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Given a right ascension in degrees and declination in degrees (say, 88 degrees RA, -63 degrees DEC) I want to find the angle between that location and the ecliptic pole (90 degrees RA, -66 degrees DEC... are these the right values? What are the decimals following the 66? Most of my values that I'm working with are similar to the one I give, so which ecliptic pole, if any, does this correspond to?) How would I find this angle, I need a convenient formula (such as a dot product to find this angle). Thank you!

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  • $\begingroup$ The question about finding the angle between two celestial positions is a duplicate of other questions, such as How to calculate the angle formed between 2 planets? astronomy.stackexchange.com/questions/26625/… $\endgroup$
    – JohnHoltz
    Commented Apr 4, 2019 at 12:10

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The angle between an ecliptic pole and another sky position is complementary to the ecliptic latitude of that position. To find ecliptic (λ, β) coordinates, one method is to convert the equatorial (α, δ) to Cartesian form: $$ \begin{align} x_\mathrm{equ} &= \cos \alpha \cos \delta \\ y_\mathrm{equ} &= \sin \alpha \cos \delta \\ z_\mathrm{equ} &= \sin \delta \end{align}$$ then rotate by ε = 23.44° on the x axis: $$ \begin{align} x_\mathrm{ecl} &= x_\mathrm{equ} \\ y_\mathrm{ecl} &= y_\mathrm{equ} \cos \varepsilon + z_\mathrm{equ} \sin \varepsilon \\ z_\mathrm{ecl} &= z_\mathrm{equ} \cos \varepsilon - y_\mathrm{equ} \sin \varepsilon \end{align} $$ and convert to spherical form: $$ \begin{align} \lambda &= \mathtt{atan2}(y_\mathrm{ecl}, x_\mathrm{ecl}) \\ \beta &= \arcsin z_\mathrm{ecl} \end{align} $$

If you are not interested in the ecliptic longitude λ, you can express the ecliptic latitude as $$ \beta = \arcsin (\sin \delta \cos \varepsilon - \sin \alpha \cos \delta \sin \varepsilon) $$ Then the angle off the north ecliptic pole (α=18h0m, δ=+66.56°) is 90° - β.

Working your example (α=5h52m, δ=-63°), I get (λ=75.63°, β=-86.34°), and the angle off the south ecliptic pole (α=6h0m, δ=-66.56°) is β + 90° = 3.66°.

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  • $\begingroup$ +1 Though not necessary, it looks like you are already close to including a second answer using dot product as mentioned in the last line of the question. $\endgroup$
    – uhoh
    Commented Apr 5, 2019 at 9:50

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