Hour Angle

[Philosophaie]

What is the Hour Angle? I found that the formula is:

HA = LST - RA

How does this relate to the RA dial in a Equatorial Mount?

[Astro_Baby]

Basically the hour angle - ie how many hours have elapsed since Polaris transited. You can use that as an easier way to set up the polar alignment. It will give HOur Angle and Clock Position.

Clock Position is where on a clock face Polaris would appear if it was the hour hand of the clock with relation to the polar circle in the polarscope (ie if the clock position was 18:00 then polaris would be at the base of the polar circle.

Hourangle is different. Lets say that the HA is 2 hours. That tells you its been two hours since Polaris was in transit. So set your mounts RA until the POlaris marker is at the bottom most position (ie 6 o clock). Now unlock the RA setting circle and rotate the mount through its RA motion until the RA circle shows 2 hours forwards. Lock the RA motion - Voila wherevere the POlaris target marker is in the polarscope is where Polaris should be.

Hope thats some help.

[Merlin66]

A-B, I think the question may have been more general than polar alignment.

Philosophaie has asked similar questions over the last week or so...

Here we go...

The Local Siderial Time (LST) represents the RA of the meridian (the imaginary line going north south through your location). ie if the LST was 10h 30min, then a star with an RA position of 10h 30min would be on your meridian. If a star had an RA of 8h 30min, then it would lie (10h 30m - 8h 30m) = 2h away from the meridian. This difference is called the hour angle, HA. It is obvious that HA becomes 0h at the meridian and 6h at the horizon.

So:

HA = LST - RA