Declination and Right Ascension what do they Mean?
March 15th 2011 06:03
Category: Cosmic Objects
Celestial Sphere
Finding your way in the night sky can be a little confusing especially if your just starting out.
Where should I look and what I’m I looking for?
Not knowing where and what your looking for can get frustrating, especially where constellations are concerned. If you manage to stumble on a constellation does it look any like what’s on the star atlas.
When I started to get interested in astronomy I was at age 10, I did not have a clue where the constellations I had been reading about where located. The other problem was my star atlas was for Northern hemisphere objects consequently some of the constellations I could see were upside down in the Southern hemisphere.
Often when reading articles on astronomy they refer to using termes like Declination and Right Ascension.
I did manage to get hold of a star atlas but that was not much help. The star atlas had terms like declination and right ascension,( say what?)
These terms are what astronomers use to define the location and direction of an object in the sky.
It seems odd that the positions of stars located light-years away in the depths of space are incorporated in a system that's tied to latitude and longitude here on Earth.
CURRENT MOON
Astronomers in the middle ages supported the long accepted view of the ancients. They believed the Earth was motionless and at the center of creation. The sky, they thought, was exactly what it looks like: a hollow hemisphere arching over the Earth like a great transparent dome and the stars were pinholes in the curtain of the dark.
The Earth is at the center of the celestial sphere, an imaginary surface on which the planets, stars, and nebulae seem to be stamped.
On the celestial sphere, lines of right ascension and declination are similar to longitude and latitude lines used on Earth. When a telescope's right-ascension axis is lined up with the Earth's axis, as shown here, the telescope can turn on it to follow the rotating sky.
Whenever you want to specify a point on the surface of a say a world globe, you'll probably using spherical coordinates. As far as the Earth is concerned these lines are referred to as latitude and longitude.
If take the lines of latitude and longitude and imagine that you could stretch them upwards and outward from the Earth and attach them on the inside of the sky sphere, you end up having the lines are now called declination and right ascension.
Right Ascension R.A. is measured in hours, minutes, and seconds. This is because as the Earth rotates, we see different parts of the sky throughout the night.
Declination is measured in degrees, arcminutes, and arcseconds. There are 60 arcmin in a degree, and 60 arcsec in an arcmin.
Decination tells you how high overhead your object eventually will rise. So your object at 48 degrees declination would pass directly over a point on the Earth at 48 degrees north latitude each night. If you were standing at, say, 38 degrees north latitude, the object would reach its greatest elevation (height) 48-38=10 degrees north on the sky from overhead.
If you were standing on Earth's equator, 0° latitude, is the celestial equator, 0° declination. If you stand on the Earth's equator, the celestial equator passes overhead. If you were to stand on the North Pole, latitude 90° N, and overhead will be the north celestial pole, declination 90°.
Lets choose another latitude Fargo North Dakota 46° N the corresponding declination line crosses your zenith (In general terms, the zenith is the direction pointing directly "above" a particular location) in this case declination 96°. (declinations north and south of the equator are called and rather than North and South.)
According to my Star Atlas the Orion constellation is RA range 04h 30m to 07h 30m, Dec range –15° to 15°
Chart centre: RA 06h 00m, Dec 0°
My location in on the Gold coast of Queensland The Orion constellation will be visible at 7:22 pm looking towards the North west at an altitude of 50 degrees Orion will be in that part of the sky relative to my location at the approximately the same time every evening for the next few weeks.
There are some very good programs that will show where the R.A and Dec of the objects you wish to see. I have included a link below to Planetarium Software you may find helpful.
*Click Here to Open New window* *Sky Chart Planetarium Software *
Working out the declination and right ascension of an object can be time consuming but I hope you have an understanding of what these terms mean.
Computer programs like Starry Night make the calculations a breeze.
My telescope has a GO TO provision for inputting declination and right ascension on a key pad and the telescope moves to the relative position of the object I wish to observe.
If I spot an interesting object I can then give my declination and right ascension co-ordinates to other observers in order for them locate the object.
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I hope you found it of some interest, I try to keep the technical stuff to a minimum it even confuses me at times.