Basic astronomy -- the North Star, Polaris, and its relation to latitude

Looking north on the 39th parallel (Source: Stellarium for Windows)

Do you know how to find Polaris, the North Star? It's a useful skill, in case you ever get lost without your GPS-enabled cellphone or a compass.

Look for the Big Dipper, as it's known in English. Can you find it in the image above? (Hint: it's to the left of the yellow line.) The two rightmost stars are called the "Pointer Stars" -- draw an imaginary line through them pointing above the Dipper to the nearest brightest star, and that is Polaris, the North Star.

Here's a diagram to make it more clear.

Source: wpclipart.com

Polaris is called the North Star because it is very close to the North Celestial Pole, the imaginary point above the geographic north pole, and so it is almost due north of any location above the equator.

Before the adoption of the magnetic compass, mariners used the stars to navigate. Polaris was their friend, because it reliably told them which way was north. And, during the time of slavery in the United States, escaping slaves would "follow the North Star" to freedom in the North or even to Canada.

The angle of Polaris above the horizon corresponds very closely to geographic latitude. As you can see in the image above, Polaris is 39° above the horizon when you are standing on the 39th parallel (Denver, Colorado, let's say).

As you head north, Polaris climbs ever higher in the sky. Here's how it would look on the 72nd parallel, somewhere in northern Canada.

Source: Stellarium for Windows

By the way, I am using the program Stellarium to create these images, but the view would be the same if you physically went to these places. The horizon is distorted in the scene above, because the program is modeling a wide-angle lens for the snapshot.

Where do you suppose Polaris would be if you could stand right on the North Pole? I'll tell you at the end, in case you can't figure it out.

Let's head south now, to Panama City, Panama, which is at 9° north latitude.

(Source: Stellarium for Windows)

Now Polaris is very close to the horizon, and our friend the Big Dipper is not even visible. It's below the horizon.

This next image removes the terrain and buildings and adds the lines of celestial latitude (the circular ones) and longitude (the straight ones), at 5°N latitude. You can see that Polaris is very close to the central point, the North Celestial Pole.

(Source: Stellarium for Windows)

Maybe you can guess what happens if we keep moving south. On the equator (0° latitude), Polaris would be right on the horizon, and if we keep heading south, Polaris disappears entirely from view.

So, how did ancient mariners navigate without the North Star as a guide? For the most part, they avoided being in the open sea far from a coastline, unless they could find the North Star. In fact, European seamen were not keen on sailing below the equator in open water until the advent of the magnetic compass in the 1300s. (For more details about the history of navigation, check out http://www.pbs.org/wgbh/nova/ancient/secrets-of-ancient-navigators.html and Wikipedia. )

Now, let's consider why the North Star moves closer to the horizon the further south we go. It's a question of the geometry of the Earth.

If the Earth were flat, as some people allege, then the North Star's relative position really would not change very much at all, and would always be above the horizon -- even in the Southern Hemisphere. Observations belie this explanation.

(Source: As noted in image)

[I anticipate some Flat Earthers will object that Stellarium images used here are "fake," because they are computer generated and not actual photos. But the position of Polaris relative to the horizon can be readily verified by anyone living at the locations cited here.]

Another possibility is the Earth might be cylindrical, with the equator running parallel to the central axis. But that would suggest people living at different longitudes would see the same stars in the sky at the same time, which is not the case.

The simplest explanation is that the Earth is round like a ball, with the North Star almost directly above the Geographic North Pole (that is, directly overhead if you were standing there). As we head south, the North Star "sinks" lower in the night sky, and when we pass the equator, Polaris is now below the horizon, blocked by the earth itself.

This diagram may clarify matters. Zenith is the point directly above your head. The angle phi (φ) is the latitude (the angle between the line connecting your head to the center of the Earth and the line connecting the equator to the center of the Earth). From the geometry, you can see that angle φ is also the angle between the plane of the horizon (the disk touching the surface at your position) and the position of Polaris.

(Source: Space-awareness.org)

The geometric proof is left to the reader as an exercise. Hint: Consider the right triangles in the diagram.

One last item about Polaris -- it's only been the "North Star" for the last two thousand years or so. Much as a spinning top does as it slows down, the Earth precesses ("wobbles") relative to its orbital plane. The imaginary line connecting the two geographic poles and extending into space describes a large circle in the northern sky and a corresponding one in the southern sky. A full precession cycle takes about 26,000 years.

4,000 years ago the North Star was Thuban in the constellation Draco. In year AD 12,000, Vega in Lyra will have the honor.

Here's a YouTube video illustrating precession. Skip to timecode 2:05, where it shows the changing North Stars over time. You're welcome to start from the beginning, but the video also illustrates a related topic, the precession of the equinoxes, which may confuse matters.

In the video, NCP stands for North Celestial Pole, which changes over time, and NEP is the North Ecliptic Pole, which does not. It's the imaginary point directly above the plane of the earth's orbit.

(Source: Steven Sanders, YouTube channel. Animation is used in Spitz planetarium shows.)

That's the end of this lesson. Please leave questions and comments below, resteem , upvote, follow and have a great day!