# Calculating apparent positions of sun and moon

I would like to calculate with high-accuracy the apparent geocentric positions of this three: Spica, the sun and moon. I will apply the following corrections for Spica:

1. proper motion
2. precession
3. rotation of ecliptic
4. stellar aberration
5. annual parallax
6. gravitational deflection
7. nutation

My questions:

1. Is there any particular order in which these corrections must be done?
2. Do I correct for gravitational deflection for sun and moon too?
3. Do I correct for light-time movement and aberration for moon?

Thank you.

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tony873004
Gold Member
my guesses:

1. No
3. how high is high-accuracy? It's not too hard to test the speed of light by comparing over the course of a few months the positions of Jupiter's moons (or any planets' moons). But if you simply want to know where to point your telescope to observe Spica, the Sun, or the Moon, you can comfortably ignore the speed of light.

Hello and thanks for replying. I need geocentric apparent positions of the sun, moon and planets to the nearest tenth of an arc-second. So please indicate which corrections of the seven listed I can drop. Thank you.

SpaceTiger
Staff Emeritus
Gold Member
Gravitational deflection will be negligible unless they lie on top one another. The others will depend on what data you're working from and how far in advance you need them calculated.

SpaceTiger said:
Gravitational deflection will be negligible unless they lie on top one another. The others will depend on what data you're working from and how far in advance you need them calculated.
I need geocentric positions for Spica, sun and moon from 1800 to 2200 (or so) to the nearest tenth of an arc-second. "What data I am working from" -- I do not understand this.

SpaceTiger
Staff Emeritus
Gold Member
"What data I am working from" -- I do not understand this.
Stellar aberration, proper motion, parallax, and gravitational deflection are all effects that appear in data, so in order for them to be important you have to be working from something. If, instead, you want to calculate the positions theoretically or with empirical fits (given certain initial conditions), then you only have to make sure you're considering precession, nutation, and other physical changes in the coordinate system and planetary orbits.

SpaceTiger said:
If, instead, you want to calculate the positions theoretically
For the sun and moon I will be using the JPL and VSOP/ELP (the latter for the users who do not have the JPL files). For Spica of course I have ICRS coordinates and proper motion from Simbad.

Now I am wondering which of these various corrections can be neglected because I do not require a precision less than 0.1"... That is what I am asking the guru-s here.

OK let me try something here:

1. proper motion -- in the period 1800 to 2200 (400 years) the proper motion changes the equatorial coordinates by at least 12 arcseconds so I have to do this corrections.
2. precession -- certainly I have to calculate
3. change of longitude due to rotation of ecliptic -- I have to do some calculations -- since Spica lies pretty close to the ecliptic, I wonder if the 47" /cy (=188" in 4 cys) change creates a significant change in the ecliptic longitude
4. stellar aberration is of the order of 20" so I have to do this for Spica and the sun but do I have to do this for the moon?
5. annual parallax for Spica is 12.44 milliarcseconds. So this can be neglected.
6. nutation -- yes I need to calculate as it can be of the order of several arcseconds.

So are my above judgments correct?

Now how about gravitational deflection? If a star is right behind the sun, then how much would its light be deflected?