Newbie question about Right Ascension

[JHeels]

Thanks in advance for any help, and sorry if I am failing to get the obvious and if this is covered elsewhere!  

I'm struggling to grasp the application of Right Ascension for every-day use. I have a pretty good understanding of how it was arrived at as a coordinate system (and declination is pretty much a 1 for 1 with latitude on an earth map, so that part I understand). What I can't seem to find any information on, is how to actually use the coordinates in the 'here and now'.  For example, if I am in (for the sake of argument), Philadelphia on December 16th at 8:00 PM local time, how do I apply the known Right Ascesion, Declination of Vega to determine its location in the sky.   Do these coordinates basically supplement sky charts, or can they be used independent of other locational tools (and if so, how)?  Thanks!

[D4N]

RA is the longitude of the celestial sphere.

To calculate a position if a star manually you would normally use tables to find the Greenwich hour angle of Aries, apply your longitude and the sidereal hour angle of the star to get the local hour angle of the star. Using this and the declination you can then calculate the azimuth of the star for your position using either tables or trigonometry.

Then you can calculate the altitude of the star using the above info and cosine formula.

It's all basically spherical trigonometry but it's a lot easier to just use an app like sky safari

Sent from my iPad using Tapatalk

[Gazabone]

I'm looking forward to seeing replies to this thread as my knowledge is pretty limited, but I'm happy to get things started and wait for someone to shoot me down in flames;

First thing is that RA/Dec is static in space but the earth revolves within the theoretical celestial sphere. As far as I understand it, if you're looking for a structure that is not visible to the naked eye or star hopping is realistic or you just want to use coordinates, find something that you can see/recognise and using a star chart find a star on the same dec and preferably not too far away and then find out how far away the object is that you want then use RA to "swing across" to it. Provided you've polar aligned your mount well, if you get the dec correct, skewing your scope in RA means you can't miss your target, you just have to recognise it when you get there. Most people seem to think that setting circles aren't accurate enough to use but somebody earlier today on another post says he uses them, a lot depends on how big the circles are on your mount.

The most important thing if using coordinates is to polar align accurately (including using a polar finder scope).

I'll now sit back a wait to be shot down in flames, but that's how I understand it and hope it helps you.

[Dannae]

Hi JHeels,

You can indeed determine a star's location just using the RA and Dec independently but you would need to know where the celestial 'Greenwich Meridian' or Point Zero is at that time in order to account for RA. The Greenwich Meridian or Point Zero on the celestial sphere is known as the 'First point of Aries' though it's in neighbouring Pisces! also known as the Vernal Equinox. The right ascension of a star is the time it takes to culminate (be at its highest in the sky) after the culmination of the First Point of Aries.

Lets say you want to find Vega which has a RA of 18h 36mins. The first point in Aries isn't that easy to locate so you are better with another star that is visible and you know its RA, so using Deneb with an RA of 20h 41mins means that Vega culminates 2h 5mins before Deneb does. It will always be just over 30 degrees anticlockwise of Deneb (1hr of RA = 15 degrees). If your reference star has a lower RA than Vega it will culminate clockwise to the length of time dependent on the difference between the 2 star's RA.

You need to draw an imaginary line between your known star (Deneb) to the North star then another line that starts at the North star but at the angle of RA difference (in the Deneb example this is 30 degrees anticlockwise to the first line). Vega will be the star on that line.

Hope my maths is ok and my clock and anti-clock aren't mixed up but does this answer your question? Thank goodness for Skysafari

Regards

Dannae

[JHeels]

OK, I think I understand it better now (especially referencing other objects as part of the calculation).  I do appreciate the ease of computer assistance with this sort of thing, but I would prefer to first be able to perform location tasks 'old school' as part of the overall learning process.

Thanks for the replies!

[jnb]

I'll have a go at a coherent and useful explanation.

First thing to notice is that right ascension increases to the east, ie as you look at the sky something overhead has a lower right ascension than something just to the east of it.

The second thing to notice is that it is measured in hours and minutes.

The third thing to notice is that the sky rotates (yes I know it doesn't but it appears to) from east to west.

Now imagine that it is 22:00 and something with a right ascension of 22 hours is overhead. When will something just east of that with a right ascension of 23 hours be overhead? the answer is 23:00.

Using right ascension in hours and minutes is exactly the same as using longitude on the Earth but measuring in the time it takes the earth to turn.

Or looking at it another way. Imagine hovering in space looking at the Earth. You could use the Earth as a clock by replacing longitude on the Earth with the hours and minutes it takes to rotate. Then as the Earth rotates you can tell the time be reading the longitude below you. From the Earth exactly the same applies except we draw that reference frame onto star maps rather than the Earth and look up not down.

[Floater]

Well it's certainly helped me, Dannae. Thanks.

I've been struggling with the concept for some time. Like JHeels, I'm sorted on the what and why and drummed it into myself over many viewings of different tutorials. But I couldn't figure out what use it was in being able to 'just look up' and find the star at such-and-such a RA. And even with your explanation, I need to know the RA of another star and add/subtract before moving to the one I want to find.

As you wrote, thank goodness for Sky Safari.

Nevertheless, thanks.

p.s. Posted as you wrote, JNB. More help. Thanks to you, too.