KStars gravitational lensing

[Lorenzo]

Dear all,

Thank you for your swift reply! Following Giovanni’s suggestion, I was 
able to see the shift of nearby stars caused by passage of the Sun. As 
noted by Giovanni, the deflection is quite small (~1.7’’, matching 
Eddington’s measurement), so it is difficult to appreciate it, but 
zooming in makes it clear.

Apparently there is this extra factor of 2, so I guess the formula 
implemented in the code is 8GM/Rc^2, while the correct one is 4GM/Rc^2 
(and the (wrong) Netwonian calculation gives 2GM/Rc^2).

I truly appreciate your help—thank you all again!

Best,

LB


On 14-02-2025 10:33, Giovanni Ceribella wrote:
> Dear Akarsh, dear Jasem,
> 
> As a simple user, I can add that the feature still exists and works in 
> my installation KStars 3.7.5. The corresponding option is in the 
> settings window, under the "advanced" menu and in the "general" tab 
> (just below the "Correct for atmospheric refraction" option). Turning it 
> on visibly applies a radial distortion to the stars next to the Sun, 
> which is the expected effect.
> 
> However, If I take the star HD207520, which Kstars shows as grazing the 
> limb of the sun at 7UT of today, its distance (as measured by Kstars) 
> from the center of the sun is 15'54'' without the GR effects and 15'58'' 
> with it. But:
> 
>     In [1]: import astropy.units as u
>     In [2]: from astropy.constants import M_sun,c,G
> 
>     In [3]: impact_parameter = (16. /60 /180*np.pi) * 1 * u.AU
>     In [4]: impact_parameter = impact_parameter.to(u.m)
> 
>     In [5]: schwarzschild = 2*G*M_sun/c**2
> 
>     In [6]: angular_delta = 2*schwarzschild/impact_parameter
> 
>     In [7]: angular_delta.value
>     Out[7]: 8.483180496594135e-06
> 
>     In [8]: angular_delta.value*180/np.pi*3600
>     Out[8]: 1.749781581489136
> 
> So the deflection as measured by Kstars is ~twice as large than the 
> actual one. I could not check if this is an issue of the distance 
> measuring algorithm and the presented view is correct, or if the view 
> itself is off by that amount.
> 
> Best regards,
> 
> Giovanni
> 
> Il 14/02/25 07:07, Akarsh Simha ha scritto:
>> I implemented this feature, but I did not verify it, nor is it 
>> maintained properly at present. The main source of computational 
>> errors isn't so much calculating the lensing itself (the formula is 
>> fairly straightforward), but in ensuring we have the correct position 
>> of the sun, if I recall correctly. I wouldn't have the time to look 
>> into it, but I would assume that it is unmaintained and therefore not 
>> functioning as intended.
>>
>> This correction is explained in the Explanatory Supplement to the 
>> Astronomical Almanac, and one could take up re-implementing it 
>> according to that book.
>>
>> Regards
>> Akarsh
>>
>>
>> On Thu, Feb 13, 2025 at 8:39 PM Jasem Mutlaq <mutlaqja at ikarustech.com> 
>> wrote:
>>
>>     Hello everyone,
>>
>>     I'm forwarding an email I received from Dr. Lorenzo regarding
>>     the "Gravitational lensing effects around the Sun" feature in
>>     KStars. I actually never used this at all, anyone has any ideas?
>>     --
>>     Best Regards,
>>     Jasem Mutlaq
>>
>>
>>
>>     ---------- Forwarded message ---------
>>     From: *Lorenzo* <bzzlnz at unife.it>
>>     Date: Fri, Feb 14, 2025 at 12:24 AM
>>     Subject: KStars gravitational lensing
>>     To: <mutlaqja at ikarustech.com>
>>
>>
>>     Dear Jasem,
>>
>>     I am Lorenzo, a post-doc in astrophysics at University of Ferrara,
>>     Italy. I'm preparing a lecture for high school students on
>>     gravitational
>>     lensing, and I just found out KStars. In particular, the website
>>     mentions: "Switch on and switch off gravitational lensing effects
>>     around
>>     the sun to understand Eddington’s experiment". I am trying to
>>     visualise
>>     the deflection of starlight caused by the Sun on May 29, 1919, from
>>     Principe Island (i.e., Eddington’s experiment), however, I am
>>     unable to
>>     observe the expected deflection of nearby stars. Could you please
>>     clarify if this feature is functioning as intended, or if there are
>>     specific settings I need to adjust? I've already downloaded some
>>     extra
>>     stars catalogues. Do you confirm that I just have to press the "R"
>>     key
>>     to activate the lensing effects?
>>
>>     Thank you for your time and assistance.
>>
>>     Best,
>>
>>     Lorenzo
>>