Calculation of Delta T is incorrectly applied
Affects | Status | Importance | Assigned to | Milestone | |
---|---|---|---|---|---|
Stellarium |
Fix Released
|
High
|
gzotti |
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
Related branches
- Alexander Wolf: Approve
-
Diff: 3522 lines (+693/-585)59 files modifiedplugins/AngleMeasure/src/AngleMeasure.hpp (+5/-3)
plugins/ArchaeoLines/src/ArchaeoLines.cpp (+11/-5)
plugins/ArchaeoLines/src/ArchaeoLines.hpp (+1/-1)
plugins/EquationOfTime/src/EquationOfTime.cpp (+12/-10)
plugins/EquationOfTime/src/EquationOfTime.hpp (+2/-2)
plugins/Exoplanets/src/Exoplanet.cpp (+1/-1)
plugins/MeteorShowers/src/MeteorShower.cpp (+3/-3)
plugins/MeteorShowers/src/MeteorShowers.cpp (+4/-3)
plugins/MeteorShowers/src/gui/MeteorShowerDialog.cpp (+2/-1)
plugins/Novae/src/Nova.cpp (+1/-1)
plugins/Novae/src/Nova.hpp (+5/-0)
plugins/Observability/src/Observability.cpp (+121/-99)
plugins/Observability/src/Observability.hpp (+25/-18)
plugins/PointerCoordinates/src/PointerCoordinates.cpp (+2/-2)
plugins/Satellites/src/Satellite.cpp (+2/-3)
plugins/Satellites/src/Satellites.cpp (+1/-1)
plugins/Satellites/src/gSatWrapper.cpp (+1/-1)
plugins/Satellites/src/gsatellite/gVector.cpp (+1/-1)
plugins/Satellites/src/gsatellite/gVector.hpp (+3/-2)
plugins/Scenery3d/src/Scenery3d.cpp (+3/-3)
plugins/Supernovae/src/Supernova.cpp (+1/-1)
plugins/TelescopeControl/src/clients/TelescopeClient.cpp (+4/-2)
plugins/TelescopeControl/src/servers/Socket.cpp (+4/-2)
plugins/TextUserInterface/src/TextUserInterface.cpp (+3/-3)
src/core/StelCore.cpp (+147/-111)
src/core/StelCore.hpp (+54/-26)
src/core/StelMovementMgr.cpp (+3/-3)
src/core/StelMovementMgr.hpp (+1/-1)
src/core/StelObject.cpp (+5/-12)
src/core/StelObserver.cpp (+6/-12)
src/core/StelObserver.hpp (+4/-1)
src/core/StelUtils.hpp (+1/-1)
src/core/TrailGroup.cpp (+3/-3)
src/core/modules/Atmosphere.cpp (+1/-1)
src/core/modules/Comet.cpp (+12/-12)
src/core/modules/Comet.hpp (+2/-6)
src/core/modules/Constellation.cpp (+1/-1)
src/core/modules/LandscapeMgr.cpp (+2/-1)
src/core/modules/MinorPlanet.cpp (+3/-3)
src/core/modules/Orbit.cpp (+17/-26)
src/core/modules/Orbit.hpp (+7/-7)
src/core/modules/Planet.cpp (+61/-58)
src/core/modules/Planet.hpp (+17/-15)
src/core/modules/SolarSystem.cpp (+10/-8)
src/core/modules/SolarSystem.hpp (+3/-3)
src/core/modules/StarWrapper.cpp (+1/-1)
src/core/modules/StarWrapper.hpp (+1/-1)
src/core/modules/ZodiacalLight.cpp (+1/-1)
src/core/modules/ZoneArray.cpp (+2/-2)
src/core/planetsephems/sidereal_time.c (+10/-10)
src/core/planetsephems/sidereal_time.h (+2/-2)
src/gui/ConfigurationDialog.cpp (+4/-2)
src/gui/DateTimeDialog.cpp (+16/-23)
src/gui/SearchDialog.cpp (+1/-1)
src/gui/StelGui.cpp (+1/-1)
src/gui/StelGuiItems.cpp (+20/-22)
src/gui/dateTimeDialogGui.ui (+3/-3)
src/scripting/StelMainScriptAPI.cpp (+37/-26)
src/scripting/StelMainScriptAPI.hpp (+16/-11)
description: | updated |
tags: | added: archaeoastronomy |
Changed in stellarium: | |
importance: | Undecided → High |
Changed in stellarium: | |
importance: | High → Undecided |
status: | New → Opinion |
Changed in stellarium: | |
status: | Confirmed → In Progress |
Changed in stellarium: | |
status: | Fix Committed → Fix Released |
Hell fbilki, /en.wikipedia. org/wiki/ %CE%94T Any modern ephemeris uses a 24*60*6o SI seconds per day, and thus one needs to adjust the time to map real time.
You say that DeltaT has nothing to do with the rotation of the earth. That is not true. DeltaT is the accumulative difference between the rotation of the earth (close to 24 hours, not not fully) and a 'day' of precisely 24*60*60 SI seconds. see also https:/
But I don't think your point is only that, so let me know, perhaps I can help.
All the best,
Victor