Talk:Epoch (astronomy)

Epoch (astronomy)

Shouldn't be this defined in Terrestrial Time?

Yes, so why not change it now? Nike 06:53, 17 Dec 2004 (UTC)

question

Is it known how long the current epoch will be adhered to? Do we know what will come next? Are these questions relevant; and if not, why not? Can this be clarified in the article? (As may be apparent, I don't have a clue.) -- Cimon avaro; on a pogostick. 11:58, 14 January 2006 (UTC)

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. --Nike 03:45, 16 January 2006 (UTC)

question again

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

#

What's with all the #s? I've never seen this usage anywhere. #Julian epoch is just bizarre. Also, why are there links to articles which no longer exist?

Although the article only mentions one Julian epoch, I have also seen J1900 and J1950. --Nike 08:51, 3 August 2006 (UTC)

I get it, those are supposed to be links to sections within the article. You need to use a piped link. That would look like this: [[#J2000.0|J2000.0]] --Nike 09:00, 3 August 2006 (UTC)

What are Besselian vs. Julian epochs? How do they differ from each other aside from what letter is prefixed to their number? --Haruo 10:03, 6 October 2006 (UTC)

What about the article is unclear in explaining this? --Nike 11:38, 6 October 2006 (UTC)

What I want to know is why the phrase "Since the right ascension and declination of stars are constantly changing due to precession..." is repeated verbatim in every section. BIEB!! 13:07, 25 October 2006 (UTC)

My guess is that this article is actually a series of stubs that were stitched together in the hope that someone would clear the result up into some coherent whole; sadly, nobody seems to have done so yet, and the result is just a mess... - IMSoP 01:03, 5 November 2006 (UTC)

Epoch and Equinox

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). Radec 08:15, 21 January 2007 (UTC)

I found the text trying to explain the distinction between Epoch and Equinox completely confusing. 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. — Elliot Winkler 06:30, 3 September 2012 (UTC)

I found this revision which helps explain the difference between them. Well, a little bit. :/ (unfortunately nothing is referenced, so it's kind of worthless) — Elliot Winkler 06:44, 3 September 2012 (UTC)

After re-reading 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. — 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. — Preceding unsigned comment added by 66.65.53.212 (talk) 01:03, 16 October 2015 (UTC)

Article is unhelpful for links from Comet Infobox

I was directed to Epoch by the Comet Infobox on the Comet McNaught article, which states "Epoch: 2454113.2961 (January 20, 2007)". It was not immediately apparent from this article that the stated epoch was in fact a Julian Day, and that I should refer to that article for an explanation of the number 2454113.2691. If a subject expert is revising this article, it would be helpful if they would consider this usage. PaulKishimoto 17:57, 21 January 2007 (UTC)

I believe this is a problem with the Comet Infobox and not this page. 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? —Preceding unsigned comment added by Trigamma (talk • contribs) 22:58, 4 January 2008 (UTC)

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. — Joe Kress (talk) 08:17, 5 January 2008 (UTC)

B1875

We need more info about B1875. 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? – 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 noon and midnight rather than 12 pm and 12 am". So if you don't like 12h, the simplest solution is to replace it with "noon". 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.) 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. 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 used when a date was first added to the article, which is month, day, year. 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. 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 own style manual as well as a manual for dates and numbers and a guideline for citations. I think it's highly unlikely we would completely adopt the manual Marco.bs mentioned, or another like it, such as the 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. 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.

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. 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".)