In astronomy and navigation, the celestial sphere is an imaginary sphere of arbitrarily
large radius, concentric with a particular celestial body. 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 or a hemispherical screen. 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.
Introduction 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 celestial body 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 celestial body 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. 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.
For some objects, this is over-simplified. Objects which are relatively near to the observer
will seem to change position against the distant celestial sphere if the observer moves far
enough, say, from one side of the celestial body 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 celestial body's center, A star'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. The celestial sphere can be thus be thought
of as a kind of astronomical shorthand, and is applied very frequently by astronomers.
For instance, the Astronomical Almanac for 2010 lists the apparent geocentric position
of Earth's Moon, 1 Jan 2010 at 0h Terrestrial Time, in equatorial coordinates, as right
ascension 6h 57m 48s.86, declination +23° 30' 05".5. Implied in this position is that
it is as projected onto the celestial sphere; any observer at any location looking in that
direction would see the "geocentric Moon" in the same place against the stars. For many
rough uses, this position, as seen from the Earth's center, is adequate. For applications
requiring precision, the Almanac gives formulae and methods for calculating the topocentric
coordinates, that is, as seen from a particular place on the Earth's surface, based on the
geocentric position. This greatly abbreviates the amount of detail necessary in such almanacs,
as each observer can handle their own specific circumstances.
Celestial coordinate systems These concepts are important for understanding
celestial reference systems, the methods in which the positions of objects in the sky
are measured. Certain reference lines and planes on the celestial body, when projected
onto the celestial sphere, form the bases of the reference systems. These include the
celestial equator, axis, and the orbit. At their intersections with the celestial sphere,
these form the celestial equator, the north and south celestial poles and the ecliptic,
respectively. As the celestial sphere is considered infinite in radius, all observers see the
celestial equator, celestial poles and ecliptic at the same place against the background stars.
Directions toward objects in the sky can be quantified by constructing, from these bases,
celestial coordinate systems. Similar to the terrestrial longitude and latitude, the equatorial
system of right ascension and declination specifies positions relative to the celestial
equator and celestial poles. The ecliptic system of celestial longitude and celestial
latitude specifies positions relative to the celestial body's orbit. There are more coordinate
systems besides the equatorial right ascension/declination system and the ecliptic system.
History The ancients assumed the literal truth of
stars attached to the celestial sphere of Earth, revolving about the Earth in one day,
and a fixed Earth. The Eudoxan planetary model, on which the Aristotelian and Ptolemaic models
were based, was the first geometric explanation for the "wandering" of the classical planets.
The outer most of these "crystal spheres" was thought to carry the fixed stars. Eudoxus
used 27 concentric spherical solids to answer Plato's challenge: "By the assumption of what
uniform and orderly motions can the apparent motions of the planets be accounted for?"
Star globe
A celestial sphere can also refer to a physical model of the celestial sphere or celestial
globe. Such globes map the constellations on the outside of a sphere, resulting in a
mirror image of the constellations as seen from Earth. The oldest surviving example of
such an artifact is the globe of the Farnese Atlas sculpture, a 2nd-century copy of an
older work. See also
Celestial coordinate system Celestial spheres
Firmament History of astronomy
Sky Spherical astronomy
Stellar parallax, a type of short-term motion of distant stars
Proper motion, a type of longer-term motion of distant stars
Notes
References Bowditch, Nathaniel. The American Practical
Navigator. Bethesda, MD: National Imagery and Mapping Agency. ISBN 0-939837-54-4.
MacEwen, William A.; William Hayler; Turpin, Edward A.. Merchant Marine officers' handbook:
based on the original edition by Edward A. Turpin and William A. MacEwen. Cambridge,
Md: Cornell Maritime Press. pp. 46–51. ISBN 0-87033-379-8.
External links MEASURING THE SKY A Quick Guide to the Celestial
Sphere Jim Kaler, University of Illinois General Astronomy/The Celestial Sphere Wikibooks
Rotating Sky Explorer University of Nebraska-Lincoln Interactive Sky Chart SkyandTelescope.com
Monthly skymaps for every location on Earth