In 1884, it has been decided that the zero meridian is one that passes through the royal astronomical observatory at Greenwich near London.
180 0 meridian (International Date Line) is exactly opposite to the prime meridian (such points are called antipodal points).
82.5 0 East meridian is the standard time meridian of our country, it is 5 hrs 30 min ahead of the Greenwich mean time (GMT) and is situated at Naini near Allahabad.
The earth is divided into 24 longitudinal zones, each being 15 0 or 1 hrs apart in time or 1 0 in four minutes are called standard time zones.
Russia has as many as 11 time zones.
Both USA and Canada have five time zones viz. the Atlantic, eastern, central, mountain and pacific.
International date line:- the 1800 meridian running over pacific ocean deviating at Fiji, Samoa and Gilbert islands. (deviates at land masses)
While crossing date line from west to east – repeats a day.
While crossing date line from east to west – losses a day.
Generally a total of seven eclipses (including solar and lunar eclipse) take place in a year.
Solar eclipse (moon comes between) –
It can be partial or total.
Happen only on a new moon day.
But it doesn’t occur in every new moon day.
Lunar eclipse (earth comes between) –
Happens only on a full moon day.
But not on every full moon day.
Structure of earth’s interior –
The crust.
Thinnest layer.
Least density.
Thickness varies from 8 km to 40 km.
The rocks are rich in lighter minerals like Si and Al.
Layer is also called Sial.
Average density = 2.7 gm/cm3.
The mantle.
About 2900 km in thickness.
Minerals are in semi solid state.
Upper mantle (Asthenosphere) is about 250 km thick.
Layer is also called Sima.
Average density = 5.68 gm/cm3.
The core.
The innermost layer.
3500 km in radius.
Also known as Nife.
Average density = 17.2 gm/cm3.
Discontinuities: The various layers are separated by discontinuities, which are advent in seismic data.