A little bit puzzled about RA DEC and planet coordinates

[adder001]

Hi Guys,

First time here and a new telescope owner. I have always been into astronomy and finally I have purchased an Orion Starmax 90mm Mak-Cass and I have mounted it on an EQ2 mount.

So far I am really impressed.

After a steep learning curve about the EQ mount and polar alignment and RA and Dec which I think I have finally mastered there is just one thing that has been puzzling me that I just cant seem to find the answer to.

It is the coordinates of things like planets, moons, sun etc in respect to RA and DEC.

I understand that the RA and DEC of a star can be fixed with respect to Polaris but how can a planet or some other object that is moving have a coordinate of RA and DEC, surely it has to have a time of day/night element to it as well as its RA and DEC position changes with time, which makes me think that really you cant just use RA and DEC on its own to pinpoint a planet.

Many thanks in advance for any replies 

[Davey-T]

Planets do indeed move faster than the stars in RA at least which is why mounts have seperate planetary, lunar and solar tracking rates.

Dave

[Owmuchonomy]

As Dave says. The main solar system objects move along the ecliptic at various rates and in some cases for superior planets will show a period of retrograde motion. A tracking mount can adjust with this variation in RA as long as you select the correct rate. So your reasoning is sound. The Sun moves along the ecliptic roughly 4 minutes per day whilst the Moon races along at nearer 50 minutes per day.

[Bizibilder]

The co-ordinates given in an ephemeris for a particular day (for a planet) are usually their RA and Dec at midnight (start of the day in question).  However if you need greater accuracy then you need to look up their RA and Dec values for a specific time that you are interested in.  Most sky atlas software will do this for you (e.g. Cartes du Ceil and Stellarium).

[adder001]

That's great, thanks for the replies.

So it is as I suspected that the time is needed for the planets. At least I think I have a grasp of this now, it is all starting to make sense, trouble is every time I think I've understood one topic about 100 more new ones enter my head .

A lot of this is quite new to me such as the ecliptic, celestial equator, Celestial Pole etc. so I am learning as I go along as my interest in astronomy has just been knowing the constellations, planets etc. as a kid and other general knowledge that I have picked up over the years.

Davey-T Thanks for the info on the mounts having separate tracking for lunar and planets. I presume that mine does not have those. It is a 2nd hand Skywatcher EQ2 Mount.

But anyway at least that question that has been bugging me for a while now has been answered. I'm sure there will be plenty more as i continue learning.        

[Charic]

....... trouble is every time I think I've understood one topic about 100 more new ones enter my head  :smiley:ke sense, .

 I'm sure there will be plenty more as i continue learning.        

The more you ask, the more you learn, and  the easier  it is  to forget ? so keep asking!

I don't use the co-ordinates myself, but still find it  fun learning new aspects within the hobby.

[Tinker1947]

If you don't have Stellarium now is the time to download it.....

http://www.stellarium.org/

[astronymonkey]

Although the planets move a general ra an dec will get you on target for a given date. Its like gps when on a journey ie you know you are in a certain place for 30mins as you pass through.

Cheers

[ollypenrice]

Are you thinking of RA and Dec simply as equivalents of Longitude and Latitude? That's all they are. The key difference is that when we look at a globe we are looking at these imaginary lines from the outside while we envisage RA and Dec from the inside.

A ship sailing more or less (but not precisely) round the equator does what a planet does.

Olly

[adder001]

I have just got stellarium for my android tablet. It is pretty good. Cant get it to work on my pc which has Linux as the operating system which is a pity but it is good on the tablet.

Interesting when a planet is clicked on and you advance the time that the ra and dec do indeed change so brilliant. Total confirmation of what everyone has said.

[adder001]

To Olly, yeah thats pretty much how I thought of it which is why a moving object like a planet could not have a fixed ra dec value without knowing the time value also. With stars etc I see that j2000 is the value that is used so everything now makes sense