By Thomas P.

Focusing on Polaris.
Polaris, the brightest and last star on the «tail» of the Little Bear, does not unintentionally constitute a point of reference for navigation. It is really worth seeing a bit more analytically its position in the celestial dome because its unique characteristics are exactly due to this position.

Navigating with the stars

We know that all the stars of the celestial dome, which are exactly above us, as well as the Sun and the Moon, rise, culminate and set during 24 hours. This «phenomenal» movement in the sky is mainly due to the rotation of the Earth. Actually, because the Earth rotates from the west to the east, the sky and consequently all celestial bodies look as if they rotate reversely. So, it seems that they carry out a complete circle every 24 hours and they are, therefore, in the same time at the same point. This, of course, in reality is the result of a complete rotation of the Earth within 24 hours, since it is only the Earth that revolves and not the stars.

So, all the celestial bodies are on a celestial sphere which encloses the Earth and rotates around it. The poles of this celestial sphere are called the north and South Pole accordingly. The rotation axis of the sphere is the rotation axis of the Earth which connects the earthy North and South Pole and by being extended it meets the north and South Pole accordingly.
In other words, as far as the north hemisphere, which is of interest to us, is concerned, the extension of the rotation axis of the Earth, beyond the North Pole towards the sky, will meet the celestial North Pole.

Navigating with the stars

Well, very close to this celestial North Pole (exactly above the earthy North Pole) there is Polaris. It is this exact position which makes it so significant.
Therefore, we can consider that Polaris does not revolve at all and remains almost steady in its position since it is very close to the rotation axis. On the contrary, all the other stars seem to revolve around it. The closer they are to it the smaller their orbit.
So, it is made clear why Polaris points to the earthy north and why its position is steady every hour and season throughout the year.

Defining the Latitude

But it is very easy to use Polaris since it is almost still and steady in its position, in order to define our latitude.
When we are in the North Pole, Polaris is just over us. The height of Polaris from the horizon, that is, the corner that is formed from the imaginary straight line which begins from our position and reaches Polaris with the horizontal level, is 90 degrees. This is our latitude in the North Pole.
As we go down towards the Equator, the height of Polaris «lowers» in the sky, so it looks lower in the horizon. Consequently, the corner of observation is constantly reducing and, as a result, when we reach Equator we can see it very low, in the level of the horizon. There, the corner of the horizon with the imaginary straight line which begins from our position and reaches Polaris, is 0 degrees; the same as that of latitude, that is, of Equator. Actually, when we are on the Equator the level of our horizon is parallel with the rotation axis of the Earth, in the extension of which is Polaris. This is why Polaris is discerned there, low in the horizon whereas, if we move further down from Polaris, we cannot see it at all because of the round shape of the Earth.

Navigating with the stars

So, when we are in the north hemisphere, the height of Polaris on the horizon is in accordance with our distance from the Equator. The closer we approach the North Pole, the higher we see it in the horizon because we approach more the rotation axis of the Earth. Therefore, the observation corner is growing in order to reach its maximum price in the North Pole, which is 90 degrees.
Be careful though. It is not the position of Polaris that changes since, as we have already pointed out, this is always steady but our position and consequently our horizontal level, because of the sphericity of the Earth. So, the angle of observation changes because as we move from the Equator to the North Pole, our horizontal level is constantly changing, whereas, the imaginary straight line which connects our position with Polaris remains theoretically steady.
In other words, the angle of the horizon with Polaris is altering according to where we are, as far as our transfer from the North Pole to the Equator and reverse is concerned. This exact angle corresponds to our latitude.
Now, if we move in the direction from the east to the west in parallel with the Equator, our horizontal level in relation to Polaris does not change. Therefore, the observation angle of Polaris does not change either.
There are an infinite number of such places on the surface of the Earth from where the observation corner of Polaris is exactly the same. If we connect all these with imaginary lines, concentric circles embracing the Earth and being in parallel with the Equator will be formed. The places which are in the same imaginary circle have the same latitude as well.

Orientation with Polaris

As we have already mentioned in the first part of this article, with the help of the Little Bear and the Great Bear constellations we can locate Polaris.
In the point where the imaginary vertical straight line, which begins from it and crosses the horizon, there is the north and, in fact, the real north. This practically means that if we turn towards Polaris, at the back it is the south, on our right hand, in An angle of 90 degrees with Polaris, it is the east while on our left hand and under the same angle there is the west.

...keep Ribbing!


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