Why ECEF does not coincide with ECI on January 1, 2000 at 12.00?

Question

I assumed that ECEF frame coincides with ECI frame on January 1, 2000 at 12.00.

To see that, on MATLAB, I used the dcmeci2ecef() function as:

%this function does not return identity matrix
dcmeci2ecef('IAU-2000/2006',[2000 1 1 12 0 0])

I expected that this function returns identity matrix. However the result is not close to the identiy.

After, by trial and error, I obtained the identity matrix on January 1, 2000 at 17.17 when I write:

%the function which returns identity matrix
dcmeci2ecef('IAU-2000/2006',[2000 1 1 17 17 17])

Can you explain the reason behind this?

I believe In which direction does the x axis of the ECI at 1.1.2000 at 12:00 (J2000) go and where is the 0 deg meridian? is a superset of this question, and the answer applies. The short answer is that the prime meridian, midnight UTC, January 1, and 2000AD were already defined, and the J2000 epoch is set for convenience for the frame definition, not to align the equinox direction and the prime meridian. — Erin Anne, Mar 20 '23 at 07:09
https://physics.stackexchange.com/questions/755685/why-ecef-does-not-coincide-with-eci-on-january-1-2000-at-12-00 — Starship - On Strike, Mar 20 '23 at 09:19
@Starshipisgoforlaunch you’ve linked to a deleted question, fyi — fyrepenguin, Mar 20 '23 at 13:20
I know, my point being does this belong on physics or here. @fyrepenguin — Starship - On Strike, Mar 20 '23 at 13:25
@Starshipisgoforlaunch It was closed on Physics for being an engineering question. I mentioned in a comment that it would be on-topic here, linking to the ECEF tag, hoping that the OP would search the existing questions before asking a new one... — PM 2Ring, Mar 20 '23 at 13:57
@Starship I don't like to suggest stacks that I'm not familiar with. I just did a quick search, and they only have 1 post that mentions ECEF, in passing. https://engineering.stackexchange.com/q/14822 — PM 2Ring, Mar 20 '23 at 14:53
Okay, I was just wonder as it was closed as an "engineering" question and so it seems like it automatically eblongs on "engineering" — Starship - On Strike, Mar 20 '23 at 14:54
The X axis point to lat/lon 0,0. Jan 1 2000 was just an arbitray choice for the celestial coodinate system, it's independent of the Earth's rotation. — Greg Miller, Mar 20 '23 at 16:17
@Starshipisgoforlaunch questions can be on topic on multiple sites at the same time, though they shouldn't be cross-posted even if they are valid, topically. — fyrepenguin, Mar 21 '23 at 02:27

Why ECEF does not coincide with ECI on January 1, 2000 at 12.00?

0 Answers0