I'm not absolutely sure if it is correct, but I use information from this site: http://star-www.st-and.ac.uk/~fv/webnotes/chapt9a.htm
And I convert equatorial coordinates (RA = α, Dec = δ) to ecliptic coordinates (λ, β) using this formula.
sin(β) = sin(δ) cos(ε) - cos(δ) sin(ε) sin(α)
cos(λ) cos(β) = cos(α) cos(δ)
ε = Axial tilt (23,5°)
I know that due to precession only λ changes (β remains same for ecliptic coordinates). So I add the angle of precession to the λ. Then I convert the new λ and β back to the equatorial coordinates.
sin(δ) = sin(β) cos(ε) + cos(β) sin(ε) sin(λ)
cos(λ) cos(β) = cos(α) cos(δ)
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