Talk:Equation of time

Discussion

Notice the difference between this:

${3{\rm {{hr}56{\rm {min}}}} \over 8}\times 2$

(with no spaces between digits and letters) and this:

${3\ {\rm {{hr}\ 56\ {\rm {min}}}} \over 8}\times 2$

That's part of what my recent edits did.

(On most browsers) 28° shows a superscript circle indicating "degrees"; I changed 28 deg to that. Similarly, in TeX, I changed this:

$28\ {\rm {deg}}$

to this:

$28^{\circ }$

"Displayed" TeX should normally be indented; thus this

$3\ {\rm {{hr}\ 56\ {\rm {min}}}}$

differs from this:

$3\ {\rm {{hr}\ 56\ {\rm {min}}}}$

Michael Hardy 23:34, 5 Sep 2004 (UTC)

---

The external link to the article by Brian Tung (currently dated and copyrighted 2002), is I think, important, because at the end it contains a link to a C program for the analemma or equation of time, which uses a more accurate formula than many use. Particularly, it is more complex than simply working out the Equation of Time due to the two components (eccentricity and obliquity) on their own, independantly of each other, then adding the two results. The movement of the Sun eastwards among the stars due to the orbital motion of the Earth, is itself uneven due to eccentricity of the orbit; this needs to be taken into account when working out the component of the Equation of Time due to the obliquity. As Brian Tung states in the last paragraph of his page, the formula which many use (working out the two effects independantly then adding them linearly) works reasonably well for small eccentricities and obliquities, but becomes noteably inaccurate for extreme orbits and inclinations.

Roo60 13:54, 2 Apr 2005 (UTC)

Comments on the Mathematical Description section.

Should the definition of GMST be the angle between the mean vernal equinox and the meridian (not the mean Sun). Otherwise it does not seem to be an approximation to GAST as claimed.

This might be linked to what might be a problem in ref 27. In that reference, ST starts out as something that increases by about 24 hours in a day, but when the apparent version gets approximated by the mean version, ST seems to absorb the -15UT term (the 15 converting hours to degrees) and so only changes significantly over a year.

I also wonder about the definition of Delta t = Lambda - alpha. Lambda is the ecliptic longitude of the mean sun. Delta t is claimed to be close to EOT = alpha_M - alpha where alpha_M is the right ascension of the mean sun. Surely the difference between the right ascension and the ecliptic longitude of the mean sun is significant (of order the effect of the obliquity of the ecliptic)? Possibly this is a different mean sun to that referred to near the start of the article, one that goes round the ecliptic at constant rate rather than crossing longitude lines at constant rate? If so, it would be good to explain this and explain why Lambda is different to GMST - UT + offset (it doesn't seem to be defined precisely enough for it to be clear that it is different from GMST - UT + offset).

I won't attempt to fix these issues myself because, although I'm fairly convinced there's a problem, I'm not sure how it should be fixed. Gingercatnine (talk) 16:46, 5 July 2022 (UTC)

Removed redundant graph

The lead contained two graphs of the equation of time, which were practically identical except that one was sign-reversed from the other. I can't see how a reader would benefit from viewing both graphs, but I can see how they might be confused by the apparent discrepancy between them. I have therefore removed one of them. CodeTalker (talk) 15:11, 24 March 2023 (UTC)

@CodeTalker: I restored the graph for now since it is still mentioned in the article, but if you want to make edits so that there is no confusion, feel free to remove it again. Also, your last edit had a lot of unexplained changes, which led me to believe that you may have accidentally edited an old version of the article. --Lasunncty (talk) 23:38, 4 April 2023 (UTC)

Unmatched parentheses

The initial (top-most) equation for Δtey appears to be missing an opening parenthesis. There is a closing parenthesis after the 3.5932, but no matching opening parenthesis. Looking down in the "Major components of the equation" section, it looks as though the last component of the Δtey equation should be 9.863sin((2D) + 3.5932), or just 9.863sin(2D + 3.5932). That said, the basis of the equation is beyond me: my interest was in calculating the sunrise and sunset times in a GPS-based mobile phone app. It may be helpful to state that the values of D etc... are in degrees, as mathematical functions such as sin/cos in programming languages often expect angles to be in radians. I didn't initially look at the less precise but more compact forms just below the initial equation (that effectively made it clear that D was in degrees), as I was happy to use the more precise equation. Added later: It looks as though the value angle values as shown are in fact in radians, so the D and 3.5932 are values in radians. — Preceding unsigned comment added by 2404:4402:3010:8300:D119:49B0:AFBD:8C4D (talk) 13:22, 22 August 2023 (UTC)

David 05:48, 19 Aug 2023 (UTC)

Fixed. --Lasunncty (talk) 08:04, 19 August 2023 (UTC)