Basics: 1. The Celestial Coordinate Systems.

"A basic requirement for studying the heavens is determining where in the sky things are". Having a coordinate system for the sky will help us specify any position just like we do on earth by specifying Latitude and Longitude. It is very nicely visualized in this video:

The above video shows how to visualize the CELESTIAL SPHERE and the coordinates. For astronomy various coordinate systems exist. The coordinate systems differ only in their choice of the fundamental plane, which divides the sky into two equal hemispheres along a great circle. For example, the fundamental plane of the geographic system is the Earth's equator. Each coordinate system is named for its choice of fundamental plane. The following table shows a list of them:

Coordinate system	Fundamental plane	Poles	Coordinates
Horizontal (Alt Az)	Celestial Equator	Celestial Poles	Altitude -Azimuth
Equatorial	Ecliptic	Ecliptic_pole	Declination – Right Ascension
Galactic	Galactic Plane	Galactic Poles	--
Super Galactic	Supergalactic Plane	--	--

The Horizontal and Equatorial System are sufficient for our use now and if required we will discuss about Galactic and Super galactic system. For now we will shrink our scope to Horizontal and Equatorial System only. The following video helps in visualizing the celestial coordinates.

The Horizontal Coordinate System: The Horizontal coordinate system uses the observer's local horizon as the Fundamental Plane. This conveniently divides the sky into the upper hemisphere that you can see, and the lower hemisphere that you can't (because the Earth is in the way). The pole of the upper hemisphere is called the Zenith. The pole of the lower hemisphere is called the nadir. The angle of an object above or below the horizon is called the Altitude (Alt for short). The angle of an object around the horizon (measured from the North point, toward the East) is called the Azimuth. The Horizontal Coordinate System is sometimes also called the Alt/Az Coordinate System. The Horizontal Coordinate System is fixed to the Earth, not the Stars. Therefore, the Altitude and Azimuth of an object changes with time, as the object appears to drift across the sky. In addition, because the Horizontal system is defined by your local horizon, the same object viewed from different locations on Earth at the same time will have different values of Altitude and Azimuth.Horizontal coordinates are very useful for determining the Rise and Set times of an object in the sky. When an object has Altitude=0 degrees, it is either Rising (if its Azimuth is < 180 degrees) or Setting (if its Azimuth is > 180 degrees).

The Alt Az or Horizontal System

                                

The Equatorial Coordinate System: The Equatorial coordinate system is probably the most widely used celestial coordinate system. It is also the most closely related to the Geographic coordinate system, because they use the same fundamental plane, and the same poles. The projection of the Earth's equator onto the celestial sphere is called the Celestial Equator. Similarly, projecting the geographic Poles onto the celestial sphere defines the North and South Celestial Poles.

However, there is an important difference between the equatorial and geographic coordinate systems: the geographic system is fixed to the Earth; it rotates as the Earth does. The Equatorial system is fixed to the stars, so it appears to rotate across the sky with the stars, but of course it is really the Earth rotating under the fixed sky. The latitudinal (latitude-like) angle of the Equatorial system is called Declination (Dec for short). It measures the angle of an object above or below the Celestial Equator. The longitudinal angle is called the Right Ascension (RA for short). It measures the angle of an object East of the Vernal Equinox. Unlike longitude, Right Ascension is usually measured in hours instead of degrees, because the apparent rotation of the Equatorial coordinate system is closely related to Sidereal Time and Hour Angle. Since a full rotation of the sky takes 24 hours to complete, there are (360 degrees / 24 hours) = 15 degrees in one Hour of Right Ascension.

The Equatorial System

                                    

The following video is helpful in understanding the celestial coordinate system and the apparent motion of stars in the sky because of movement of the earth.

Google Earth is a very useful tool to see sky maps at a particular time and also see their position by seeing the coordinates. As shown in the video, choose your location on ground by selecting 'earth' view first. Then switch to sky. Google earth will see the current time of your computer and show you the objects visible in your sky at that particular time. Its awesome. You don't need to buy any professional software like 'Sky' or 'Starry Night' or 'Redshift' etc. Google earth alone is very sufficient for basics. Watch the video for how to use Google earth as an astronomical tool.