Understanding the Night Sky

Now once you have realised what all you can see from your observation site, it is important to start looking. First thing we all need to understand is that in order to observe the stars and other objects in the sky, we first need to identify our directions. In astronomy the directions we follow are cardinal direction which is North, East, South and West. To identify these directions, we can refer to many methods but only one method will give us the accurate answers which to using pole star.

Pole star or Polaris is a star located in the constellation of Ursa Minor and is placed in such a way that the axis of earth when extended towards the North Pole of Earth, meets this star at a distance of about 480 LY. So when the Earth rotates around its axis, the star does not change its position as it lies on top of the axis of rotation of Earth. This makes the star unique. However this star is not the brightest stars in the sky. It is not even among the top 10 or top 50 bright stars. In fact it is 58th brightest star in the northern sky. The pole star also has a unique character. The altitude of the star in the sky is the same as your latitude. This made the star very important for navigational purpose.

Since the star is faint, it can barely be seen from the city skies. But from a dark sky the problem is different. There are so many faint stars that it is difficult to be sure which one is Pole Star. Here we can take help from the history books where our ancestors had done the research for us. Our ancestor were very keen observers of the sky as they had nothing to do with social media and other distractions of towards world. They would go out and sleep in the open or just wander around looking at the stars. They found that two constellations in particular can be used to locate pole star. One of them is Ursa Major while the other is called Cassiopeia.

Ursa Major Constellation is shaped as “question mark” or the spoon. It is visible in the summer months from northern hemisphere. The first two stars of the question mark when joined star 2 with star 1 and extended further, reaches to a region of faint stars but only one bright star almost aligning with the line. This bright star is Polaris or Pole Star. Remember it is not the brightest star in the sky but only in that small region.

Cassiopeia constellation is shaped is numeric three, mirrored three, English alphabet W or M as it moves around the star Polaris. It is visible in the winter months from northern hemisphere. The constellation is combination of two triangles, one acute and the other obtuse triangle with Stars 1, 2 and 3 forming acute angle triangle and Stars 3, 4 and 5 making obtuse angle triangle. To locate the pole star using Cassiopeia, take the obtuse angle triangle and divide the angle 3 4 5 (the obtuse angle in the triangle into two parts. As the angle is divided, the dividing line when drawn and extended meets Polaris or the pole star. This is how the Pole star is identified in the night sky. Once the pole star is identified, drop a perpendicular from the star towards the ground and identify the landmark where the earth and sky appear to meet (also known as Horizon) and North Direction.

Horizon is a line where the Earth and Sky appear to meet. It is like a circle around you and you are alway at the centre of the circle. Which ever direction you see, you will always find the earth and sky meeting at some distance and as you move towards it, you will see that the horizon also moves with you. Horizon can be identified in three ways:

1. Apparent Horizon: Apparent horizon is the horizon we see in our daily life. Weather we are in city or on mountain, out views are obstructed by some man made object like building, tower, etc or natural object like hill, trees etc. Due to these obstructions, the horizon does not remain circular but it takes the shape of the obstruction and hence is called apparent horizon.
2. Absolute or True Horizon: An absolute or true horizon is observed in an ideal condition when you are in a boat in the middle of the ocean and all you see around is just the water and no sign of land around. Or you are in a flat desert with no sand dunes or anything to block your views. The circle formed where the Earth and Sky appear to meet will be the Absolute Horizon. It is extremely rare to find a place which can offer absolute horizon.
3. Negative Horizon:Negative horizon is observed when the observer is standing at a height like top of a mountain peak or from an airplane where there is nothing to block the views and also the ground is below which means the sky will be meeting the ground below them. This additional height causes the horizon to from below them and hence called as negative horizon.

So as you stand facing North direction, on your right will be east, behind you will be south and on your left will be west. So now you know from where the Sun will rise in the morning and where the stars will set.