Talk:Epoch (astronomy): Difference between revisions - Wikipedia

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==Epoch (astronomy)==

Shouldn't be this defined in [[Terrestrial Time]]?

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:Usually it's every 50 years, although I'm not certain that this is always the case. The article refers to [[B1875.0]], which is only 25 years before [[B1900.0]]. --[[User:Nike|Nike]] 03:45, 16 January 2006 (UTC)

==question again==

Should there be a reference to Epoch/Unix time here - 1453330082 [[Special:Contributions/66.194.64.130|66.194.64.130]] ([[User talk:66.194.64.130|talk]]) 22:48, 20 January 2016 (UTC)

==#==

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This page may be mis-titled completely. When speaking of terms such as 'B1950.0' and 'J2000.0', especially in the context of celestial coordinates and precession, you are speaking of EQUINOX, not EPOCH. For example, the EPOCH of observation on a star may be 1991.25, but the coordinates given for its posistion may be specified in EQUINOX J2000.0 (The Hipparcos and Tycho stellar catalogs are a good example of this). [[User:Radec|Radec]] 08:15, 21 January 2007 (UTC)

:I found the text trying to explain the distinction between Epoch and Equinox completely confusing. [[User:MartinSpamer|MartinSpamer]] ([[User talk:MartinSpamer|talk]]) 12:18, 28 August 2010 (UTC)

:: I totally agree. I hate to be blunt but the entire section, and this entire article really, reads like a textbook. It's clearly written by someone who is an expert, but there are too many details. — [[User:Mcmire|Elliot Winkler]] 06:30, 3 September 2012 (UTC)

:: I found [http://en.wikipedia.org/w/index.php?title=Epoch_(astronomy)&oldid=174691201 this revision] which helps explain the difference between them. Well, a little bit. :/ (unfortunately nothing is referenced, so it's kind of worthless) — [[User:Mcmire|Elliot Winkler]] 06:44, 3 September 2012 (UTC)

:: After re-reading [http://www.usno.navy.mil/USNO/astronomical-applications/astronomical-information-center/icrs-narrative this page on the IRCS], as well as a few other sources I came across, it seems to me that "epoch" is just a fancy word for "date". Either it refers to a date that observations were made, or the year in which an equinox occurred. So you can have an epoch of observation and an epoch of equinox. And it is totally possible that when used together they can be different. For instance, that page mentions the Hipparcos observations, which were recorded within the ICRS (which is defined by vernal equinox year 2000), but at an epoch of J1991.25 (meaning if you traveled back in time to Julian day 2448349.0625, or April 2, 1991 1:30:32 TT, all of the observations would be accurate at that exact point in time). I think this is kind of what this article is trying to say but it's not doing a very good job of it. — [[User:Mcmire|Elliot Winkler]] 08:19, 3 September 2012 (UTC)

:: It's 1:30 p.m., not a.m. So, Julian day 2448349.0625 TT = April 2, 1991 13:30:00 TT. <small class="autosigned">— Preceding [[Wikipedia:Signatures|unsigned]] comment added by [[Special:Contributions/66.65.53.212|66.65.53.212]] ([[User talk:66.65.53.212|talk]]) 01:03, 16 October 2015 (UTC)</small>

== Article is unhelpful for links from Comet Infobox ==

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I believe this is a problem with the [[Template:Comet|Comet Infobox]] and not this page. [[User:Icez|Icez]] 02:15, 5 February 2007 (UTC)

== Julian Year ==

The article states

::''Julian year 2000 began on 2000 January 1 at exactly 12:00 TT. ''

Does that imply ''according to the Julian calendar''? <small>—Preceding [[Wikipedia:Signatures|unsigned]] comment added by [[User:Trigamma|Trigamma]] ([[User talk:Trigamma|talk]] • [[Special:Contributions/Trigamma|contribs]]) 22:58, 4 January 2008 (UTC)</small>

:That and other references within the article to Julian epochs are poorly worded. For J2000.0, 2000 January 1 (at noon) is in the Gregorian calendar. Other Julian epochs differ from this epoch in Julian years of 365.25 days each. Hence the Hipparchus epoch of J1991.25 is 8.75 Julian years before J2000.0. This requires significant rewording of the article. — [[User:Joe Kress|Joe Kress]] ([[User talk:Joe Kress|talk]]) 08:17, 5 January 2008 (UTC)

== B1875 ==

We need more info about B1875. [[Special:Contributions/65.94.47.63|65.94.47.63]] ([[User talk:65.94.47.63|talk]]) 08:54, 2 July 2011 (UTC)

== 12h rather than 12:00? ==

Why is noon/midday written as 12h rather than 12:00 as in [[ISO 8601]]? – [[User:Kaihsu|Kaihsu]] ([[User talk:Kaihsu|talk]]) 05:58, 23 April 2014 (UTC)

:I don't know why the editors who wrote this article chose to write "12h".

