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SkySafari 5 Pro | Hour Angle Does Not Correct The Refraction

When enable the refraction in precession setting ,
The HA of Polaris in info does not correct the refraction

Skysafari 5 pro

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8 comments

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    Gleeman

    So does my Skysafari 4 plus of Android
    I guess all versions have the same behavior?
    That's a mistake(or bug),
    The hour angle (HA) in info of object should correct the refraction of air when you enable the refraction in precession setting.

  • 0
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    Bill Tschumy

    The hour angle is the difference in right ascension between the object's RA and the RA of the meridian.  When expressed in hours and minutes, the HA the time elapsed since the celestial body’s last transit of the observer’s meridian.

    This is not affected by refraction.

     

  • 0
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    Gleeman

    Hi Bill, thank you for your explanation,but i don't get it,
    Why does the refraction not affect the hour angle of the object , when the object has different refraction from NCP?

    i'm not the expert, but I just saw the article below:
    Title: The Correction of Declination and Hour Angle for Atmospheric Refraction
    Authors: Levin, A. E.
    Journal: Memoirs of the British Astronomical Association, vol. 24, pp.18-28
    http://adsabs.harvard.edu/full/1924MmBAA..24...18L

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    Bill Tschumy

    Perhaps you are confusing the "Hour Angle"  with the "Position Angle" (I occasionally confuse them as well).   The latter would be the angle of an object around a clock face where the NCP is the center of the clock.  I agree that that would change with refraction.

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    Gleeman

    Hi Bill

    I believe that I'm not confusing the HA (https://en.wikipedia.org/wiki/Hour_angle) with the PA

    as far as i know, the polar scope of equatorial mount is a good  example of concept for using HA to find the polaris

    the attached image is a ios app called "Polar Scope Align Pro"

    regards,

     

  • 0
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    Bill Tschumy

    That very Wikipedia article makes it clear that refraction can't come into play here.

    ================================================================

    The local hour angle (LHA) of an object in the observer's sky is

    {\text{LHA}}_{{{\text{object}}}}={{\text{LST}}}-\alpha _{{{\text{object}}}} (If result is negative, add 360 degrees. If result is greater than 360, subtract 360 degrees.)
    or
    {\text{LHA}}_{{{\text{object}}}}={{\text{GST}}}-\lambda _{{{\text{observer}}}}-\alpha _{{{\text{object}}}}

    where LHAobject is the local hour angle of the object, LST is the local sidereal time, \alpha _{{{\text{object}}}} is the object's right ascension, GST is Greenwich sidereal time and \lambda _{{{\text{observer}}}} is the observer's longitude (positive west from the prime meridian).[3] These angles can be measured in time (24 hours to a circle) or in degrees (360 degrees to a circle)— one or the other, not both.

    Negative hour angles indicate the time until the next transit across the meridian; an hour angle of zero means the object is on the meridian.

    ================================================================

    So the HA of an object is the Local Sidereal Time (RA of the meridian) - the RA of the object.  The right ascension is *never* affected by refraction.

    In the case of Polaris and the north celestial pole, the PA and HA are somewhat conflated.  I do agree that where Polaris is placed in the Polar Scope Align app is dependent on refraction.

  • 0
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    Gleeman

    Hi Bill
    from my understanding，
    the definitions of "hour angle" is “the angular distance along the celestial equator from the observer's meridian to the hour circle of a given celestial body”

    the "LST- Object RA" equation holds only when there's no correction required.

    Regards.

  • 0
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    Bill Tschumy

    I'm sorry, I didn't have time to argue this point any longer.  We stand by our belief that HA is independent of refraction.

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