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Angular Measurements and the Celestial Sphere

In this video, I talk about how we navigate our way around the sky using angular measurements, and I introduce the basics of the Equatorial Coordinate system. • Celestial Sphere Concept: The stars appear fixed on a celestial sphere, a useful model for understanding their positions despite the stars’ actual motion. • Angular Separation Measurement: Instead of physical distance, angular separation is used to measure the positions of stars on the celestial sphere. • Coordinate System Introduction: A fixed coordinate system is needed to map the positions of stars on the celestial sphere for navigation. • Angular Separation Definition: Angular separation is the angle between two directions, independent of the distance to the objects. • Hand as an Angular Measurement Tool: A fist at arm’s length represents approximately 10 degrees, while a pinky finger at arm’s length represents about 1 degree. • Zenith and Horizon Relationship: It takes nine fists held at arm’s length from the Zenith to reach the Horizon, as the Zenith is at a 90-degree altitude. • Angular Measurement with Pinky: The Sun and the Moon appear to be the same size in the sky because they are both about half the size of your pinky held at arm’s length. • Degrees and Arc Minutes: There are 360 degrees in a circle, and each degree can be divided into 60 arc minutes, which is a very small angular measurement. • Visualizing Small Angles: You can visualize small angles by marking 60 evenly spaced tick marks across your pinky nail held at arm’s length, with each mark representing one arc minute. • Angular Measurement: Explanation of arc minutes and arc seconds as very small units of angular measurement. • Angular Size Perception: Description of how an object’s angular size depends on its physical size and distance from the observer. • Celestial Sphere: Introduction of the concept of the celestial sphere, a fictional construct used to represent the apparent positions of stars in the sky. • Celestial Sphere: An imaginary sphere around Earth on which all celestial objects appear to be located. • Celestial Coordinates: A system for locating objects on the celestial sphere using declination (measured from the celestial equator) and right ascension (measured from the vernal equinox). • Equatorial System: A system for locating objects on the celestial sphere using declination and right ascension, with the celestial equator as the reference plane. • Celestial Sphere and Coordinates: The celestial sphere is a fictitious concept used to map the sky, with right ascension (measured in hours) and declination (angle north or south of the celestial equator) as coordinates. • Star Trails and Celestial Sphere: Star trails, captured through long-exposure photography, illustrate the concept of the celestial sphere and the apparent motion of stars due to Earth’s rotation. • Right Ascension and Declination: Right ascension is measured eastward around the celestial sphere, with 24 hours corresponding to 360 degrees, and each hour representing 15 degrees. Declination measures the angle north or south of the celestial equator. • Star Trail Analysis: The length of star trails in an image can indicate the exposure time of the photograph. • Celestial Pole and Latitude: The altitude of the celestial pole above the horizon corresponds to the observer’s latitude. • Circumpolar Stars: Some stars never rise or set because they are located near the celestial pole. • Celestial Equator Visualization: A long-exposure image of star trails projected onto a 2D surface, with the Sun’s path superimposed, illustrates the celestial equator. • Star Trail Patterns: Star trails curve around the celestial poles (north and south) but appear straight towards the east, reflecting Earth’s rotation and latitude. • Next Lecture Topic: A more detailed examination of the celestial sphere using Stellarium and Sky Safari. Credits: Juan Carlos Casado: https://twanight.org/gallery/star-tra... Matthew Saville: https://fstoppers.com/profile/matthew...