The next thing to understand is how the sky appears to us and how do we find objects in the sky.

The Celestial Sphere

We all understand that Earth is a sphere and we have the “Poles” on either side of this sphere and the Equator running through it. WE use a system of co-ordinates called Latitude and Longitude to figure out exactly where a point in the sphere is. The Latitude specifies distance vertically (from the equator) and the longitude specifies the distance horizontally (along the equator). Similarly, when we think about the sky it is the exact same thing. It is a sphere (like the earth), it has an equator (the celestial equator) and has a system of coordinates which we will discuss next.

We never see the whole sky but see a part of it from where we are standing. As the Earth rotates, the sky changes and so does all the objects that are in the sky. In some cases (like stars etc) it is not very evident and in some cases like planets, sun. moon it is very evident. If you imagine yourself standing outside your house and looking at the sky, the sky looks like an upside-down bowl set upon the horizon as if on a table. The point directly overhead is called the zenith. Opposite to the Zenith, the point below your feet is called the nadir. The line drawn from the Zenith to the nadir is called the meridian. This is basic English right!

The Celestial Co-ordinate System

This system of coordinates is called the Celestial Coordinate systems and they are of many kinds. But that is for the purist or maybe when I get an expert I will explain all of them. For now, we will use the commonly used Equatorial Coordinate System. Basically it says like we have the Latitude, in the sky we have something called “Declination” and like we have the Longitude, in the sky we have something called “Right Ascension”. So every object in the sky can be located by its 2 coordinates – the RA (right Ascension) and the dec (Declination).

Right Ascension

RA is like the longitude on the Earth’s sphere. It is measured in  hours minutes and seconds and a typical RA measure is 20:55:27 meaning 20 hours, 55 minutes and 27 seconds. RA can be used to determine a star’s location and to determine how long it will take for a star to reach a certain point in the sky. For example, if a star with RA = 01:30:00 is at your zenith, then a star with RA = 20:00:00 will be at zenith (20-1.5) 18.5 hours later

Declination

Dec is like the latitude on the Earth’s sphere. An object on the celestial Equator has a dec of 0°.  An object at the north pole has a dec of +90°. So a declination of an object tells you how high the object is in the sky wrt the celestial equator. An object at the south pole has a dec of −90°. If an object is at my zenith (the top most point) then the object will have its dec same as my latitude. Hence objects who have a dec > (90 deg – Laitiude of the place) are called circum polar objects and are visible throughout the day. E.g: Polaris the North Star which is always seen from northern hemispheres as it is at the north Pole and has a dec of 90 deg.

Conclusion

So basically, you look at a star map and if an object has an RA and dec co-ordinates given, we can figure out where it will be in the sky. If a dec is greater than (90 deg- Latitude of your location) then that object will always be visible to you and is called circumpolar objects (eg: North Star).  Next I will come back on constellations. There is some pretty interesting stuff about them. I learnt that just by looking a t the constellations you can figure not only the time, but which season is on etc. The thing that excites me is all this does not require even a single telescope or binocs. You can just explore all of this with your eye. The only thing you need is a clear sky.

More to come..