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Observing the Sky Solar System Satellites Navy Research Resources

Observing the Sky: The Celestial Sphere

To make it easier to understand the motions of objects in our skies, astronomers often depict the heavens as a sphere surrounding the Earth. Everything we see in space is drawn on this celestial sphere, ignoring the fact that the Sun, Moon, and stars are really at different distances from us. You can even buy celestial spheres that look like globes with stars and constellations instead of countries and seas.

We use a coordinate system to help us find things on the celestial sphere, just as we use a coordinate system of longitude and latitude to find our way around on Earth. The Earth's North and South Poles are extended out to the sphere to mark the north and south celestial poles. The equator is projected out to become the celestial equator.
 
Diagram of the celestial sphere If we plot the Sun's location in our sky for each day of the year, it will trace a circle on the celestial sphere. This circle shows the Sun's path among the stars throughout the year, and is called the ecliptic. This is the same word used to describe the Earth's path around the Sun because the Earth's motion is what makes the Sun seem to move among the stars. The ecliptic is 23.5' to the north of the celestial equator on the Summer Solstice, and 23.5' to the south of the celestial equator on the Winter Solstice. The ecliptic crosses the celestial equator twice a year, on the equinoxes. Remember that the tilt of the Earth's axis is 23.5' and it is the reason the Sun's path is also tilted from the celestial equator.
 
Like the longitude and latitude system here on Earth, the celestial sphere has a system to pinpoint locations. Declination tells us how far an object is north or south of the celestial equator, measured in degrees. These are not the same degrees we use to measure temperature, but the ones we use to measure a circle, which is 360' around. Right ascension tells us how far an object is to the east of the Vernal Equinox, that point midway between winter and summer when the ecliptic (the Sun's path) crosses the celestial equator. Latitude is measured to both the east and west of its starting point, but right ascension is measured only to the east. Also, right ascension is not measured in degrees, but in hours, minutes, and seconds. Each hour is equal to 15'.
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