = 180° = 12 hours).
• Porbandar (Gujarat) – Tinsukia (Assam): 30° → about 2 hours, with Tinsukia ahead.
• IST = local time of 82°30'E (NCERT). Prime Meridian = 0° through Greenwich. Antimeridian = 180° [2].

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 a country with a wide east–west spread has many local times. It needs one standard time or several time zones.

• India's wide east–west spread (about 30°, roughly 2 hours) causes real problems under one IST.

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

• This is the basis for the demand 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 link between longitude and time is used in navigation and mapping.

• Comparing local noon with Greenwich time gives a place's longitude. 1 hour of difference = 15° [2][3].
• The world's system of meridians and time zones is built on this method.

Related concepts

• Longitude and time

Read more

• Longitude and time: local time