[Arasan1571]

(1)Stars are held together by the gravitational pull they exert on each other.In fact,galaxies are the collections of billions of such stars held together.(2)Pressure gradient force,coriolis force(increases with latitude affecting the wind direction),and to some extent frictional forces(below 2000 feet) due to tall buildings,small mountains etc. affect the wind direction.(3)The atmosphere filters out much of the sun's harmful ultra violet radiation and burns up objects(particularly meteors) approaching from space,preventing most of them from reaching earth.Significant amounts of oxygen in the atmosphere sustain most life on earth.Water vapour in the atmosphere is responsible for rainfall and other forms of precipitation,being one of the main sources of pure water.(4)The mantle is 2900 Km thick and makes more than 82 percent of earth's volume.it consists of molten rock called magma.(5) The tropic of cancer(23.5 degree latitude N) and the tropic of capricon(23.5 degree latitude S) are the limits of the apparent movement of the sun in the north-south direction.This apparent movement is caused due to the tilt of the earth's axis.(6)The galaxies are distinguised by their overall shape.Spiral galaxies have arms wound round a central nucleus.Barred spiral galaxies have arms spiralling round a bar-shaped core.Elliptical galaxies are oval in shape and have no arms.Irregular galaxies have no specific shape.