Add option to display ecliptic grid of date

[Shriramana Sharma]

Hello. I'm using Stellarium 0.12.4 on Kubuntu Precise 64 bit. (I think it is a great software except for some minor bugs which I hope to report over time and get fixed by the kind devels! So thank you!)

Now this is possibly related to bug 512086 but I do not think it is exactly the same as that so I'm filing this separately.

(That bug reports some possible error in displaying the ecliptic line whereas) I would like to have an option under:

Sky/Viewing options > Markings

to display the ecliptic grid of date. Currently we have options to display the Equatorial Grid (I presume of date) and the J2000 Equatorial Grid. We also have an option to display the J2000 Ecliptic Grid. But we do not have an option to display the Ecliptic Grid of date. Please add such an option. Due to precession, obviously the Ecliptic Grid of date can be displaced (in longitude and slightly in obliquity) w.r.t. the J2000 Ecliptic.

While fixing this please clarify the wording in this window as follows:

Equatorial grid -> Equatorial grid of date
Equatorial J2000 grid -> Equatorial grid of J2000
Ecliptic grid -> Ecliptic grid of date
Ecliptic J2000 grid -> Ecliptic grid of J2000

Thank you!

[Shriramana Sharma (jamadagni)]

An additional thought -- I do not see much requirement for any user to display both the equatorial/ecliptic grids of J2000 and of date at the same time. That is, you either require the equatorial grid of J2000 or of date but not both. Likewise for ecliptic grid. So perhaps you could make this a radio option or perhaps a combobox to the right of a simple "Ecliptic grid" with options: "Of date", "J2000" and likewise for "Equatorial grid". This would help reduce clutter in the options window.

[Alexander Wolf (alexwolf)]

This issue partially fixed at revision 6495 - http://bazaar.launchpad.net/~stellarium/stellarium/trunk/revision/6495

[Nick Fedoseev (nick-ut2uz)]

Perhaps I missed something?
While J2000 and JNow are virtual grids which are changed in parallel with moving the North Pole over star map, the ecliptic grid is a grid which physically belongs to the Sun plane. There is the only one "ecliptic grid" available, no matter which year is discovered, 2000 or 2015.

[Shriramana Sharma (jamadagni)]

Well actually there are two points:

1) The ecliptic is not entirely constant but changes very slowly over the centuries w.r.t. the invariable plane which is much more stable. This is one of the reasons that the obliquity is not constant (apart from the nutation etc).

2) Even if the above slow change over the centuries is ignored, the equinox which determines the zero point of ecliptic longitude keeps changing because of the movement of the equator in precession, so the ecliptic grid (which depends on the zero longitude) will change as well.

[Nick Fedoseev (nick-ut2uz)]

Well, the ecliptic changes in equatorial coordinates, because those coordinates are changed with North Pole movement and other effects. But how the ecliptic may change in it's own coordinates? I understand that it changes very slow and the ecliptic plane in 2000 is not the same as in 4000. But why zero point of this system depends of equator precession, which belongs to another (geo) system?

[Alexander Wolf (alexwolf)]

A fix has been committed as revision 7748 of the trunk branch of Stellarium's Bazaar repository at Launchpad: http://bazaar.launchpad.net/~stellarium/stellarium/trunk/revision/7748