New post
0

SkySafari 6 Pro | Why Is Accuracy Of Comet 74P Better In SkySafari 5 Pro (Answer: SS6 Now Uses JPL, Epoch Retrieved For 74P Is From 2011 - And This Is Isolated)

74P/Smirnova-Chernykh will transit across M66 over April 30 to May 2. It appear the position of 74P is off in SkySafari 6 pro but appear to be correct in SkySafari 5. What am I doing differently in SkySafari 6 pro vs SkySafari 5 or is there a problem with the program or something else.

Thank you

 

 

 

4 comments

  • 0
    Avatar
    Dean

    I'm seeing the same difference between SkySafari 5 and 6. (SkySafari 5 & 6 for Mac OS and iOS display the same discrepancy.)

    Minor body orbit data updated on all versions.

    Here are two screen caps that compare the two:

  • 0
    Avatar
    Bill Tschumy

    We will look into this.  We get our orbit data from slightly different sources in SS6 as compared to SS5.  It looks like this is coming from JPL (Jet Propulsion Lab) in SS6 and the epoch of the orbit data for this object is 2011.  In SS5 we are getting a more modern epoch of 2018 (I believe this is from Lowell Observatory).

    Not clear why JPL is used an old epoch but that is the reason for the discrepancy.  Has anyone verified which is actually correct by observation?

  • 0
    Avatar
    Dean

    Bill Tschumy wrote:

    "Not clear why JPL is used an old epoch but that is the reason for the discrepancy.  Has anyone verified which is actually correct by observation?"

     

    Sorry Bill, I'm an armchair astronomer here, so I can't verify.

     

    I guess I've never completely understood the epoch thing... I'm assuming that's the Precession setting in SkySafari. (?)

    In SkySafari 6, if I manually set the Precession epoch to 2011 (to match the JPL orbit data epoch), and then toggle back and forth between 2011 and "Current Epoch" (2018.3), the shift in the RA/Dec coordinate grid measures approximately 5' 16" and is not large enough to account for the position change of 74P/Smirnova-Chernykh (approximately 1º 20' 2.2" at the time of M66 transit shown in my screen captures earlier).

    I would have assumed that if I set the epoch to 2011 and the comet's orbit path coordinates were created using epoch 2011, then the RA/Dec coordinates shown in the comet's info page should closely match those of M66 (when measured using epoch 2018) during the transit. But they don't. Which makes me wonder if there's not a different explanation for the discrepancy.

    I probably just don't understand how the epoch setting is being used, but are you sure that the 2011 epoch is the reason for such a large deviation?

    (I'm making the assumption that SkySafari 5 is accurate and the transit will actually occur. I've only been able to find one mention of the transit online: https://www.ast.cam.ac.uk/~jds/ under the heading "Highlights and News".)

  • 0
    Avatar
    Bill Tschumy

    No, in this case "epoch" has nothing to do with precession.  It is basically the time for which the orbital elements are valid for.  Orbital elements change over time due to perturbations of other solar system bodies.  In general you want to find the elements with an epoch close to the time you are interested in.  If you select the comet, show the Object Info and scroll to the bottom, you can see the epoch of the elements we are using.

    I would assume that SS5's prediction is more accurate, but I wouldn't swear to it.  There may be a good reason the JPL is still using the older epoch.

    In general we have been very happy with our change to using JPL in SS6.  I think this is an isolated quirk and shouldn't cause you to prefer SS5 in general.

    Is this comet in the news right now?  It seems pretty dim for there to be much interest in it

Please sign in to leave a comment.