# Faster/more accurate ways to get Earth-Sun vectors in different reference frames than transforming get\_sun results **URL:** \ **Category:** Astropy\ **Tags:** question\ **Created:** [December 14, 2023, 1:34am UTC](https://community.openastronomy.org/t/faster-more-accurate-ways-to-get-earth-sun-vectors-in-different-reference-frames-than-transforming-get-sun-results/847 "2023-12-14T01:34:09Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![MichaelBonnet](https://dub1.discourse-cdn.com/flex005/user_avatar/community.openastronomy.org/michaelbonnet/32/484_2.png) [@MichaelBonnet](https://community.openastronomy.org/u/MichaelBonnet)\ **Post date:** [December 14, 2023, 1:34am UTC](https://community.openastronomy.org/t/faster-more-accurate-ways-to-get-earth-sun-vectors-in-different-reference-frames-than-transforming-get-sun-results/847/1 "2023-12-14T01:34:09Z")
I have a simple function for getting the Earth-Sun position vector in ICRS reference frame at a particular time: ```auto import datetime import astropy.units as u import numpy as np from astropy.coordinates import ICRS, get_sun from astropy.time import Time def get_earth_sun_position_vector_icrs_at_epoch(epoch: datetime.datetime) -> np.ndarray: earth_sun_gcrs = get_sun(Time(epoch)) earth_sun_icrs = earth_sun_gcrs.transform_to(ICRS) earth_sun_vector_icrs_at_epoch = earth_sun_icrs.cartesian.xyz.to(u.km).value return earth_sun_vector_icrs_at_epoch ``` The conversion from GCRS to ICRS is incredibly expensive, time-wise. 80,000 iterations took over ten minutes. Is there a faster way to get Earth-Sun vectors at given epochs in different reference frames than this way? Also - when providing only a Time to get\_sun, is the vector from the center of the Earth to the Sun, or from some other point to the Sun? ---
**Author:** ![ayshih](https://dub1.discourse-cdn.com/flex005/user_avatar/community.openastronomy.org/ayshih/32/84_2.png) [@ayshih](https://community.openastronomy.org/u/ayshih)\ **Post date:** [December 14, 2023, 4:03am UTC](https://community.openastronomy.org/t/faster-more-accurate-ways-to-get-earth-sun-vectors-in-different-reference-frames-than-transforming-get-sun-results/847/2 "2023-12-14T04:03:21Z")
First, there is a fundamental error in your code. `earth_sun_gcrs = get_sun(...)` is the location of Sun center in the `GCRS` frame, so `earth_sun_icrs = earth_sun_gcrs.transform_to(ICRS)` is the location of Sun center in the ICRS frame. That is, the vector you return is merely the vector from the origin of ICRS (the solar-system barycenter) to Sun center, with no connection to the Earth at all. Second, if what you want is the apparent Earth-Sun vector relative to ICRF axes, including the effects of stellar and planetary aberration, then all you need to do is to skip the transformation to ICRS. The origin of `GCRS` is Earth center, so `earth_sun_gcrs.cartesian` is the Earth-Sun vector relative to ICRF axes, including the above observer effects. Alternatively, if what you want is instead the true/geometric Earth-Sun vector relative to ICRF axes, you shouldn’t use `get_sun()` at all. Instead, you can call `get_body_barycentric()` to get the true location of a solar-system body in ICRS, e.g.: ```python from astropy.coordinates import get_body_barycentric epoch = Time(epoch) earth_icrs = get_body_barycentric('earth', epoch) sun_icrs = get_body_barycentric('sun', epoch) earth_sun_vector_icrs_at_epoch = (sun_icrs - earth_icrs).xyz.to_value(u.km) ``` Hope that helps! ---
**Author:** ![MichaelBonnet](https://dub1.discourse-cdn.com/flex005/user_avatar/community.openastronomy.org/michaelbonnet/32/484_2.png) [@MichaelBonnet](https://community.openastronomy.org/u/MichaelBonnet)\ **Post date:** [December 14, 2023, 5:36pm UTC](https://community.openastronomy.org/t/faster-more-accurate-ways-to-get-earth-sun-vectors-in-different-reference-frames-than-transforming-get-sun-results/847/3 "2023-12-14T17:36:47Z")
It does make sense, thank you. It does bring up related questions for my project though. You say ICRF axes use the solar system barycenter - it was my understanding that ICRS is closely analogous to MJ2000 axes/origin. Is that incorrect? And how might I do the same sort of thing, but getting the TEME earth-sun vector? ---
**Author:** ![ayshih](https://dub1.discourse-cdn.com/flex005/user_avatar/community.openastronomy.org/ayshih/32/84_2.png) [@ayshih](https://community.openastronomy.org/u/ayshih)\ **Post date:** [December 14, 2023, 6:07pm UTC](https://community.openastronomy.org/t/faster-more-accurate-ways-to-get-earth-sun-vectors-in-different-reference-frames-than-transforming-get-sun-results/847/4 "2023-12-14T18:07:17Z")
You can use ICRF axes with any origin. Setting aside some details: - ICRS = ICRF axes with the origin at the solar-system barycenter - GCRS = ICRF axes with the origin at Earth center - HCRS = ICRF axes with the origin at Sun center - etc. My understanding of MJ2000 is that it’d be very similar to GCRS. If you have the Sun’s location as a `SkyCoord`, you can transform to Astropy’s `TEME`, which has the origin at Earth center, so the vector will be the Earth-Sun. I’ll caution that there are multiple definitions of TEME, so it’d be prudent for you to confirm whether Astropy’s definition of TEME is what you need. ---
**Author:** ![MichaelBonnet](https://dub1.discourse-cdn.com/flex005/user_avatar/community.openastronomy.org/michaelbonnet/32/484_2.png) [@MichaelBonnet](https://community.openastronomy.org/u/MichaelBonnet)\ **Post date:** [December 14, 2023, 10:34pm UTC](https://community.openastronomy.org/t/faster-more-accurate-ways-to-get-earth-sun-vectors-in-different-reference-frames-than-transforming-get-sun-results/847/5 "2023-12-14T22:34:16Z")
This has helped clear up a lot. The code you have for getting the true/geometric Earth-Sun vector relative to ICRF axes - how might I modify it to use GCRS, to be Earth-centered? Like this?: ```auto from astropy.coordinates import get_sun epoch = Time(epoch) earth_sun_gcrs = get_sun(epoch) earth_sun_vector_gcrs = earth_sun_gcrs.cartesian ``` ---
**Author:** ![ayshih](https://dub1.discourse-cdn.com/flex005/user_avatar/community.openastronomy.org/ayshih/32/84_2.png) [@ayshih](https://community.openastronomy.org/u/ayshih)\ **Post date:** [December 21, 2023, 3:02pm UTC](https://community.openastronomy.org/t/faster-more-accurate-ways-to-get-earth-sun-vectors-in-different-reference-frames-than-transforming-get-sun-results/847/6 "2023-12-21T15:02:00Z")
Again, if what you want is the true/geometric Earth-Sun vector relative to ICRF axes, you shouldn’t use `get_sun()` at all. Note that as the difference of two positions, this vector is the same irrespective of the notion of the coordinate-frame origin, e.g., SSB for ICRS or Earth center for GCRS. However, if what you want is the Earth-Sun vector relative to ICRF axes, but including the effects of stellar and planetary aberration for an observer at Earth center, then yes, your new code block gives you that.