Paramount MX User Guide
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P a g e
coordinate system, the coordinates of all celestial objects remain fixed* from hour-to-hour, day-to-day
and so on.
An object's equatorial coordinates remain the same regardless of from where on Earth the object is
viewed. This allows astronomers to create star maps that apply to any place on Earth, or publish the
anticipated position of an upcoming comet so that astronomers everywhere know where it is located
among the stars.
The equatorial coordinate system used to specify the positions of celestial objects is directly analogous
to the latitude-longitude coordinate system used on Earth. In fact, if you were to expand the latitude
and longitude grid of the Earth so that it is out beyond all stars, you would have a sphere with identical
geometry to the celestial sphere.
We suppose that all stars and deep-sky objects are located on a very large sphere (out beyond all stars).
We call this the celestial sphere. For purposes of describing the positions of celestial objects, we
consider all stars and deep sky objects to be on the celestial sphere, when actually they are all
positioned at varying distances from the Earth.
* Equatorial coordinates change over long periods of time due to precession (wobbling of the Earth).
TheSkyX computes this change in stars’ position for the current date. Precession, however, does not
change the relative positions of objects with respect to one another.
Right Ascension
The geometry of the right ascension (RA) lines on the celestial sphere is the same as the longitude lines
on Earth. Longitude lines divide the Earth into 360 degrees. Right ascension lines divide the celestial
sphere into 24 hours, based on one revolution of Earth. Therefore one hour of right ascension equals 15
degrees (360 divided by 24). See the definition of Local Sidereal Time for additional information on why
24 hours are used for right ascension instead of 360 degrees.
Zero degrees longitude passes through Greenwich, England and is the designated reference line for
longitude. What, then, is the reference line for zero hour’s right ascension? Astronomers use the
vernal
equinox
, the location where the Sun crosses the celestial equator during its apparent annual motion
against the background stars, as a "starting point" for right ascension.
The term "right ascension" comes from the fact that when viewed from the equator, all stars rise (or
ascend) at right angles to the horizon, so their times of rising are called their times of right ascension.
