Confused about Gamma Draconis

[Grogfish]

So, I was lucky enough to go the Royal Observatory in Greenwich today. Very enjoyable and interesting place to go, but I was very confused about a vertically mounted telescope (literally straight up) which sadly I didn't take a picture of, but I did take a snap of the label which I've attached, key bit as follows:

"Located almost directly above, the star Gamma Draconis can be observed clearly with minimal distortion (refraction) caused by the Earth's atmosphere. Astronomers used observations of this star to correct for other stars lower in the sky whose positions appeared to change in the distorted light."

 

And, looking further, Wikipedia says that:

"It is by far the brightest star having a zenith above a point near London which led to its vaunting in these places as the "zenith star"."

Now, I am very much the amateur, so I'm sure it's me, but... :

1. Gamma Draconis is sometimes near "almost directly above" as stated at Greenwich, but it moves around the sky - it's not polaris afterall?? Presumably there is a critical time this is true, which is very useful for some reason??
2. What would a refraction correction even look like? That sounds like a vanishingly small impact to any measured position, so why was it necessary? 
3. The Wikipedia statement is bizarre and again seems to be missing out a key bit of information - surely every star has a zenith, whether you're observing from London or anywhere else? 

Is anyone able to explain this all to me in ever-so-simple language? Thanks!

 

[vlaiv]

At any given place of the earth - zenith plots "a circle" on celestial sphere as earth rotates in 24 hours.

Depending on how precisely you define zenith - there will be limited "supply" of stars of sufficient brightness that lie on that circle.

You need to be able to identify your star as it drifts thru field of view of stationary telescope - for that reason it is good for it to be fairly bright.

We know that there is no shift in position for stars that are directly at zenith.

You can then take two measurements:

1. Position of some star when our reference star is at zenith

2. Position of same star when our reference star is somewhere else, together with position or reference star at that moment.

In first measurement - reference star will be at exact point, and other star will be shifted due to refraction of the atmosphere

In second measurement - both will be shifted due to refraction.

You can calculate their angular distance (subtract their positions in spherical coordinate system).

With many such measurements you can plot curve of their distances vs reference star position - which you can then use to derive shift based on altitude of the star.

Fact that you have one point where you know that curve of distances is anchored helps you get absolute values rather than just shape of the curve.

By the way - apparent position can be quite a bit different than actual position:

 

 

[vlaiv]

I just thought of even simpler way to calculate atmospheric refraction effects

You don't need two stars at all - you just need one bright star that passes thru zenith and good stopwatch

We know sidereal rate quite precisely. We can measure where such star sets at horizon. From those two points we can calculate angular length of arc that star covers and then we can divide it with 1/4 of sidereal day to get "speed" at which star is supposed to move across the sky.

When star is at zenith with start our stopwatch and then take that star position measurement in regular time intervals. It motion should be uniform, but it will in fact "slow" as it is approaching horizon. If we plot this curve - we will get actual / measured position graph that we can use to derive how apparent position varies with altitude.

[rojay]

Once you have the curve that vlaiv described, you can use it to convert your measured altitude angle of a star into its true altitude, and then work out the RA and declination. This would be part of the process for compiling accurate catalogues of star positions.

I guess the curve would be useful for navigators too, making observations of the sun or stars with a sextant.

[beka]

On 30/04/2022 at 21:52, Grogfish said:

What would a refraction correction even look like? That sounds like a vanishingly small impact to any measured position, so why was it necessary? 
 

Atmospheric refraction can actually significant and observable. For example it can be several minutes between the times the Sun actually rises or sets and the time we see it doing so. The effect of atmospheric refraction is greatest near the horizon because we see objects through a greater length of atmospheric thickness as compared to strait up to the Zenith.

Best