Celestial Sphere, Poles and Equator

The Celestial Sphere

The Celestial Sphere is an imaginary sphere with the observer at the centre.
All objects in the observer’s sky can be thought of as located on the inside surface of the celestial sphere.
The celestial sphere is a useful tool for observing celestial objects, despite it not being a representation of the actual scale of the universe.

The North Celestial Pole and the South Celestial Pole are the points in the sky directly above Earth’s North and South Poles, respectively.
The celestial poles are the projection of Earth’s axis of rotation onto the celestial sphere.
The Celestial Equator is an imaginary line above the Earth’s equator. It divides the celestial sphere into the northern and southern hemispheres.
The celestial equator is perpendicular to the celestial poles.

Celestial objects appear to move across the sky because of the Earth’s rotation, which is from west to east.
The paths of celestial objects depend on the observer’s latitude.
For an observer at the poles, celestial objects move horizontally, parallel to the horizon.
For an observer at the equator, celestial objects rise perpendicularly from the horizon and set straight down.

The Ecliptic is the apparent path the Sun follows across the celestial sphere over the course of a year.
The ecliptic is tilted at an angle of about 23.5° to the celestial equator due to the tilt of Earth’s axis of rotation.
The zodiac is a belt of the celestial sphere extending about 8° either side of the ecliptic that includes the apparent paths of the Sun, Moon, and the planets excluding Pluto.
The zodiac is divided into twelve equal parts, each named after the constellation that once appeared in its section.

Declination and Right Ascension are the celestial equivalent of latitude and longitude on Earth.
Declination is measured in degrees, from +90° (north celestial pole) to -90° (south celestial pole).
Right ascension is measured in time (hours, minutes, and seconds), from 0h to 24h. One hour of right ascension corresponds to 15°.
The zero point of right ascension is the position of the Sun at the vernal equinox, also known as the First Point of Aries.