Right Acsension/Declination

[Dom1961]

I know EQ mounts use RA/Dec but could someone explain to me how to measure these when looking at the sky or using an app such as star chart for android. Where do you measure RA from ? And when measuring Dec is it from the earths equator? Im in the northern hemisphere does this make any difference?

[pipnina]

You shouldn't look for objects based on those numbers, most EQ mounts don't have setting circles accurate enough and the R.A. axis moves throughout the night. This means that your mount needs to be motorised unless you want to re calibrate every time you want to find a new object.

Star charts do show R.A and Dec co-ords. Every line division on the page represents a number of degrees. (e.g. the one I use uses 10 degree steps) But again, don't use em!

Of course, if your mount is computerised (HEQ5 pro, for example), It probably handles that stuff by itself and it will probably be more than happy to go to given R.A./Dec co-ords if the date/time and location are set correctly and the mount is polar aligned.

Hope this helps!

        ~pip

[Dom1961]

If it is a manual eq do i just have to star hop?

[ronin]

RA is a measure of time, the value is the time relative to 00:00 at the Meridian when an object is overhead at the Meridian.

By this means you can "define" where objects are.

In an odd way.

What happens is that you need the RA of the object and the local time.

You really need to set a marker on the mount to the Local time then go to the RA value of the object.

Meaning that every so often you set/reset the "local time" or you have an RA circle that is slowly driven around at one revolution per 24 hours, actually a sidereal day, but keep it simple.

Dec is more simply horizontal or vertical but I equally suspect it is not as easy as that since all measurement are in polar coordinates and these differ from the "normal" up/down ones we think in.

The "mistake" is to think of RA as a coordinate in the normal sense.

It is a Time measurement.

Besides the first RA definition it can also be the Local Time on the line of Longitude when the object is on this line of longitude compared to 00:00 at Greenwich. Both are the same.

In the UK you are "lucky" as our local time is close to GMT, it in a way removes one item from the mess.

[D4N]

I wouldn't agree that RA isn't a coordinate, it is. Just on the celestial sphere not on Earth which is why you have to account for the rotation of the Earth by applying time to it. This is where you get your hour angles from, L(local)HA, G(Greenwich)HA and S(sidereal)HA. These are used to account for the fact that the earth keeps spinning.

Dec is also a coordinate, measured from the celestial equator to the celestial poles.

Dec is equivalent to latitude on Earth and RA is equivalent to Longitude. However RA is measured continuously rather than in East / West.

If you look at Polaris that is pretty close to 90 degrees North declination. Zero is where Orion's Belt is.

Alpheratz is pretty close to zero RA.

As previously stated though most settings circles seem to be mainly decorative and of little use for finding things.

You are better off learning to star hop or get a nice reflex finder and a star chart

Sent from my iPad using Tapatalk

[pipnina]

If it is a manual eq do i just have to star hop?

I have a manual EQ (unmotorised EQ2) and using setting circles is impossible! I've tried!