The Celestial Sphere

Constellations can help us point out regions in the night sky, but precisely describing an exact location of a faint object in the sky can be challenging. Watch the teacher video Horizon Coordinate System for an introduction to one method we can use to specify the location of faint stars, planets, and nebulas. (You can play with the simulations from this video for yourself: Lat/Long Explorer and Alt/Az Demo.)
The “horizon coordinate system” described in the previous video 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 “equatorial coordinate system.” To properly understand the equatorial system, you really need to have a solid understanding of the concepts of latitude and longitude on the earth. Review the article Units of Longitude and Latitude.
Now read about the equatorial coordinate system and study the animations on the page Celestial Equatorial Coordinate System. You can also experiment with this simulation: RA/Dec Demonstrator.
The equatorial 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.
As a final review, watch the teacher video Celestial Coordinate System, which uses simulations to describe the concepts and terminology of the celestial sphere.
Are you having a hard time picturing all this stuff on the actual sky? Check out the teacher video Horizon & Celestial Coordinates, where I summarize the differences between the two systems.