<p>As I pointed out in the comments, the difference appears to be due to the use of predicted values of <span class="math-container">$ \Delta T $</span> rather than a value updated with the most recent data from the <a href="https://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html" rel="nofollow noreferrer">International Earth Rotation Service bulletins</a>.</p>
<p>The screenshot below is from the Five Millennium Catalog of Solar Eclipses by Meeus and Espenak.  The fourth column is <span class="math-container">$ \Delta T $</span>.  As you can see, it is not an exact match, so the values were likely getting updated at some point, and seemed to stop in 2019.  As many people know, the USNO Data Services went offline in 2019 for an upgrade planned to be 6 months, but ended up lasting several years.  So, it looks like the update process to use more modern data was broken in the process.  I have sent them a message through the e-mail on the site, but I've had spotty success with that in the past.  I will update this answer if I receive a response, but I doubt any changes could be made before the eclipse actually happens.</p>
<p><a href="https://i.sstatic.net/C9EUi.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/C9EUi.png" alt="enter image description here" /></a></p>

CC BY-SA 4.0
2024-03-25T16:44:58.030Z
So the moral of the story is: use the listed TD time and the current Delta T to calculate the UTC yourself.
PM 2Ring
CC BY-SA 4.0
2024-03-25T21:34:49.637Z
The whole point of having delta t listed separatley is so you could use the most recent data.  But the whole point of the IERS bulitens being machine readable is so that can be done automatically.  So, really the USNO dropped the ball on this one.
Greg Miller
CC BY-SA 4.0
2024-03-27T13:20:50.320Z
Just wanted to say thanks for clarifying that this was the issue. In case you're curious why I was trying to do this myself: I made an app for the eclipse. And thanks to your help, it's now accurate to within 1s! It's available for free (no ads) on the Apple AppStore: https://apps.apple.com/us/app/2024-eclipse-live/id6480014046
hanno
CC BY-SA 4.0
2024-03-27T15:24:58.033Z
@hanno You might find this Javascript implementation useful, the code is public domain. https://celestialprogramming.com/apps/SolarEclipseViewer/viewer.html
Greg Miller
CC BY-SA 4.0
2024-03-25T14:00:41.353Z
2024-03-25T14:00:41.353Z
Greg Miller
<p>I'm trying to compare some eclipse calculations to that by the U.S. Naval Observatory, specifically, <a href="https://aa.usno.navy.mil/calculated/eclipse/solar?eclipse=12024&amp;lat=44.2214&amp;lon=-76.4893&amp;label=&amp;height=83&amp;submit=Get%20Data" rel="nofollow noreferrer">these results</a>. One thing I just don't understand is how they come up with a Delta T of 72.8s. Shouldn't it be more like 37s + 32.184s = 69.182s these days, including both leap seconds and TT-TAI offset. Is this Delta T on the website something different or am I missing something?</p>
<p>Edit: Thank to everyone's help, this got sorted out (see below). I can now <a href="https://apps.apple.com/us/app/2024-eclipse-live/id6480014046" rel="nofollow noreferrer">predict eclipses down to 1s accuracy</a>. Yay!</p>

