# COSMIC DISTANCE LADDER PART 1

Updated: Sep 26, 2021

- Oliver Pereira, MSc Part II

Have you come across pictures of various stars with captions that mention their distances as hundreds of light years? Such numbers are so fantastically huge on our everyday scales of meters and kilometers, that we cannot really visualize how far away these objects really are. And yet the stars seem like tiny dots, like holes in some distant curtain. There is no perception of depth in the night sky. The ancients identified the planets as distinct entities from stars. The earliest models of the universe, such as a geocentric universe, could predict planetary positions. Eratosthenes used the difference in the angle of the sun’s rays from two different cities at noon to calculate the Earth’s radius. Aristarchus used this radius along with the time taken by the moon to cross the Earth’s shadow during a lunar eclipse to calculate the distance of the moon from the earth. This also made it possible to estimate the radius of the moon. The average distance of the earth from the sun is the “astronomical unit”, the next fundamental distance. This required measuring the solar parallax. Parallax is just using simple trigonometry to measure distance. If you hold your thumb in front of your eyes and look at it with one eye open against a clear background, you notice that your thumb shifts in position with respect to the wall when you switch eyes. This represents the practical implication of the parallax method.