:The English Wikipedia has not adopted ISO 8601 to express time of day. Thus, if one just writes "12:00", it is unclear if one is referring to noon or midnight, because some articles use the 12 hour clock and others use the 24 hour clock. This article does not state whether it uses a 12 or 24 hour clock, so "12:00" is ambiguous.

:In the case of the 12 hour clock, [[MOS:TIME]] instructs "Use {{xt|noon}} and {{xt|midnight}} rather than {{!xt|12 pm}} and {{!xt|12 am}}". So if you don't like 12h, the simplest solution is to replace it with "noon". [[User:Jc3s5h|Jc3s5h]] ([[User talk:Jc3s5h|talk]]) 15:46, 23 April 2014 (UTC)

:In astronomy, the time (and Right Ascension) is usually written as 00h 00m 00.00s. So Noon is written in full as 12h 00m 00s, 11:27 pm is 23h 27m 00s, and Midnight is 00h 00m 00s. (As a side note, Declination is written as 00d 00m 00s.) [[User:Dsgd47|Dsgd47]] ([[User talk:Dsgd47|talk]]) 21:46, 24 September 2015 (UTC)

::As far as I know astronomy does not have one single authoritative source that every single astronomer will acknowledge as the authority on time notation. In some fields, midnight can also be 24:00. Thus, when using the word "midnight", one must specify whether it is the midnight at the beginning of a certain date, or the end of a certain date. [[User:Jc3s5h|Jc3s5h]] ([[User talk:Jc3s5h|talk]]) 10:19, 25 September 2015 (UTC)

==Date format==

Since the date format in the article is not consistent, I will follow [[MOS:DATEVAR]] and use the format [https://en.wikipedia.org/w/index.php?title=Epoch_(astronomy)&diff=prev&oldid=907665 used when a date was first added to the article], which is month, day, year. [[User:Jc3s5h|Jc3s5h]] ([[User talk:Jc3s5h|talk]]) 14:28, 6 August 2018 (UTC)

I agree the format for dates is inconsistent. However, being an article about astronomy, there is an internationally recognized standard for epochs mandated by the International Astronomical Union (see for example https://www.iau.org/static/publications/stylemanual1989.pdf, page S29) that shall be followed.

[[User:Marco.bs|Marco.bs]] ([[User talk:Marco.bs|talk]]) 10:54, 7 August 2018 (UTC)

:Style manuals for astronomical journals apply to the journals that issued the manual, not Wikipedia. Wikipedia has its [[WP:MOS|own style manual]] as well as a [[WP:MOSNUM|manual for dates and numbers]] and a [[WP:CITE|guideline for citations]]. I think it's highly unlikely we would completely adopt the manual [[User:Marco.bs|Marco.bs]] mentioned, or another like it, such as the [https://journals.aas.org/authors/manuscript.html#_Toc2.2 author instructions for the American Astronomical Society]. For example, few Wikipedia editors would want to use a 29 year-old citation style from IAU rather than Wikipedia's citation templates. But, in the past, isolated recommendations from astronomy journal style manuals have been adopted, such as the symbol for astronomical unit, au.

:Currently the year month day date format (e.g. 2018 August 7 or 2018 Aug 7) suggested by IAU and AAS conflicts with the acceptable formats listed at WP:MOSNUM. If this style of date were used in citation templates, the article would be disfigured with red warnings because the date format would be considered improper. If you feel the year month day format should be allowed in astronomy articles, please take it up at [[WT:MOSNUM]]. [[User:Jc3s5h|Jc3s5h]] ([[User talk:Jc3s5h|talk]]) 11:51, 7 August 2018 (UTC)

==typo?==

I assume the following is a typo? 1950?

"Julian years, e.g., J2000.0 for January 1.5, 1950, TT"

Also, dates in the form "January 0.9235, 1950 TT"

are beyond me. I consulted TT (terrestrial time) to learn

what that phrase means and find no examples or templates there

to explain such a date. "Julian Day" explains its own

use of fractional days, but I haven't found anything

to confirm any of my guesses as to how to specify such a day

with a month.

[[Special:Contributions/74.79.156.16|74.79.156.16]] ([[User talk:74.79.156.16|talk]]) 20:05, 1 October 2018 (UTC)

:Yes, "Julian years, e.g., J2000.0 for January 1.5, 1950, TT" is a typo and I fixed it.

:I am not aware of a widely accepted standard for how to specify a fractional day with a month name. I see that the ''Explanatory Supplement to the Astronomical Almanac'', 3rd ed., in the glossary entry for J2000.0 uses 2000 January 1.5 TT, but our [[WP:MOSNUM]] does not endorse this notation. [[User:Jc3s5h|Jc3s5h]] ([[User talk:Jc3s5h|talk]]) 21:00, 1 October 2018 (UTC)

January 0.9235, 1950 TT = December 31.9235, 1949 TT = December 31, 1949 22:09:50.4 TT. (Historically, the "January 0" concept arose because astronomers start the day 12 hours before non-astronomers do. Astronomers didn't want the year number to decrement "unnecessarily".)