Celestial sphere

Celestial balloon Jost Burgi. 1594.

The celestial sphere or celestial vault is an ideal sphere, without a defined radius, concentric with the terrestrial globe, in which the stars apparently move. It allows to represent the directions in which the celestial objects are; this is how the angle formed by two directions will be represented by an arc of a greater circle on that sphere.

Theoretically it is considered that the Earth's is the Axis of the world (the rotation of the celestial sphere), and that the eye of the observer is coincident with the center of the Earth. It is a model that constitutes one of the fundamental concepts of astronomy, especially to be able to represent celestial observations.

Celestial observation

The celestial sphere is a mental construct that we create when we look at the sky. This arises from the information it receives from our eyes. The size and separation of the eyes allows us to perceive the volume of objects, but only up to a certain distance (stereoscopic vision). After that, all the objects that are perceived will give the impression of being located at the same distance, since they will be mentally projected onto the same plane.

When we use common sense, that perception changes. If we look at the sky and observe objects that are very far from us, the brain acts in the same way: it projects them onto the same plane. By moving our eyes in all directions, we perceive the sky as if it were an immense dome limited by the horizon, with us located in the center. Such a perception was what prompted the ancient philosophers to consider that the Earth was the center of the universe.

Celestial motion

The celestial sphere, with the north and south celestial poles and the celestial equator.

The movement of the celestial sphere is apparent and is determined by the rotation movement of our planet on its own axis. The rotation of the Earth, in a West-East direction, produces the apparent movement of the celestial sphere, in an East-West direction. We can perceive this movement during the day, due to the movement of the Sun in the sky, and at night, due to the movement of the stars. Both are carried out in an East-West direction. The speed with which the celestial sphere rotates is 15°/hour, so every 24 hours it completes a 360° turn.

At the same time, it performs another movement, also East-West (observing in the opposite direction to the celestial pole), as an effect of the translation of the Earth, since in 24 hours, simultaneously with the complete rotation, the planet moves the 365th. part of its orbital circle. Thus the rate of that second apparent rotation of the celestial sphere is 365 times slower than that generated by the rotation of the Earth. In fact, the speed is 2.5'/hour (2.5 arc minutes per hour) so every 24 hours it rotates 0.98°, almost 1 degree. It is imperceptible at the scale of hours but perceptible at the scale of weeks, since at the same time every 7 days the stars have advanced 7° of their apparent celestial circle because the Earth has moved about 7° of its orbit. Therefore the celestial sphere seems to rotate 360° every 365 days/year. This extremely slow movement is the reason why, for example, on the nights of mid-December we see the Orion constellation throughout the night and at its culmination at midnight, and half a year later it is invisible because it is right under the sun. sun at noon

Geographic location

Astronomers base their measurements on the existence, in that sphere, of points, circles and conventional planes: the plane of the horizon and that of the celestial equator; the pole and the zenith; the meridian, which serves as the origin for the measurement of the azimuth. It is easy to find a star or situate it with respect to these fundamental planes. When the viewer's horizon is oblique with respect to the equator, the celestial sphere is qualified as oblique. For an observer located at one of the two poles, the sphere is parallel, since his horizon is parallel to the equator. Finally, the sphere is straight for the observer located on the equinoctial line, because there the horizon cuts the equator perpendicularly. The celestial sphere is a concept, not an object; It is the virtual surface on which we see the stars projected as if they were all at the same distance from Earth.

Main elements

Stereographic projection of the celestial with the most important elements.

• Vertical direction refers to the direction that would mark a plumb. If you look down, you would head towards the center of the Earth. Looking up is the cenit.
• Cenit astronomical is the point of the celestial sphere located exactly above us, intersection of the ascending vertical with the celestial sphere.
• Nadir is the point of the celestial sphere diametrically opposed to the cenit
• La distance (usually represented by letter z) is the angular distance from the cenit to a celestial object, measured on a maximum circle (a maximum circle is the result of the intersection of a sphere with a plane passing through its center and divides it into two identical hemispheres, in the figure, the cenital distance is the arch between the cenit and the astro “A”).
• Astronomical horizon, skyline or true of a place is the plane perpendicular to the direction of the vertical, circular plane or maximum circle perpendicular to the vertical of the place passing through the center of the celestial sphere. In relation to the celestial sphere, we say that it is a diametric plane, since the horizon is a diameter of the sphere, and divides it into two hemispheres: one visible and another invisible.
• Celestial pole is the intersection of the celestial sphere with the prolongation of the axis of terrestrial rotation (also called the axis of the world) to the infinite.
• Axis of the world is the axis around which the celestial sphere would revolve.
• Ecuador is the projection of the Earth equator on the celestial sphere, plane or maximum circle perpendicular to the axis of the world that passes through the center of the celestial sphere. A meridian and celestial parallels are defined, similar to terrestrial ones:
• Meridiano celestial is the maximum circle that passes through the celestial poles and the cenit of a place.
• Sky Parallel are the smaller circles of the celestial sphere parallel to the equator. They are similar to the Earth parallels. The smaller circles result from the intersection of the celestial sphere with planes perpendicular to the rotation axis.
• Timeline is a graduated maximum circle of the celestial sphere located in the celestial equator.
• Straight East - West is the straight intersection of the celestial horizon with the celestial equator.
• North celestial pole intersection of the axis of the ascending world with the celestial sphere.
• South celestial pole intersection of axis of the descending world with the celestial sphere.