New post
0

Accuracy of Polaris Declination value

I have found a really basic error in your program, assuming that I am doing my tests correctly. Here is something you can duplicate. Click on the viewing location, then click Other . . ., then click Latitude/Longitude. Set a longitude of 0 (zero degrees). Latitude is not particularly important, but you can set it at 50 degrees North. Then set the time for exactly midnight, July 4, 2011. Stop the time flow rate so that time is not changing. Then look at the declination value for the sun (I get 22d 50m 15s) and for Polaris (I get 89d 18m 52s). The published ephemeris values for this longitude and time are respectively 22d 55m 14.9s and 89d 18m 35s. Please let me know if my data makes any sense to you.

5 comments

  • 0
    Avatar
    Marko Kudjerski

    Hello Dennis,

    I've taken a look at the data you provided and I noticed that Starry Night is reporting the position of the Sun to be 22d 55m 5s which is close to the data you provided. Polaris, indeed is being reported at 89d 18m 52s.

    This is likely to do with different Julian Dates used to present the data. Note that the declinations I gave you are reported using JNow in Starry Night. The ephemeris data you obtained may be using JNow of when the data was published. Could you provide the source of your data?

    In addition, please take a look at this article: https://support.simulationcurriculum.com/entries/20723932-archeoastronomy-mathematical-accuracy-starry-night

  • 0
    Avatar
    Dwmorelli

    Marko - - Hi again. I have read the reference information that you suggested. What I am most interested in knowing is why the predicted declination for the sun and Polaris for the current date does not agree with the predictions published by the Naval Observatory. Is your data more accurate? I would like to know the reliability of your real-time predictions of the azimuth of the sun and Polaris. My concern is that if there is an error in the predicted declination, then there is also likely an error in the predicted azimuths. Starry Night displays the real-time azimuth of Polaris to 0.06 seconds. To me, this implies an accuracy of better than plus or minus 0.1 second. When Polaris is at transit, large errors in declination produce only very small errors in computed azimuth, but when the hour angle of Polaris is at 90 degrees or 270 degrees, an error of 15 seconds in declination produces an error of over 20 seconds in azimuth (when observed at a latitude of about 45 degrees). I hope that you will be able to untangle this for me.

    Dennis Morelli

  • 0
    Avatar
    Dwmorelli

    The source of my data can be found here:

    http://aa.usno.navy.mil/software/mica/micainfo.php

    Here is a table of abbreviated data for the sun and Polaris:

    http://www.cadastral.com/2012jan.htm

    These 2 sources seem to agree exactly, but the data does not agree with that shown on Starry Night. Please let me know your thoughts after reviewing data from the cited sources. Thanks.

    Dennis Morelli

  • 0
    Avatar
    Geoff Gaherty

    Hi Dennis,

    As Marko has pointed out, I think this is a question of the epoch for which the coordinates are computed. Starry Night calculates them for two epochs, "JNow" is for the current epoch; "J2000" is for the epoch 2000.0, which is used by most current star atlases and many reference books. It's a long time since I've used the Nautical Almanac, but I would expect that it is calculated for the current epoch, but if I recall correctly, that is indicatedon the title page or introductory notes.

    Geoff Gaherty

    Starry Night Software Support

  • 0
    Avatar
    Dwmorelli

    Geoff - - Thanks for your comments. I notice that, for Polaris, both declinations are shown, but neither one agrees with the Naval Observatory data. JNow is the closest with a difference of about 15 seconds, whereas J2000 is way off. You can verify this by looking at the data shown on the cadastral website link above. I am not questioning your methodology, only your results. I hope we can agree that the Naval Observatory is the ultimate authority. This is the data they expect Polaris observers to use to calculate azimuth at any hour angle. Irrespective of the epoch used, the bottom line is that Starry Night exhibits an error in the prediction of the real-time declination of Polaris. If this declination is used by Starry Night to predict the azimuth, then the indicated azimuth of Polaris is also in error. I suspect the same may be true of other stars. Geoff, I may be way off track here, and I would appreciate any additional light that you or others can shed on this issue.

    Dennis Morelli

Please sign in to leave a comment.