Right Ascension confusion.

[iainmax]

Hi guys,

I've now have my copy of the Backyard Astronomer, Turn Left at Orion and the Cambridge Start Atlas. I'm learning more and more each day.

I'm struggling with the concept of RA and DEC. I understand so far, how they are measured and can work my way through the charts in a very amateur way. What I can't seem to grasp is this:

Say I'm looking up at Ursa Major which is measured at 11h and 50 degress declination. How exactly does that relate to where I stand on our planet? Where does 0 hours RA begin? I think what I'm asking is if there is a point at which I could look up and have an immediate reference in which I can then star hop. So sorry if i've not been very clear. I know what I need explaining but it seems difficult to articulate.

I've googled my question and the books explain it (I just seem to have a brain fog preventing me from absorbing the information)

Thanks guys.

Iain 

[BritAngler]

From Wiki....

"Right ascension is measured from the vernal equinox or the first point of Aries, which is the place on the celestial sphere where the Sun crosses the celestial equator from south to north at the March equinox and is located in the constellation Pisces. Right ascension is measured continuously in a full circle from that equinox towards the east."

[NigeB]

A couple of additional things to keep in mind.

The First Point of Aries is effectively the celestial equivalent of the Greenwich Meridian - longitudes are measured with respect to Greenwich, Right Ascensions are measured with respect to The First Point of Aries. 

But because of Earth's rotation, the direction to the First Point of Aries (and all other celestial objects) changes over the course of the day/night. So you need an extra bit of information to be able to go from Right Ascension, to a knowledge of where to point your telescope. This tends to be one of the parts people sometimes struggle with. Think of it this way:

The Right Ascension of an object is equal to the amount of time that passes between the First Point of Aries crossing your meridian, and the object of interest crossing that same meridian (the meridian is the imaginary great circle line passing through the north celestial pole and due south for a given observer).

How do you know when the First Point of Aries transited your meridian? That's what "Local Sidereal Time" (LST) tells you. If LST=0h 0m 0s, then the First Point of Aries is transiting your meridian, and will be due south (if you're in the northern hemisphere). If the Local Sidereal Time is 5 h 30 m, then you know that the First Point of Aries transited 5 h 30 min ago, and the objects on your meridian now, have a Right Ascension of 5 h 30 min.

Hence armed with a knowledge of Local Sidereal Time, you know what Right Ascension is on your meridian (due south) at a particular instant. That allows you to determine the bearing to any other value of Right Ascension - because 24 hours of R.A. corresponds to 360 degrees. 

[faulksy]

this helped me loads, very well explained

http://www.rothervalleyoptics.co.uk/rvo-beginners-guide-to-equatorial-mounted-telescopes-dvd.html

[Phil Fargaze]

I`ve found that one way of getting familiar with RA and DEC from your point of view is to use  a planisphere. Whilst being refreshingly simple and no computer needed, they have the RA and DEC marked on them so you can see how an object`s position varies in the sky according to the date and time. Some of the smaller ones are not marked with RA and DEC but the Philip`s 10" or 11" definitely are. 

(because I have them both. In fact I have three of the things! They also make for a good alternative bedtime read!)

[Charic]

In the FREEWARE program Stellarium, you can display various grids, and there is an information menu  for every target you select. In combination with the books, the info menu and the grids, you should soon find your answer.

Stellarium is a remarkable and capable program, and can be set to replicate your own environment?

[iainmax]

Thank you guys.

I think I need to learn a little more about LST and also the vernal equinox and the first point of Aries. I've had a look further into it and I believe i'm getting a much better grasp of it. 

Thanks to NigeB also. I believe I was asking (in a muddled fashion) what the celestial equivalent of GMT is. I have my answer. 

A planisphere seems to be a tool that would come in handy and is on it's way to me. My equipment setup is getting bigger each week. I can't wait to get the Rigel Quikfinder and RACI scope.

[D4N]

The one thing that is fairly stable is Polaris, this can make a good starting point as it will be in the same place every night. Polaris is at 90 N declination and will be roughly as high in the sky as your latitude.

Sent from my iPad using Tapatalk

[Dave In Vermont]

I see that Stellarium has been mentioned. So I'll take it home!

Stellarium is a software-program that you set for your own location - for which you need to know your latitude and longitude - IF your city isn't listed in it's program. It is officially classed as a 'Planetarium' program as it will show you views similar to a brick & mortar planetarium: A spherical representation of the sky as it appears when you're outside looking up and around. I personally refer to programs like this as a 'Star-Charting' program. Name it "Rover." It's yours!

You can set it up to show you what you're interested in finding, or locating, or whatever you wish. It's a very large program that you can spend weeks setting-up as you discover more & more functions it has available. Or you can do a rough set-up in 10 minutes. It's up to you. Similar programs can easily run you £200, but Stellarium is completely FREE. Yes - you read that right. So.....

Without further ado - STELLARIUM:

http://www.stellarium.org/

As the developer's (one of them is a member of SGL) are always adding new things, and fixing minor problems, the current instructions are posted in a Wiki. Which is here:

http://www.stellarium.org/wiki/index.php/Stellarium_User_Guide

And a slightly outdated Pdf. is located - perfectly usable - here:

http://barry.sarcasmogerdes.com/stellarium/stellarium_user_guide-new.pdf

You can set it up to run a computerized telescope, print out star-charts, show you where and when and what satellites are visible, meteor-shower dates and radiants - everything but wash your dishes.

Here's what I was using it for tonight:

Have fun!

Dave