There are less than a handful of hobbies I enjoy, Astronomy is one of this hobbies, I got my first real telescope in 1986, I was waiting with great expectations to be able to observe the return of Haley after a little more than 75 years since it's last visit, the telescope was a small refractive one not very powerful or practical by today's standards, but a good item.

    Human beings had been wondering for centuries about what is their place on the universe, are we alone here in this tiny blue planet? Or, are we just one in many, many more planets, currently populated with life, and intelligence?

    Not too long ago we were brave enough to even send our invitation and greetings to whoever could and wishes to answer it... Yeaph, the Voyager missions, most people do not know about it, others do not remember it anymore, regardless; sometimes I sit down thinking about human behavior toward others, the levels of tolerance between people from even different zip codes, religion, gender or race... I had experience on firsthand the "love shared" toward people from different nationalities, languages. What would be like for a visitor to stop by, and stay over for a little while?... maybe to fall in love, but learning the hard way about all those protocols you take for granted... no woman in this planet would understand his confusion... so, yes, try to imagine that.

       Oh and I almost forget again, the positions are expressed in degrees or time... like for example a day is 360 degrees, then of course this make an hour 15 degrees, similarly one minute is then 15 arco minutes  and a second 15 arco seconds. This may sound hard to remember, but there is no need for remember it, just imagine the transparent face of a clock in front of you and you can use it as a tool put it between you and the plane you are looking at or at the star if you want to measure it... and  there is the Radian, a radian is about 3.8 hours, and the other way around a degree is about 4 minutes, an arco minute is 4 seconds... Chances are we will not need more precision than that at less  we are trying to drop an asteroid into a target with the precision of a guided missile... if so we have a lot of love to chose from hehehe :)

      Anyhow, Just as I try to explain that cute woman at Austin Pea's Library, we have some very special ways to keep track of objects on the space. Many of these systems allow us to locate any given object, at any given time, past, present or future. And some other more tricky calculations allow inferring the effects of their proximity to other objects based on their mass, gravitational effects, trajectory, etc.

       The simple way to track an object on the sky; let's say a star, an asteroid, or a satellite is by assigning a coordinate system to it. The most common is by altitude and azimuth, this system is called horizon coordinate.

      This is how it works... lets say you are standing facing North, (this system uses the true north), the plane formed by the horizon can be marked starting on 0 for the north, and continue to your right (clock wise) all the way around until you find yourself back face to north. This are marked on degrees 0-360. East is at 90 degrees, South is at 180 degrees, West is at 270 degrees and back to 0 or 360 degrees when faced north again.

      Now, if you look directly above you, perpendicular(90 degrees straight up) to the horizon plane we drew in our minds, the point above you is called Zenith. and the direct opposite point in straight line under you feet is the Nadir. Well, we are almost set, draw a line from Nadir to Zenith, and keep yourself aware of the circumference in the horizon. and marked it the plane NESW.

      Now let's say  you want  to assign  the coordinate to a object located somewhere in the sky. First trace a line between the object and the closest point in the horizon, this point in the horizon is located at some angle from North counting clockwise, and is called Azimuth.

     While the drew from ground up on the horizon to the object is the altitude or ascension angle.

The coordinate looks like this: (13º 23" 34' , 154º 33" 20').

     We read it, Ascension thirteen hours, twenty three minutes thirty four seconds of arc, and one hundred fifty four degrees thirty three minutes twenty seconds azimuth.

Also we can express this coordinated in it's decimal equivalent.

     In order to do this just take the seconds and divide by 60, add this to the amount of minutes, and divide again by 60, add this to the amount of hours and we got:

(13.39278a , 154.55556A).

     Now, this is good to get a fix on most objects, but my coordinate need to be good to find the objects from other places on Earth. in order to do this we need to assign a date and time to the coordinate point.

     However if you spot something really interesting and you would like to share it with the rest of the world, it is better to used a more standardized system. The most widely used is the Equatorial system, which is very similar and use the same idea that the horizon coordinated.

     Let's go back into our minds for a moment and place ourselves back inside our coordinated system, we are once more facing north (0 degrees Azimuth Elevation 0). Now, accelerate time around us, look at the stars moving East to West fairly quick and disappearing in the horizon at our left... awesome isn't it!!!!

