  ##### Distance measurement in astronomy Radar
The distance from the Earth of objects in the Solar System can be measured using radar.

Parallax
The difference in direction of a star viewed from the two ends of a line with a length equal to the radius of the Earth’s orbit is called the PARALLAX of the star.

Cepheid variables
The brightness of a Cepheid variable star varied in a particular way and Henrietta Leavitt discovered an important connection between the period and brightness now known as the period-luminosity relationship. The period-luminosity relation means that if you can measure the period of a Cepheid variable you can find its luminosity. Knowing how bright the star really is and then measuring how bright it appears to be will then give the distance of the star from the Earth.

 Astronomical unit One Astronomical unit (AU) is defined as the mean distance of the Earth from the Sun (1.5x1011 m) The light year This is the distance that light travels in free space in one year = 9.5x1015 m The Parsec 1 parsec is the distance at which an object subtends an angle of one second using the radius of the Earth’s orbit as the baseline. 1 Parsec = 3.06x1016 m = 2.04x105 AU = 3.26 light years 1 Mega parsec (Mpc) = 3.26x106 light years = 3.097x1022 m