# Current Value for Delta T

Forums Computing Current Value for Delta T

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• #574740
John O’Neill
Participant

Hi,

I am looking for an online source for the current value of Delta T (to nearest second). It should be an observed value, not a prediction. The USNO website used to provide this, but many of their website site internal links appear to be broken. I am writing a program to calculate moon phases.

John

#583177
Dr Andrew Smith
Participant

Not sure if DUT1 = UT1 -UTC us what your after but look at the International Earth Rotation Service IERS. It may have what you want.

Regards Andrew

#583178
Dominic Ford
Keymaster

Your question doesn’t make sense!

If you’re after (TAI-UTC), then that’s not an observed quantity. It’s defined by when leap seconds are inserted. A simple list of leap seconds over recent years will give you an answer that is 100% accurate by definition:

https://www.ietf.org/timezones/data/leap-seconds.list

The current value is precisely 37 seconds.

But… if you’re after UT1-UTC (I can’t quite think why you would be – it’s only relevant to rising and setting times!), the full gory details are all here with awful lot of decimal places (see Bulletin A):

https://www.iers.org/IERS/EN/DataProducts/EarthOrientationData/eop.html

However, UT1-UTC is maintained at a value less than 0.5 seconds by inserting leap seconds whenever it drifts outside that range. If, as you say, you are only interested in accuracy to the nearest second, then you can assume that UT1-UTC always equals zero.

#583180
Participant

Hi John,

You might find this link useful – https://www.usno.navy.mil/USNO/earth-orientation/eo-products/long-term – this gives historic and predictions for delta T.

Peter

#583182
John O’Neill
Participant

Hi,

Yes Peter, that’s the USNO link I relied on for years, but in my brower (Firefox), it’s broken.

Just to be clear, the quantity, I was after, is Delta T = (TT – UT), where TT is Terrestrial Time and UT is Universal Time.

In J.Meeus Astronomical Algorithms 2nd Ed page 77 he states “The exact value of the difference Delta-T = TD – UT can be deducted only from observations”. I assume, for my purposes, TD and TT are interchangeable.

John

#583183
Dominic Ford
Keymaster

Jean Meeus’s statement that it can only be deduced from observations is (unless I’m mistaken) only true for UT1 – a time standard which strictly follows the Earth’s rotation.

The time that the clock on your wall reads is UTC, which is pinned to International Atomic Time (a realisation of TT), but has leap seconds added to keep it closely aligned to UT1.

The quantity you’re almost certainly looking for is (TT-UTC), which is exactly 37 seconds currently, and changes in 1 second steps when leap seconds are added to UTC.

Whilst observations certainly are needed to determine (TT-UT1), it’s not a very useful quantity for most people, because UT1 is only used for a few specific applications that directly involve the Earth’s rotation. The ones that come to mind are calculating precise sidereal time, and calculating the Earth’s rotation angle during eclipses. Oh, and of course deciding when to put leap seconds into UTC! 🙂

If the problem you’re working on involves the times when the Moon is at certain positions in its orbit, then that’s a problem where the Moon’s orbit is defined in TT, and you want to report times in UTC. So UT1 doesn’t come into it.

#583187
Nick James
Participant

If you really want to put delta-T in a program then there are polynomial approximations such as the ones here. As has been said though you are unlikely to require this for lunar phases. It is important for high precision ephemerides of relatively close objects (including satellites)  since it does affect the parallax correction and, as Dominic says, it is crucial for calculating total eclipse paths. It is probably worth including in telescope pointing algorithms too. Should anyone ever decide to remove leap seconds from UTC it will become important for the general public in a few thousand years!

#583188
John O’Neill
Participant

Hi,

Thank you for the replies. I did find a link to the value of Delta T that (pre-Covid) was updated every month:

ftp://cddis.gsfc.nasa.gov/pub/products/iers/

See the file deltat.dat

The latest value they give is for 1st Feb 2020 (69.3752 seconds).

In my lunar phase program I give the result to one minute (just like the BAAH), which is why I was looking for Delta T. It’s nice to be able to compare the predicted and actual values.

John

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