Changing The Standard Equinox and Epoch
To calculate the visibility of a celestial object for an observer at a specific time and place on the Earth, the coordinates of the object are needed relative to a coordinate system of current date. If coordinates relative to some other date are used, then that will cause errors in the results. The magnitude of those errors increases with the time difference between the date and time of observation and the date of the coordinate system used, because of precession of the equinoxes. If the time difference is small, then fairly easy and small corrections for the precession may well suffice. If the time difference gets large, then fuller and more accurate corrections must be applied. For this reason, a star position read from a star atlas or catalog based on a sufficiently old equinox and equator cannot be used without corrections, if reasonable accuracy is required.
Additionally, stars move relative to each other through space. Apparent motion across the sky relative to other stars is called proper motion. Most stars have very small proper motions, but a few have proper motions that accumulate to noticeable distances after a few tens of years. So, some stellar positions read from a star atlas or catalog for a sufficiently old epoch require proper motion corrections as well, for reasonable accuracy.
Due to precession and proper motion, star data become less useful as the age of the observations and their epoch, and the equinox and equator to which they are referred, get older. After a while, it is easier or better to switch to newer data, generally referred to a newer epoch and equinox/equator, than to keep applying corrections to the older data.
Read more about this topic: Epoch (astronomy)
Famous quotes containing the words changing the, changing, standard and/or epoch:
“Armies, though always the supporters and tools of absolute power for the time being, are always the destroyers of it too; by frequently changing the hands in which they think proper to lodge it.”
—Philip Dormer Stanhope, 4th Earl Chesterfield (16941773)
“Minnaloushe creeps through the grass
Alone, important and wise,
And lifts to the changing moon
His changing eyes.”
—William Butler Yeats (18651939)
“If the Revolution has the right to destroy bridges and art monuments whenever necessary, it will stop still less from laying its hand on any tendency in art which, no matter how great its achievement in form, threatens to disintegrate the revolutionary environment or to arouse the internal forces of the Revolution, that is, the proletariat, the peasantry and the intelligentsia, to a hostile opposition to one another. Our standard is, clearly, political, imperative and intolerant.”
—Leon Trotsky (18791940)
“Our own epoch is determining, day by day, its own style. Our eyes, unhappily, are unable yet to discern it.”
—Le Corbusier [Charle Édouard Jeanne] (18871965)