For minor planet Pholusorbital elements have been given including the following data: An example is as follows:
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. Epochs and equinoxes are moments in time, so they can be specified in the same way as moments that indicate things other than epochs and equinoxes.
The following standard ways of specifying epochs and equinoxes seem most popular:. Besselian years, used mostly for star positions, can be encountered in older catalogs but are now becoming obsolete. The Hipparcos catalog summary,  for example, defines the "catalog epoch" as J A Besselian year is named after the German mathematician and astronomer Friedrich Bessel — Meeus  defines the beginning of a Besselian year to be the moment at which the mean longitude of the Sun, including the effect of aberration and measured from the mean equinox of the date, is exactly degrees.
This moment falls near the beginning of the corresponding Gregorian year. The definition depended on a particular theory of the orbit of the Earth around the Sun, that of Newcomb , which is now obsolete; for that reason among others, the use of Besselian years has also become or is becoming obsolete. Lieske  says that a "Besselian epoch" can be calculated from the Julian date according to. This relationship is included in the SOFA software library.
When using Besselian years, specify which definition is being used. To distinguish between calendar years and Besselian years, it became customary to add ". Since the switch to Julian years in the mids, it has become customary to prefix "B" to Besselian years. So, "" is the calendar year , and " A Julian year is an interval with the length of a mean year in the Julian calendar , i. This interval measure does not itself define any epoch: But, standard conventional epochs which are not Besselian epochs have been often designated nowadays with a prefix "J", and the calendar date to which they refer is widely known, although not always the same date in the year: Like the Besselian epoch, an arbitrary Julian epoch is therefore related to the Julian date by.
Before that, the equinox of B Different astronomers or groups of astronomers used to define individually, but today standard epochs are generally defined by international agreement through the IAU , so astronomers worldwide can collaborate more effectively.
It is inefficient and error-prone if data or observations of one group have to be translated in non-standard ways so that other groups could compare the data with information from other sources. How to make a drill bit in a pinch. The video content is inappropriate. The video content is misleading. The ad is too long. The ad does not play. The ad does not inform my purchase. The video does not play. There is too much buffering.
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