Overview
This lecture introduces how to understand the night sky's appearance and motion from Earth using the concepts of the celestial sphere and horizon diagrams, with special focus on the perspective from the Northern Hemisphere.
The Earth's Motion and Perspective
- Earth is roughly spherical and rotates on its axis, giving the illusion that the sky moves around us.
- Observers in different Earth locations (Northern/Southern Hemisphere, poles, equator) see different parts of the sky.
- From our perspective in Westchester County, NY (Northern Hemisphere), the sky appears to rotate daily.
The Celestial Sphere Model
- The celestial sphere is an imaginary sphere surrounding Earth onto which the sky is projected.
- The north and south celestial poles are projections of Earth's poles on the celestial sphere.
- The celestial equator is Earth's equator projected onto the celestial sphere.
- Polaris, the North Star, is located nearly at the north celestial pole.
Horizon and Horizon Diagrams
- The horizon is a 360° line separating the sky from the land/sea, limiting your field of view.
- Horizon diagrams are 2D representations helping visualize which stars are visible above the horizon.
- In these diagrams, North is left, South is right, East is into the screen/paper, West is out.
Star Paths: Rising, Setting, and Circumpolar Stars
- Rising: An object moves from below to above the horizon.
- Setting: An object moves from above to below the horizon.
- Some stars (circumpolar) never set or rise; they're always visible or always hidden depending on latitude.
- In the Northern Hemisphere, most stars reach their highest point above the southern horizon.
Example Star Motions
- The constellation Orion rises in the east, reaches its highest point in the south, then sets in the west.
- The Big Dipper (near the north celestial pole) is circumpolar in the northern hemisphere—never sets.
- The Southern Cross (near the south celestial pole) is never visible from the northern hemisphere.
- At the equator, all stars rise and set straight up and down, and both celestial hemispheres are visible.
Using and Interpreting Horizon Diagrams
- Directions in diagrams are based on the observer’s point of view; only stars above the horizon are visible.
- Practice is needed to interpret horizon diagrams and relate them to real sky observations.
Key Terms & Definitions
- Celestial Sphere — Imaginary sphere surrounding Earth onto which all celestial objects are projected.
- Celestial Poles — Extensions of Earth's North and South Poles onto the celestial sphere.
- Celestial Equator — Earth’s equator projected onto the celestial sphere.
- Horizon — The line separating sky from land/sea, marking the visible limit.
- Circumpolar Star — A star that never rises or sets; always above the horizon.
- Rising/Setting — Motion of objects crossing the horizon, becoming visible or hidden.
Action Items / Next Steps
- Complete lecture tutorial "Position" on page 1 of the lecture tutorial book.
- Practice interpreting and using horizon diagrams as described in the tutorial.