The Celestial Sphere

The Celestial Sphere
The Celestial Sphere. (Image Courtesy of the Lunar and Planetary Institute)

What is the Celestial Sphere?

In astronomy and navigation, the celestial sphere is an imaginary sphere of arbitrarily large radius, concentric with Earth. All objects in the observer’s sky can be thought of as projected upon the inside surface of the celestial sphere, as if it were the underside of a dome. The celestial sphere is a practical tool for spherical astronomy, allowing observers to plot positions of objects in the sky when their distances are unknown or unimportant.


The Earth rotates … giving it the appearance that the stars are the ones that rotate:

Because astronomical objects are at such remote distances, casual observation of the sky offers no information on the actual distances. All objects seem equally far away, as if fixed to the inside of a sphere of large but unknown radius, which rotates from east to west overhead while underfoot, the Earth seems to stand still. For purposes of spherical astronomy, which is concerned only with the directions to objects, it makes no difference whether this is actually the case, or if it is the Earth which rotates while the celestial sphere stands still.

The celestial sphere can be considered to be infinite in radius. This means any point within it, including that occupied by the observer, can be considered the center. It also means that all parallel lines, be they millimetres apart or across the Solar System from each other, will seem to intersect the sphere at a single point, analogous to the vanishing point of graphical perspective. All parallel planes will seem to intersect the sphere in a coincident great circle (a “vanishing circle”). Conversely, observers looking toward the same point on an infinite-radius celestial sphere will be looking along parallel lines, and observers looking toward the same great circle, along parallel planes. On an infinite-radius celestial sphere, all observers see the same things in the same direction.


Parallax:

Objects which are relatively near to the observer (for instance, the Moon) will seem to change position against the distant celestial sphere if the observer moves far enough, say, from one side of the Earth to the other. This effect, known as parallax, can be represented as a small offset from a mean position. The celestial sphere can be considered to be centered at the Earth’s center, The Sun’s center, or any other convenient location, and offsets from positions referred to these centers can be calculated. In this way, astronomers can predict geocentric or heliocentric positions of objects on the celestial sphere, without the need to calculate the individual geometry of any particular observer, and the utility of the celestial sphere is maintained. Individual observers can work out their own small offsets from the mean positions, if necessary. In many cases in astronomy, the offsets are insignificant.


How to find Objects in the Sky:

The equatorial coordinate system is a widely-used celestial coordinate system used to specify the positions of celestial objects. It may be implemented in spherical or rectangular coordinates, both defined by an origin at the center of the Earth, a fundamental plane consisting of the projection of the Earth’s equator onto the celestial sphere (forming the celestial equator), a primary direction towards the vernal equinox, and a right-handed convention.

The origin at the center of the Earth means the coordinates are geocentric, that is, as seen from the center of the Earth as if it were transparent and nonrefracting. The fundamental plane and the primary direction mean that the coordinate system, while aligned with the Earth’s equator and pole, does not rotate with the Earth, but remains relatively fixed against the background stars. A right-handed convention means that coordinates are positive toward the north and toward the east in the fundamental plane.

A star’s spherical coordinates are often expressed as a pair, right ascension and declination, without a distance coordinate. Because of the great distances to most celestial objects, astronomers often have little or no information on their exact distances, and hence use only the direction. The direction of sufficiently distant objects is the same for all observers, and it is convenient to specify this direction with the same coordinates for all. In contrast, in the horizontal coordinate system, a star’s position differs from observer to observer based on their positions on the Earth’s surface, and is continuously changing with the Earth’s rotation. Telescopes equipped with equatorial mounts and setting circles employ the equatorial coordinate system to find objects. Setting circles in conjunction with a star chart or ephemeris allow the telescope to be easily pointed at known objects on the celestial sphere.

Right ascension (blue) and declination (green) as seen from outside the celestial sphere.
Right ascension (blue) and declination (green) as seen from outside the celestial sphere. (Image Courtesy of Wikipedia)

Declination

Declination (symbol δ, abbreviated dec) measures the angular distance of an object perpendicular to the celestial equator, positive to the north, negative to the south. For example, the north celestial pole has a declination of +90°. Declination is analogous to terrestrial latitude.


Right ascension

Right ascension (symbol α, abbreviated RA) measures the angular distance of an object eastward along the celestial equator from the vernal equinox to the hour circle passing through the object. The vernal equinox point is one of the two where the ecliptic intersects the celestial equator. Analogous to terrestrial longitude, right ascension is usually measured in sidereal hours, minutes and seconds instead of degrees, a result of the method of measuring right ascensions by timing the passage of objects across the meridian as the Earth rotates. There are (360° / 24h) = 15° in one hour of right ascension, 24h of right ascension around the entire celestial equator.

When used together, right ascension and declination are usually abbreviated RA/Dec.