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The article states that "after a year has passed, one additional sidereal "day" has elapsed compared to the number of solar days that have gone by". I believe this is precisely backwards. There will be one LESS sidereal day. Thought experiment:
Suppose the Earth made no rotation in a year. An observer might see all of the fixed stars fixed in the sky during the entire year and every year: Zero sidereal days per year. However the Sun will have appeared to rise and set exactly once during that year: One solar day per year. Thus yearly:
Nope, one extra sidereal day each year. A quick way to see this is that the sidereal day is four minutes shorter than the civilian (solar) day, hence it takes 366 of them to fill up 365 solar days. Or look at the direction that the stars move each day, very slightly "westwards", hence causing the motion of the stars across the sky to be slightly more rapid than the motion of the sun (ie. packing more sidereal days into the year). Back to your thought experiment, the conclusion is incorrect precisely because the earth is rotating, and it is rotating in the same sense as its orbital motion. It is interesting to perform a different thought experiment, where the earth rotates once per year such that the same face is always towards the sun. In such a year there are no solar days and one sidereal day. If you can visualise the situation for rotation rates in between these two thought experiments, and then more or less quickly than either, you will have the full range of possibilities for sidereal vs solar days, and will know which of those we are actually living in. Lithopsian (talk) 18:47, 24 August 2018 (UTC)[reply]
The article claims that there's only two known surviving sidereal angle clocks in the world, both at the Royal Observatory, Greenwich. Except... there is most definitely a surviving medieval astronomical clock attached to the Old Town Hall in Prague that tells sidereal time. And... this article seems to indicate that there are many more. Am I missing something?
Robo042 (talk) 20:33, 16 April 2021 (UTC)[reply]
The line of intersection of the mean plane of the equator and the ecliptic defines the direction of the vernal equinox (♈︎) and the autumnal equinox, as per the northern hemisphere.
This seems to only work correctly (and produce the given example) if you take J2000 to mean 1 January 2000 at 12:00:00 UT1, rather than 1 January 2000 at 12:00:00 TT as the page on J2000 specifies. Should this just say the full date in UT1 (or simply Julian day 2451545.0) instead of referencing J2000? Styglian (talk) 21:30, 31 August 2022 (UTC)[reply]
You are right and I have edited the article to remove the mention of J2000. The cited source by Urban and Seidelmann doesn't say anything about J2000. Jc3s5h (talk) 23:29, 31 August 2022 (UTC)[reply]
Thanks! There was one reference remaining, so I've removed that too, I hope that's ok. Styglian (talk) 09:17, 1 September 2022 (UTC)[reply]
I checked the source, Urban & Seidelmann, and the bit about Terrestrial Time isn't in there; it was added by Eric Kvaalen at 8:58 UTC on 16 November 2022. Since one JD in the expression is specified to use UT1, the implication is that the constant is also a UT1 JD date/time. I have made the change. Jc3s5h (talk) 15:12, 26 April 2023 (UTC)[reply]
@Styglian:Jc3s5h, now the sentence is useless. What time of day on January 1, AD 2000, is tU equal to zero? Eric Kvaalen (talk)
The Julian date (JD) of any instant is the Julian day number plus the fraction of a day since the preceding noon in Universal Time.
So tU is zero at exactly noon UT1 on 2000-01-01, which due to ΔT_(timekeeping) was approximately noon (UT1) + 32 leap seconds (in 2000) + 32.184s = 12:01:04.184 Terrestrial Time. Styglian (talk) 11:44, 5 April 2024 (UTC)[reply]
Let us repeat the passage from the article as it now stands:
ERA, measured in radians, is related to UT1 by a simple linear relation:[1]
Checking with the USNO's MICA software, we see that at January 1, 2000, 12:00:00.0 UT1 the Julian date was 2,451,545 UT1. According to the definition of tU given in the article,
I don't think mentioning Terrestrial Time is useful, it just makes this more confusing since it's not how the ERA is defined. Maybe mentioning exactly when tU=0 in UT1 would be helpful though. Styglian (talk) 19:53, 5 April 2024 (UTC)[reply]
That's too short a quote for me to make sense of. But I strongly suspect it's wrong. That's because UT1 is all about the actual rotation of the Earth, while Terrestrial Time is a modern replacement for Ephemeris Time, and is time that advances at a steady pace without any of the variations that occur with the real Earth's rotation. Jc3s5h (talk) 21:46, 5 April 2024 (UTC)[reply]
There seems to be a typo in the equation given in the section called "Relationship between solar time and sidereal time intervals" (or, at least, it's different from what is in the reference). Equation 3.17 on page 81 of Urban & Seidelmann (i.e., the reference given) says