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:
“This part of being a man, changing the way we parent, happens only when we want it to. It changes because we are determined for it to change; and the motive for changing often comes out of wanting to be the kind of parent we didn’t have.”
—Augustus Y. Napier (20th century)
“By this unprincipled facility of changing the state as often, and as much, and in as many ways as there are floating fancies or fashions, the whole chain and continuity of the commonwealth would be broken. No one generation could link with the other. Men would become little better than the flies of a summer.”
—Edmund Burke (1729–1797)
“This unlettered man’s speaking and writing are standard English. Some words and phrases deemed vulgarisms and Americanisms before, he has made standard American; such as “It will pay.” It suggests that the one great rule of composition—and if I were a professor of rhetoric I should insist on this—is, to speak the truth. This first, this second, this third; pebbles in your mouth or not. This demands earnestness and manhood chiefly.”
—Henry David Thoreau (1817–1862)
“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] (1887–1965)