Earth and Sky

Recall that we previously learned about the “horizon coordinate system” (altitude and azimuth), which provides a simple way for us to describe the location of an object in the sky. If you’d like to review, you can re-watch the Horizon Coordinate System video from a prior lesson. (You can also play with the simulations from the video: Lat/Long Explorer and Alt/Az Demo.)
The horizon coordinate system is great because it is easy to use, BUT the altitude and azimuth of stars change with your location on the earth and the time of day. It would be ideal if we had a coordinate system that allowed us to specify the exact location of an object in the sky so that anyone in the world could find it at any time of the year. To do this, we need to use the “celestial coordinate system” (also known as the “equatorial coordinate system”.) To properly understand these coordinates, we really need to have a solid understanding of the concepts of latitude and longitude on the earth. Read 4.1 Earth and Sky to learn how terrestrial and celestial coordinate systems, including latitude, longitude, right ascension, and declination, enable us to pinpoint locations on Earth and in the sky.
The celestial coordinate system is an extension of latitude and longitude out into space. We imagine an invisible celestial sphere in the sky onto which these lines are drawn. Rather than latitude, we refer to “declination” and instead of longitude, we call it “right ascension” - the change in names reflects the fact that we are using new celestial reference points rather than references tied to the surface of the earth. Because this system is aligned to references in the sky, the whole coordinate system rotates with the stars - so a star always has the exact same right ascension and declination - resolving the problem we faced with the horizon coordinate system. Do you want to see a cool application of this? You can actually map all of the constellations in the sky onto an ordinary globe of the earth as a student shared with me in the video Constellation Globe.
Watch the teacher video Celestial Coordinate System, which uses simulations to describe the concepts and terminology of the celestial sphere. You can also play with the simulations here: 1) Terrestrial Coordinate Explorers, 2) Celestial Coordinate Explorers, 3) Rotating Sky Explorer
Are you having a hard time picturing all this stuff in the actual sky? Check out the teacher video Horizon & Celestial Coordinates, where I summarize the differences between the two systems.