Take DeltaT into computations
To increase simulation accuracy, the time shift DeltaT=TD-UT should be taken into account. Planet positions are computed in TD, Earth's rotation in UT. Earth is slowing down erratically.
Blueprint information
- Status:
- Complete
- Approver:
- None
- Priority:
- Medium
- Drafter:
- None
- Direction:
- Approved
- Assignee:
- None
- Definition:
- Approved
- Series goal:
- Accepted for 0.12
- Implementation:
-
Implemented
- Milestone target:
-
0.12.0
- Started by
- Matthew Gates
- Completed by
- Alexander Wolf
Related branches
Related bugs
| Bug #575621: Delta T and lunar acceleration | Fix Released |
| Bug #802811: ancient solar eclipses are incorrect | Fix Released |
| Bug #1106656: Display DeltaT value | Fix Released |
| Bug #1106658: Use different formulae for DeltaT calculations | Fix Released |
Sprints
Whiteboard
For simulation of events near to the present time this is not a big concern, but for ancient historical observations it can be really significant. By the time we get back 2000 years, Stellarium's idea of the rotation of the Earth is almost 3 hours off.
Because the change in Delta-T over time is non-uniform, and historical observations of events which can be used to check delta-T are fairly sparse, there is uncertainly in the exact value for a given day. There are several methods published for determining delta-T. These methods generally take the form of one or more polynomial functions which take td as an argument and return the difference between TD mid-day and solar mid-day for that date. These polynomal functions are matched to observations of Delta-T for the time since it has been measured, and historical observations before then. Future values of Delta-T are extrapolated from current trends and/or predicted based on models of tidal interactions and other factors.
Status: I have implemented the method described here: http://
This takes the form of function(s) in StelUtils.
Next steps:
- verify that Stellarium's current simulation time is in fact TD
- implemented method of adjusting Earth's rotation according to delta-T for the given date
- find / test against historical observations
Ideas:
- Multiple methods for determining Delta-T would be nice - perhaps switchable in ssystem.ini
- Delta-T functions might also be needed for other bodies, again specifiable in ssystem.ini
- Display of Delta-T for a given date would be nice, but probably not turned on by default.
Work Items
Work items:
[matthew-
[snofriacus] implemented method of adjusting Earth's rotation according to delta-T for the given date: DONE
[appl-victor-reijs] find / test against historical observations: DONE
[alexwolf] Display of Delta-T for a given date: DONE
[alexwolf] Multiple methods for determining Delta-T: DONE
Delta-T functions might also be needed for other bodies: POSTPONED