CC BY-SA 4.0
2024-03-24T14:33:55.837Z
Do they explain their Delta T somewhere what it is supposed to mean in the first place? Delta T is not a name, but just a formula symbol which could be used for a plethora of things.
planetmaker
CC BY-SA 4.0
2024-03-24T14:35:26.513Z
Well, that's more or less what my question is about. I couldn't find that information. In their novas software, delta t is pretty uniquely used for the time offset between UT1 and TT.
hanno
CC BY-SA 4.0
2024-03-24T15:04:04.227Z
There are lots of signs that this is a badly done website. The Delta T value is not documented. Times in UT1? Please! This would not be the first time (and unfortunately, almost certainly not the last time) that the US government has created a badly done website. Keep in mind that the work is done by the cheapest bidder. Your best bet is to find some other solar eclipse calculator.
David Hammen
CC BY-SA 4.0
2024-03-24T15:14:33.860Z
I'll be somewhere to see this as it's the last one until 2044 for the continental US. I have two hotel rooms booked (not cheap, dang it!) in San Antonio. (A lot cheaper though than Fredericksburg or Kerrville, which are both well over \$1000 per night per room in a **cheap** hotel.) From San Antonio, I'll drive an hour or two in some direction between southwest and northwest, aiming to go to a place where weather forecasters say the skies will not be cloudy all day. And I'll use data from some other eclipse calculator.
David Hammen
CC BY-SA 4.0
2024-03-24T15:17:22.480Z
$ \Delta T $ a very common term used with Besselian Elements used to predict solar eclipses, and is defined in the glossary of the Explanatory Supplement as "the difference between Terrestrial Time (TT) and Universal Time (UT): $ \Delta T $ = TT - UT1".
Greg Miller
CC BY-SA 4.0
2024-03-24T15:19:39.987Z
@GregMiller I understand that. But it's off by a few seconds and the question asks why that is. I suspect the reason is that this website was made in the past and the Delta T was a prediction at the time and they never got around to using an updated value.
hanno
CC BY-SA 4.0
2024-03-24T15:21:19.930Z
@GregMiller The problem with that has been highlighted in the question. UT1 and UTC differ by at most 0.8 seconds, and currently TT-UTC is 69.184 seconds. This does not jibe with the 72.8 seconds on the linked website.
David Hammen
CC BY-SA 4.0
2024-03-24T15:23:13.727Z
I wasn't trying to answer the question, my comment was directed at those wondering what $ \Delta T $ was.  I agree the value use by the USNO is puzzling.
Greg Miller
CC BY-SA 4.0
2024-03-24T15:48:18.640Z
I remember seeing this thread where the value of $ \Delta T $ was off in an old program.  It was using an old predicted value, and not computed from the latest data.  So I'll bet it's something similar where the program the USNO has a file of predicted values, and they just haven't been updated.  Based on the values given for other eclipses, it looks like it was last updated in 2017. https://www.cloudynights.com/topic/914689-xavier-jubier-eclipse-timing-vs-eclipse-orcestrator-off-by-4-seconds/
Greg Miller
CC BY-SA 4.0
2024-03-24T17:32:21.673Z
Here's the Fred Espenak [page for this eclipse](https://eclipsewise.com/solar/SEprime/2001-2100/SE2024Apr08Tprime.html). It's also using a slightly "generous" value for DeltaT. Espenak & Meeus developed a series of [polynomial expressions for DeltaT](https://eclipsewise.com/help/deltatpoly2014.html), which are kind of necessary for long-term eclipse prediction.
PM 2Ring
CC BY-SA 4.0
2024-03-24T17:39:28.513Z
Also see https://www.eclipsewise.com/help/rotation.html
PM 2Ring
CC BY-SA 4.0
2024-03-24T18:06:19.177Z
@DavidHammen Defining  ΔT = TT − UT1 is pretty standard, complementing DUT1 = UT1 − UTC, which is published weekly in IERS [Bulletin A](https://datacenter.iers.org/data/latestVersion/bulletinA.txt). However, [Horizons](https://ssd.jpl.nasa.gov/horizons/manual.html#obsquan) uses a slightly different definition, DeltaT = TDB - UT, where UT is UT1 prior to 1962, and UTC afterwards. However, Horizons does *not* extrapolate future leap seconds, so their DeltaT (and UTC values) for far future events may be way off.
PM 2Ring
CC BY-SA 4.0
2024-03-24T18:40:02.187Z
@PM2Ring note the "copyright 2014" in the image on the page.  So, that is also a predicted value, and not updated with the latest data.
Greg Miller
CC BY-SA 4.0
2024-03-24T20:12:43.667Z
@Greg Of course. It's from [The Thousand Year Canon of Solar Eclipses 1501 to 2500](https://eclipsewise.com/pubs/TYCSE.html). I wasn't implying that it uses the latest data. Also note that Fred's predictions use JPL DE405. But they should still be pretty accurate, especially if you use his TT values (which he calls TD).
PM 2Ring
<coordinate><time><solar-eclipse>
Delta T used by US Naval Observatory
hanno
# Delta T used by US Naval Observatory

(Asked by: username_0 on 2024-03-24)

I'm trying to compare some eclipse calculations to that by the U.S. Naval Observatory, specifically, [these results](https://aa.usno.navy.mil/calculated/eclipse/solar?eclipse=12024&lat=44.2214&lon=-76.4893&label=&height=83&submit=Get%20Data). One thing I just don't understand is how they come up with a Delta T of 72.8s. Shouldn't it be more like 37s + 32.184s = 69.182s these days, including both leap seconds and TT-TAI offset. Is this Delta T on the website something different or am I missing something?

Edit: Thank to everyone's help, this got sorted out (see below). I can now [predict eclipses down to 1s accuracy](https://apps.apple.com/us/app/2024-eclipse-live/id6480014046). Yay!

