By engaging with all the videos within this series, you will effectively complete a full undergraduate course in astronomy, equipping yourself with the knowledge and skills necessary to navigate the night sky with confidence, learning all the basics and many advanced topics! Now, to measure the distances to nearby stars, which are crucial to astronomy. Accurate distances to celestial objects are vital. Without knowing their distance, many observations, hypotheses, and calculations would be flawed. Stars, even the nearest ones, are incredibly distant, measured in light-years or parsecs. A light-year is the distance light travels in a year, approximately 5.88 trillion miles. The primary method for measuring stellar distances is stellar parallax. This method uses the Earth’s orbit around the Sun to observe the apparent motion of nearby stars against more distant stars. As Earth orbits, a nearby star appears to shift position, creating a parallax shift. How does parallax work? 1. Geometric Baseline: The distance between the Earth and the Sun provides a baseline for triangulation. This distance is called an astronomical unit (AU). 2. Apparent Motion: By observing a star from two points in Earth’s orbit six months apart, we can measure its apparent motion. Nearby stars exhibit a larger parallax shift than more distant stars. Measuring an Astronomical Unit. To use parallax effectively, we need to know the distance between Earth and the Sun. This was historically achieved using radar ranging, where radar waves bounced off Venus at a specific angle in its orbit, creating a right triangle with known angles and one known distance. Calculating Stellar Distances Using Parallax. Once we know the distance between Earth and the Sun, we can calculate stellar distances using parallax. Once we have the AU, we can measure the parallax angle, which is typically very small, often fractions of an arcsecond. A star with a parallax of 1 arcsecond is at a distance of 1 parsec (approximately 3.26 light-years). Parsecs are a more practical unit for these tiny angles. One parsec corresponds to the distance at which one astronomical unit subtends an angle of one arcsecond. Knowing distances allows us to determine the true brightness of stars and map star clusters, helping us understand the distribution and evolution of stars within galaxies. 0:00 Introduction 1:33 Stellar Parallax 2:32 A star at a near distance 3:01 A star at a medium distance 3:17 Total angular shift of the far star 3:31 Parallax is defined to be half of the total shift 4:09 The definition of parallax distance 5:25 The Phases of Venus and the Distance to the Sun 5:57 Finding the value of the Astronomical Unit (AU) 7:45 The Astronomical Unit is Very Important 8:45 61 Cygni: the first measured parallax 13:31 The Radian: Measuring Angles the "Right" Way 15:31 What's the size of a "radian"? Overall, the segment emphasizes clear definitions, underlying geometry, and practical observing guidance so viewers can connect the concept to the real sky.