Earth's Motions, Latitude, Longitude and Time
Earth's Motions, Latitude, Longitude and Time
This topic gives 3 questions and is examined more precisely in Part-III than in General Studies. The paper asks about solstices and equinoxes, about time zones, and about the International Date Line — the last being where most candidates lose the mark.
Rotation and Revolution
| Point | Rotation | Revolution |
|---|---|---|
| Motion | Spinning on its own axis | Moving around the sun |
| Time taken | About 24 hours | About 365 and a quarter days |
| Direction | West to east | West to east |
| Result | Day and night | Seasons and the year |
| Axis | Tilted at 23.5 degrees | Tilt stays constant |
The quarter day is why a leap year comes every fourth year. The earth rotates west to east, which is why the sun appears to rise in the east.
Solstices and Equinoxes
| Date | Event | Sun overhead at |
|---|---|---|
| 21 June | Summer solstice | Tropic of Cancer |
| 22 December | Winter solstice | Tropic of Capricorn |
| 21 March | Spring equinox | Equator |
| 23 September | Autumn equinox | Equator |
On 21 June the northern hemisphere has its longest day and the southern its shortest; on 22 December the reverse. At an equinox day and night are equal everywhere, because the sun's rays fall vertically on the equator.
The word equinox means equal night, which is the definition and the memory hook together. Note that the seasons are caused by the tilt, not by distance from the sun — the earth is actually farthest from the sun in the northern summer.
✗ The northern hemisphere is warmer in June because the earth is closer to the sun | ✓ It is the axial tilt, not distance — the earth is farthest from the sun in June
Latitude and Longitude
- Latitudes are parallel circles, measured north and south of the equator, from 0 to 90 degrees; there are 181 in all.
- Longitudes are semicircles meeting at the poles, measured east and west of the Prime Meridian, from 0 to 180 degrees; there are 360 in all.
- The Prime Meridian passes through Greenwich; the Equator divides the hemispheres.
- The distance between two latitudes is roughly constant, about 111 kilometres, while the distance between two longitudes decreases towards the poles and becomes zero there.
That last point is the discriminator: latitudes are parallel and equidistant, longitudes converge. A question asking which lines meet at the poles is testing exactly this.
Time and the Date Line
360 degrees in 24 hours = 15 degrees an hour = 1 degree in 4 minutes
Every 15 degrees of longitude means one hour of time difference. India's standard meridian is 82.5 degrees East through Mirzapur, putting Indian Standard Time 5 hours 30 minutes ahead of GMT.
India could have several time zones but keeps one for convenience. The sun rises in Arunachal Pradesh about two hours before it does in Gujarat, though both keep the same clock.
The International Date Line runs roughly along the 180 degree meridian, bending to avoid land. Crossing it westward you gain a day and eastward you lose one. The bends exist so that a single country or island group does not fall on two different dates.
समरात्रि में दिन-रात बराबर — equinox का अर्थ ही समान रात है।
TRE pointer: The solstice and equinox table with its four dates is asked almost every year, and the tilt not distance reasoning question goes with it. The International Date Line direction — gain a day going west, lose one going east — is where most candidates guess. Note also that longitudes converge at the poles while latitudes stay parallel, which is the cleanest one-line discriminator in the topic.
60-Second Recap
- Rotation gives day and night; revolution with a 23.5 degree tilt gives seasons.
- 21 June and 22 December are the solstices; 21 March and 23 September the equinoxes.
- Seasons come from the tilt, not from distance to the sun.
- 181 latitudes, parallel and about 111 km apart; 360 longitudes, converging at the poles.
- 15 degrees of longitude equals one hour; IST is GMT plus 5:30 at 82.5 degrees East.
- Crossing the Date Line westward gains a day; eastward loses one.
Frequently Asked Questions
What is the difference between a solstice and an equinox?
At a solstice the sun is overhead at a tropic, giving the longest and shortest days of the year, on 21 June and 22 December. At an equinox the sun is overhead at the equator and day and night are equal everywhere, on 21 March and 23 September.
Why does India have only one time zone?
For administrative convenience, even though the country spans nearly thirty degrees of longitude. The sun rises in Arunachal Pradesh roughly two hours before it does in Gujarat, but both follow Indian Standard Time, set at the 82.5 degree East meridian.
What happens when you cross the International Date Line?
Travelling westward across it you gain a day, and travelling eastward you lose one. The line runs roughly along the 180 degree meridian but bends around land so that a single country or island group does not end up on two different dates.
Why are seasons not caused by distance from the sun?
Because the earth is actually farthest from the sun during the northern summer. Seasons are caused by the 23.5 degree tilt of the axis, which makes sunlight fall more directly on one hemisphere at a time as the earth revolves.