**Tags:** coordinate, time, solar-eclipse
> **Comments:**
> **username_1**: Do they explain their Delta T somewhere what it is supposed to mean in the first place? Delta T is not a name, but just a formula symbol which could be used for a plethora of things.  
> **username_0**: Well, that's more or less what my question is about. I couldn't find that information. In their novas software, delta t is pretty uniquely used for the time offset between UT1 and TT.  
> **username_2**: There are lots of signs that this is a badly done website. The Delta T value is not documented. Times in UT1? Please! This would not be the first time (and unfortunately, almost certainly not the last time) that the US government has created a badly done website. Keep in mind that the work is done by the cheapest bidder. Your best bet is to find some other solar eclipse calculator.  
> **username_2**: I'll be somewhere to see this as it's the last one until 2044 for the continental US. I have two hotel rooms booked (not cheap, dang it!) in San Antonio. (A lot cheaper though than Fredericksburg or Kerrville, which are both well over \$1000 per night per room in a **cheap** hotel.) From San Antonio, I'll drive an hour or two in some direction between southwest and northwest, aiming to go to a place where weather forecasters say the skies will not be cloudy all day. And I'll use data from some other eclipse calculator.  
> **username_3**: $ \Delta T $ a very common term used with Besselian Elements used to predict solar eclipses, and is defined in the glossary of the Explanatory Supplement as "the difference between Terrestrial Time (TT) and Universal Time (UT): $ \Delta T $ = TT - UT1".  
> **username_0**: @GregMiller I understand that. But it's off by a few seconds and the question asks why that is. I suspect the reason is that this website was made in the past and the Delta T was a prediction at the time and they never got around to using an updated value.  
> **username_2**: @GregMiller The problem with that has been highlighted in the question. UT1 and UTC differ by at most 0.8 seconds, and currently TT-UTC is 69.184 seconds. This does not jibe with the 72.8 seconds on the linked website.  
> **username_3**: I wasn't trying to answer the question, my comment was directed at those wondering what $ \Delta T $ was.  I agree the value use by the USNO is puzzling.  
> **username_3**: I remember seeing this thread where the value of $ \Delta T $ was off in an old program.  It was using an old predicted value, and not computed from the latest data.  So I'll bet it's something similar where the program the USNO has a file of predicted values, and they just haven't been updated.  Based on the values given for other eclipses, it looks like it was last updated in 2017. https://www.cloudynights.com/topic/914689-xavier-jubier-eclipse-timing-vs-eclipse-orcestrator-off-by-4-seconds/  
> **username_4**: Here's the Fred Espenak [page for this eclipse](https://eclipsewise.com/solar/SEprime/2001-2100/SE2024Apr08Tprime.html). It's also using a slightly "generous" value for DeltaT. Espenak & Meeus developed a series of [polynomial expressions for DeltaT](https://eclipsewise.com/help/deltatpoly2014.html), which are kind of necessary for long-term eclipse prediction.  
> **username_4**: Also see https://www.eclipsewise.com/help/rotation.html  
> **username_4**: @DavidHammen Defining  ΔT = TT − UT1 is pretty standard, complementing DUT1 = UT1 − UTC, which is published weekly in IERS [Bulletin A](https://datacenter.iers.org/data/latestVersion/bulletinA.txt). However, [Horizons](https://ssd.jpl.nasa.gov/horizons/manual.html#obsquan) uses a slightly different definition, DeltaT = TDB - UT, where UT is UT1 prior to 1962, and UTC afterwards. However, Horizons does *not* extrapolate future leap seconds, so their DeltaT (and UTC values) for far future events may be way off.  
> **username_3**: @PM2Ring note the "copyright 2014" in the image on the page.  So, that is also a predicted value, and not updated with the latest data.  
> **username_4**: @Greg Of course. It's from [The Thousand Year Canon of Solar Eclipses 1501 to 2500](https://eclipsewise.com/pubs/TYCSE.html). I wasn't implying that it uses the latest data. Also note that Fred's predictions use JPL DE405. But they should still be pretty accurate, especially if you use his TT values (which he calls TD).  

---

## 1 answers

### Answer by username_3 on 2024-03-25. Score: 4 | Accepted answer

As I pointed out in the comments, the difference appears to be due to the use of predicted values of $ \Delta T $ rather than a value updated with the most recent data from the [International Earth Rotation Service bulletins](https://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html).

The screenshot below is from the Five Millennium Catalog of Solar Eclipses by Meeus and Espenak. The fourth column is $ \Delta T $. As you can see, it is not an exact match, so the values were likely getting updated at some point, and seemed to stop in 2019. As many people know, the USNO Data Services went offline in 2019 for an upgrade planned to be 6 months, but ended up lasting several years. So, it looks like the update process to use more modern data was broken in the process. I have sent them a message through the e-mail on the site, but I've had spotty success with that in the past. I will update this answer if I receive a response, but I doubt any changes could be made before the eclipse actually happens.

[![enter image description here](https://i.sstatic.net/C9EUi.png)](https://i.sstatic.net/C9EUi.png)

> **Comments:**
> **username_4**: So the moral of the story is: use the listed TD time and the current Delta T to calculate the UTC yourself.  
> **username_3**: The whole point of having delta t listed separatley is so you could use the most recent data.  But the whole point of the IERS bulitens being machine readable is so that can be done automatically.  So, really the USNO dropped the ball on this one.  
> **username_0**: Just wanted to say thanks for clarifying that this was the issue. In case you're curious why I was trying to do this myself: I made an app for the eclipse. And thanks to your help, it's now accurate to within 1s! It's available for free (no ads) on the Apple AppStore: https://apps.apple.com/us/app/2024-eclipse-live/id6480014046  
> **username_3**: @hanno You might find this Javascript implementation useful, the code is public domain. https://celestialprogramming.com/apps/SolarEclipseViewer/viewer.html  

---

