# Astronomy

1 ) Kepler ‘s three laws

First law

The orbits of the planets are in the shape of ellipses , with the Sun (or other star ) at one focus of the ellipse Second law

There is an imaginary line (a radius vector joining planet to Sun ) which sweeps out equal areas in equal times Third law

The square of the period of orbit , divided by the cube of the radius of the orbit , is equal to a constant for that planet being orbited 2 ) Astronomical Unit (AU , the Light Year (LY , and [banner_entry_middle]

the Parsec (pc

An astronomical unit (A .U ) is the average distance between the Earth and the Sun , which is about 150 million kilometers The light year is the distance that light travel in one year (365 1 /4 days A parsec is the length of the long leg of a right triangle , whose short leg is one astronomical unit when the angle between the Sun and the Earth , as seen from an object in space (a star for example , is one arcsecond Define AU in terms of kilometres

1A .U 0km Define light year in terms of AU The speed of light is about 300000 kilometres per second , hence 1 LY 300000 t

t 60 60 24 365 ,25 sec 1 LY 300000 00 km 1 LY 00 /0A .U 63115 .2 A .U Define parsec in terms of light years

according to the figure above : 1ps 1 /sin ) A .U is small then sin ( in radian 1arcsecond 1 /3600 of a degree (1 /3600 (2 /360 ) radian 4 .8481368e-6 radian

1ps 1 / A .U 206265 A .U 206265 /63115 .2 LY 3 .26 LY 3 ) Explain what would happen to a planet ‘s motion if suddenly all gravitational forces ceased to exist . Relate your explanation to Newton ‘s 1st and 2nd Laws If suddenly all gravitational forces ceased to exist , the planets will stop revolving around their stars and , according to Newton ‘s 1st Law start moving uniformly and linearly 4 ) State Newton ‘s Law of Universal gravitation Newton ‘s Law of Gravitation states that the force of gravity between two particles of mass and has a magnitude of , where r is the distance between the centre of the two masses and G is the gravitational constant : G 6 .67 10-11 N m2 /kg2 Reference Ridpath , Ian (1997 . Dictionary of Astronomy . Oxford : Oxford University Press… [banner_entry_footer]

**Author:** Essay Raptor