Practically, determining the distance to a galaxy or a star has never been easy. However, astronomers have proved to be very innovative and clever people since they have invented several schemes through which reasonable approximations on the distances to stars can be made though there is some uncertainty. One of basic methods that has been in use is the parallax method which is based on trigonometry (Jones, 2009, p. 44). The is derived from the fact that as the earth revolves around the sun, the nearby stars appear to be shifting from the far away stars, a process known as the parallax shift. To measure the parallax, the distance to the star is observed at least twice in a year so that the second observation is made when the sun is on the opposite side from the first observation (Jones, 2009, p. 125). Thus by studying the shift distance, and determining the diameter of the orbit of the earth, the parallax angle can be calculated. The parallax units are arcseconds.
There are however some limitations when using this method to measure the distance to the stars. These include; the size of the shift and the blurriness caused by the atmosphere. Assuming that the star is at a distance D= 1 parsec from the earth, then it will only shift by an angle θ=1 arcsecond which is a very small angle thus making accurate measurements difficult to make. The lesser the shift, the far away the star is from the earth. Due to this, the method only becomes truthful for stars that are only a few hundred light-years from the earth (Jones, 2009, p. 44). To measure the distance to stars that are beyond one hundred light-years from the earth, Cepheid changeable stars, they change in brilliance after some time, are used. The true brightness is then determined and compared to the apparent brightness, thus allowing the astronomers to compute their actual distance (Jones, 2009, p. 192). Another limitation is that the atmosphere of the earth acts as a hindrance on clear views of the galaxy and stars. When looking through with an ordinary telescope, not all times will one obtain a perfect point because there are times when the visibility is interfered with by the clearness of the atmosphere (Jones, 2009, p. 24).