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HA and local Meridian

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I understood that the Hour Angle (HA) was the time since a star crosses the meridian. At the meridian, HA = 0.
However, yesterday, testing the meridian flip, I found I could be wrong.
I was tracking Sadr, waiting for the meridian flip. Before and after crossing the meridian Sadr shown HA > 0.



I would be very grateful if someone could enlighten me on this topic
3 years 5 months ago #61015
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Replied by Kevin on topic HA and local Meridian

I believe that the Flip if HA is after the meridian has been reached, not before. I've ben testing this out because I had problems with hitting the tripod legs. If you lower the number it will flip when it's closer (but after) it has reached the meridian. You can see this by looking at the flip in number of minutes. I don't think you can enter a negative number.
3 years 5 months ago #61017

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Replied by Joaquin on topic HA and local Meridian

Thanks Croz, but my question is not about the flip but about the values of Hour Angle in EKOS before and after the star pass the meridian (the blue line in Kstars.
I expected a negative value right of the meridian and a positive value left of the meridian. I see positive HA at both sides
But I am missing something...
3 years 5 months ago #61020

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Replied by Kevin on topic HA and local Meridian

Jabian, It's a good question. If you select a star on either side of the meridian, and select details it will be either + or - under position. Which is expected. Not sure why it reads positive on both sides? I guess we should just try and be positive :)
3 years 5 months ago #61022

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Replied by Joaquin on topic HA and local Meridian

Looking at Details, you can get positive HA both sides of the meridian

3 years 5 months ago #61024
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Replied by Doug S on topic HA and local Meridian

Not sure what kstars version you're running, but 3.5.0 (Beta) for Linux does not seem to exhibit any problem showing correct HA values in the position tab under details. I get positive values right of the meridian when facing south, and negative values left of the meridian when facing south. This is proper/expected behavior when using +/- 12 hour HA values (as opposed to 0-24 hour values). Cheers, Doug
3 years 5 months ago #61030

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Replied by Joaquin on topic HA and local Meridian

The first image is Stellarmate 1.5.4 stable, up to date with KStars 3.5.0 beta. Data from Telescope tab in EKOS
The second image is Win 10 with KStars 3.4.1. Data from star's Details in KStars

Just now I confirmed in the RasPi with KStars 3.5.0 beta that:
- n Cephei was crossing the meridian at HA about +15 s.
- HD199081 was crossing the meridian at HA about +31 s.
both positive.

Could this be related with hour angles given in J2000 ?
3 years 5 months ago #61033

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Replied by Ken Self on topic HA and local Meridian

HA = LST - RA
Normally we have RA fixed so HA is calculated by subtracting RA from Local Sidereal Time.
You can see that calculation is correct in the screen shots. So your Local Sidereal Time is incorrect. That could be due to either your computer clock or your longitude.
3 years 5 months ago #61036

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Replied by Joaquin on topic HA and local Meridian

Thanks Kengs,
In the second example I ran on the PC can not be the clock because I use this computer for digital radiocommunications and for this I have it synchronized with Internet Time Servers with-in a few milliseconds of the real time.
Then the only parameter left is longitude. I will ckeck it.
However, in the first example, with the Raspi I had a GPS device running. It was the only real device in the profile of simulators. So both time and longitude should have been correct...
3 years 5 months ago #61045

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Replied by alacant on topic HA and local Meridian


Hi
If your position and time time is set correctly and you have set the flip to happen 2º <em>after</em> the meridian, then that looks good.
In both your examples, Sadr had already crossed the meridian, but had not reached your 2º constraint.

If you want it to flip as soon as the HA comes zero, remove the constraint.

Before it crossed the meridian, it had a -.ve HA value.
HTH
Last edit: 3 years 5 months ago by alacant.
3 years 5 months ago #61049

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Replied by Joaquin on topic HA and local Meridian

Thanks Alacant,
Excuse my ignorance, I am not native English: what is "a -.ve HA value" ?
3 years 5 months ago #61055

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Replied by Jerry Black on topic HA and local Meridian

Before it crossed the meridian, it had a negative HA value
3 years 5 months ago #61056

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