Celestial Position Question

[tomato]

When an object reaches a position where it is moving parallel to the Altitude axis on the sky, so momentarily it is not changing it’s position in Azimuth, what is the specific astronomical description for this position?

[Mutley]

Do you mean when an object transits - crosses the meridian from east to west ?  But it would be moving parallel to azimuth, and not changing altitude . Is your post the wrong way round or am I confused?

[symmetal]

An object is moving parallel to the Alt axis when it performs a meridian transit but the azimuth position is always changing due to the Earth's rotation.

I'm not sure if you're perhaps referring to field rotation, which is maximum at the meridian and zero due East or West, where the direction of field rotation reverses.

Alan

[tomato]

I was imaging NGC 6946 recently which required me to set up the dome rotation rate to keep the scopes pointing through the aperture. I tracked its motion in Stellarium and noted that the galaxy had a retrograde motion initially until it was moving down vertically, ie no movement in Azimuth, then it started to move East to West. I know when an object is not moving in altitude this is called the meridian transit, but I wondered if the corresponding point where it  changes direction in Azimuth has a name.

[symmetal]

Ah! I'm with you now. You're imaging circumpolar objects towards the North and as the object passes through the line at 90 degrees to the meridian, which passes through the celestial poles and East and West, its azimuth movement direction will reverse.

I don't know if this line has a name but it indicates the maximum East or West deviation from the meridian the object will have.

Alan

Edited September 23, 2023 by symmetal

[tomato]

Yes, that’s it. It poses a bit of a challenge for an automated session, as I can only set the dome rotation in one direction. Currently I have to sit with it until the  object has passed through this point.