Comment 4 for bug 1380242

Hi Victor,

Thanks for looking into this. I'm glad you've figured out the situation.
I'll take another look at it when I get home and will reproduce your
readjustment of UTC so I can see it for myself. But I can picture it in my
mind's eye (and should have thought of it myself), so I don't expect to
have any problems doing so.

For what it's worth, I volunteer at an observatory that until recently was
staffed by professional astronomers. I once asked them what time argument
they used and they told me it was UTC, not UT1 or TT. If you look at the
astronomical almanac, virtually every phenomenon is listed using UT or UTC.

I think the answer is clear: *everyone* understands and uses UTC, even
average users who might not be aware of the intricacies of such a hybrid
time scale.

It's also worth remembering that Delta T will have a major effect on the
observed location of an eclipse: although the Sun-Moon-Earth geometry is
defined using TT, the position of the Moon's shadow on the surface of the
Earth is controlled by UT1 and hence UTC. A one-minute error could make the
difference between being in the line of totality and not being there.

Cheers,

Frank

On Tue, Jul 7, 2015 at 3:56 AM, Victor Reijs <email address hidden>
wrote:

> Just to check: If one changes the DeltaT formula in Stellarium, it looks
> Stellarium uses the TT as its basis. So it calculates UT (using TT-DeltaT).
> So the presented (UT) time changes in the time window of Stellarium.
> If I go back to the same UT (after changing the DeltaT formula) I see the
> same sky (I did not compare it dot for dot, but they look quite the same).
> But lets first see if you see the same as me.
>
> It took me also some time to understand the changing of the view. I
> don't know what is better keeping TT the same or UT. I think for the
> average user; keeping UT the same sound better (keeping TT the same is
> more for the expert). But not sure: Stellarium is the only programme
> that has this diversity of DeltaT formula, so comparing with other
> programmes is difficult.
>
>
> All the best,
>
>
> Victor
>
> --
> You received this bug notification because you are subscribed to the bug
> report.
> https://bugs.launchpad.net/bugs/1380242
>
> Title:
> Calculation of Delta T is incorrectly applied
>
> Status in Stellarium:
> New
>
> Bug description:
> I think you are incorrectly applying the calculation of Delta T.
>
> Delta T is defined as the difference between TT and UT (Terrestrial
> time and Universal Time). UT can be treated as Greenwich time in this
> context, and TT applies solely to the clockwork of objects within our
> solar system, relative to the solar system barycentre. Delta T has
> nothing to do with the rotation of the earth, and therefore nothing to
> do with the observed locations of objects in the sky.
>
> However, when I change Delta T options in 0.12.4 I see *everything* in
> the sky wheel around slightly. This is wrong.
>
> Delta T should only be applied to the calculation of the positions of
> solar system objects with respect to its barycentre. It should not
> affect stellar and deep sky objects at all, and in fact even changes
> solar system objects should be almost undetectable. Delta T really
> makes the most difference to eclipse watchers or those interested in
> fast moving objects like asteroids, comets or planetary moons.
>
> For example, let's take the value of Delta T as 60 seconds and assume
> that I want to know the appearance of the sky at 17:30 UT. This is how
> it is used:
>
> 1. Calculate the topocentric position of everything *outside* the solar
> system for 17:30 UT
> 2. Calculate TT as UT + Delta T = 17:30 + 0:60 = 17:31
> (Essentially, this means the Earth's rotation is 60 seconds slow
> relative to the perfect clockwork of the solar system)
> 3. Calculate the position of *solar system objects* relative to the
> solar system barycentre at 17:31 TT
> 4. Calculate the topocentric position of solar system objects at 17:30
> UT using the result of Step 3.
>
> Now if you imagine calculating the position of Mars, the difference in
> its position relative to the solar system barycentre between 17:30 TT
> and 17:31 TT will be almost indistinguishable when viewed from Earth.
> On the other hand, Earth will rotate far enough in one minute to make
> *everything* appear in a different location.
>
> Apologies for the long-winded post; I wanted to explain Delta T as
> clearly as possible. Apologies too if this has already been reported
> (I couldn't find anything).
>
> fbilki
>
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