What is the significance of hour angle in astronomical observations?

[randa177]

I have a question that I've been thinking about since February! It might be stupid but for some reason I am not getting it!

So back in February, I was observing with the 4m telescope in Chile, and each night the night assistant at the telescope would say: you reached the hour angle of (...) [I forgot exactly what it was], so you can't observe the object anymore.

I know that of course the objects appear to be moving in the sky, and at a certain time we can't see it anymore. I also know that the hour angle and the ascension add to get the sidereal time... and that is what confuses me,,, why do we need to say: at this hour angle we can't observe anymore, instead of just saying at this time we can't observe this object anymore... what new information does the hour angle tells us? Because I assume that the maximum hour angle we can reach every night is different because the time would be different, so why do we use the HA instead of the actual time?

 

[cepheid]

The hour angle of an object is the difference between the right ascension of your meridian and the right ascension of the object. (Your meridian is line that runs from due north to due south, and passes through the zenith, the the point directly overhead).

So, the hour angle of an object tells you how many hours it will be before that object crosses your meridian (or how many hours it has been since it crossed your meridian, depending on the sign). That's why it is useful.

EDIT: The right ascension of your meridian happens to be called your "Local Sidereal Time."

 

[phyzguy]

I Because I assume that the maximum hour angle we can reach every night is different because the time would be different, so why do we use the HA instead of the actual time?

This is where you are wrong. The hour angle is the angle between the meridian and the object, so the maximum hour angle you can reach (which is determined by the telescope design, how unobstructed is the horizon, etc.) would always be the same. The RA of the object at that maximum hour angle would change depending on the time and date, but the maximum HA remains the same. This is why people use it.

 

[randa177]

The RA of the object at that maximum hour angle would change depending on the time and date, but the maximum HA remains the same.

But isn't the RA of the object fixed (at least for a certain epoch (i.e: J2000))? One can get the RA from the catalogs (i.e: SIMBAD) so how can it change?

 

[phyzguy]

The RA of a given object is fixed, but the RA of the object at a given hour angle is constantly changing. Think of looking a particular place in the sky, say the meridian (due south for those of us in the northern hemisphere). This line is at a fixed hour angle, but the objects sitting on that line constantly change as the Earth rotates. So the hour angle at the meridian is fixed (0 hours in the case of the meridian), but the RA of the objects which are crossing the meridian are different depending on the time of day and the date.

 

[randa177]

Thank you all for the explanation :) It makes sense now!