Talk:Sidereal time

One LESS sidereal day per year

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:

solar days = sidereal days + 1.

--Alex — Preceding unsigned comment added by 174.29.24.16 (talk) 16:43, 24 August 2018 (UTC)

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)

Ah, yes, of course. I assumed (or failed to consider) the rotation turning the "other way". Thank you for the clear explanation. — Preceding unsigned comment added by Alexgenaud (talk • contribs) 02:03, 4 September 2018 (UTC)

Only two known sidereal clocks...?

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)

Equinox

Jun 30, 2022, 18:56 - «Presumably "Aires" was supposed to be "Aries".»
Jun 30, 2022, 18:50 - «‎top: provide clarification»

@Guy Harris and Jc3s5h: But do we really have a source on that clarification? AXONOV (talk) ⚑ 21:05, 1 July 2022 (UTC)

Urban & Seidelman (2013, p. 18).

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.

Jc3s5h (talk) 21:58, 1 July 2022 (UTC)

Earth rotation angle and J2000

According to the article on J2000, it is defined in Terrestrial Time, but the section on "Earth rotation angle" says:

where tU=JD-J2000 is the Julian UT1 date (JD) relative to the J2000 epoch (JD 2451545.0).

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)

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)

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)

There is still a reference to Terrestial Time in this section, which I think should be removed. 2C0F:F4C0:2202:1234:7F07:5048:6504:D779 (talk) 14:26, 26 April 2023 (UTC)

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)

@Styglian: Jc3s5h, now the sentence is useless. What time of day on January 1, AD 2000, is t_U equal to zero? Eric Kvaalen (talk)

@Eric Kvaalen: The Julian UT1 date is defined precisely on the linked page as:

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 t_U 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)

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:

$$\theta (t_{U})=2\pi (0.779\,057\,273\,2640+1.002\,737\,811\,911\,354\,48\cdot t_{U})$$

where t_U is the Julian UT1 date (JD) minus 2451545.0.

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 t_U given in the article,

t_U = UT1 expressed as JD - 2,451,545 = 0.

At that time the Earth Rotation Angle is 280° 27' 38.2262". Jc3s5h (talk) 12:38, 5 April 2024 (UTC)

Thanks, Styglian. So Jc3s5h, can you put back something instead of

"that is, 12:00 (midday) Terrestrial Time of January 1, 2000. (Note that this differed from 12h UTC on that day by around a minute.)"

which you and 2c0f:f4c0:2202:1234:7f07:5048:6504:d779 deleted? Eric Kvaalen (talk) 19:32, 5 April 2024 (UTC)

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 t_U=0 in UT1 would be helpful though. Styglian (talk) 19:53, 5 April 2024 (UTC)

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)

References