    If we draw a line from the Equator straight into the sky as the planet rotates, this line will travel across the sky tracing it's trajectory right in front of our very eyes. This circumference on the sky, is the Equator projected and will look tilted slightly... The angle of declination we observe is exactly 90º- Latitude, this is the latitude of our location expressed in degrees. For Example if I am in Latitude 47º, the circumference will start at Ascension 43º from the horizon.

    The circumference will go across our horizon plane east to west forming the Equatorial Plane.

    When Earth rotates, all the stars seems to follow a trajectory in the sky which is parallel with the Equatorial Plane; In order to take advantage of that and assign a coordinate system to the stars we use the natural yearly moving of the celestial bodies out there...The distance from the equatorial plane to the circumference traced by a moving object is called declination and is our first number in the coordinated system. The second it defines where in that traced circumference the object is located, We start then counting from East to West parallel to the equatorial plane, and our starting point is the Vernal Equinox. This is the very point where, the sun crosses the Equatorial plane. This is on March 21 each year. We assign this position the Ascension zero 0. We got ourselves a coordinate system, and the same rules apply, we use degrees or hours minutes and seconds to mark arcs and distances of any object. We can even use the Earth geographic coordinates in Latitude and Longitude to assist us in the calculation of a position of any object any time any where, and we can know where to look for it from any given place on earth.

     Hmmm... now as we continue looking within into our abstracted mental simulation of the Earth sky, let's make an about face and look northern ( I am in the northern Hemisphere, that make it easy), now we found ourselves facing south, the Equatorial plane is right in front of us. the line south-North go through us and cut the horizon, if we draw a line, from that point to the zenith and continue further all around 360 degrees, this new plane perpendicular to the Equatorial plane is the Meridian plane. and we can used it to measure the "speed" a star moves across the sky, which is a full cycle in 24 hours (really 24 sideral hours, but I may explain later that), any star moves across the meridian plane a full cycle of 360 degrees in those 24 hours, parallel to the Equatorial plane. The angle measured from the meridian plane westward is called Hour angle, and every revolution completed put the star back into 0 degrees, which astronomers said the star had culminated. The 360 degrees of the circumference traced by the meridian plane, is divided into two halves, and the northern portion is always positive, while the southern portion is negative. The coordinated are used as (a, H) basically the Azimuth on the Equatorial plane and the Hour angle value for the position of the object. So if you are fan of some observatories, when you look at their data, calling for some distant object located at such ascension and such degrees, or some Ascension value and some given hour angle, that is the same coordinated system. now you know :)

    To express it as hour angle, just divide it by 15... or multiply if you want the other way around...

    Hmmm... and we can also take advantage of living in a planet that is already mapped by grids, (latitude and longitude), by using your knowledge of the position of stars to help you, to locate your position in relation to those coordinates, just inverting the process... So next time you find yourself in some far away maybe in hostile land, and you had no idea where you are located... no GPS, no compass... no map... remember this, you are never alone nor are you disarmed. look around, all you need is two minutes a two times window within at least 1 hour, to look at the stars, and they do not have to be the same star as long as you remember your measurements... locate known constellations first, let's say Orion, follow see the angle of the stars, and locate the north, find Polaris for example... get the true north, now run your simulation in your mind again... look at the stars movement and locate the equatorial plane... now measure the ascension angle from the horizon to the equatorial plane... and get the different from 90 degrees... that difference is your current latitude, and is as accurate as your measures to the arco second... take a fix in the position of the stars once again and mark it with a object if you can not remember it in the fixed space, wait at least a fraction of time is easy for you to calculate or with and take a second measurement this time the hours angle of the star, will tell you the position of the star for a determinate time... remember the position of that star at 0 degrees (this is at the position of interception from equatorial plane and horizon plane, the vernal equinox remember), convert that moment from universal time to local time to your watch, by adding the time zone correction, this will give you your longitude coordinate. now you now exactly where you are...and can calculate the time correction... so if you make a point to remember a fix coordinated where friendly elements maybe located, you are in better standing now than you were before. Perhaps now we can look at the sky together, no matter if I look at you from my former home or if I see the sky from a different location here... we all can get amazed by all the wonderful objects out there... hehehe :)