One of the first details you notice when you take a long time looking to the sky is to see how, throughout the night, the stars are moving, as the Sun does during the day and throughout the year. I explained in the previous entry that the stars we see in summer are not the same as those we see in winter, the sky changes as Earth rotates around the Sun. The same happens over a day as the Earth rotates on its axis.
Well, imagine that you are in the center of a cubic room (both walls and floor and ceiling are the same size). This would imply that you would have to climb on a chair to be truly in the center but forget about the floor. Imagine that every wall and ceiling, has a different color, as shown in the image below, in which the roof is going to look white.
If you find yourself staring at the wall initially red and turn to your left you will pass to see the red wall to see the blue one, then orange, then green and finally you'll return to see the red one. However, if you do the same movement but always looking at the ceiling you'll see the same thing, an immaculate white ceiling.
The same applies to the starry sky, the "celestial sphere" of which we spoke. However, the rotational movement we do in the room is the Earth that it does in this case and our "axis" does not directly coincide with its, just standing right at the North Pole (or South Pole) would match the two axis but I think it won't be hard for anyone to imagine, therefore, the section of the celestial sphere that is above the North Pole can be seen all the year, to greater or lesser extent, from the northern hemisphere, while part that is above the South Pole can be seen all the year, to a greater or lesser extent, from the southern hemisphere.
I know that playing with spheres is a bit complicated for our mind, so I will put several examples of different situations using pictures that I post below.
In this image the black circle represents the Earth while the blue one represents the celestial sphere. Suppose that we are in the position marked as 1. The dashed line represents the line that we would see as the horizon and, therefore, above it would see the sky. A section of the sky are visible all year round, it is the ceiling of the room we talked about. Another section is only visible at certain times of the year because the Sun during the day and the Earth, the ground, overnight block us from seeing some of the stars of this section of the celestial sphere. However, there is also a section of the sky that, much as the Earth revolves around the sun, we'll never see, are those stars which are as far south in the celestial sphere that the ground prevents us from seeing them throughout the year.
Clearly, as we see in the picture below, when we change our latitude the dimensions of the sections named before change. However, while we remain in the northern hemisphere, there is always a section that can be seen throughout the year and one that can not be seen at any time.
Also, as we cross to the southern hemisphere these sections "always visible" and "always hidden" are exchanged, as can be seen in the picture below.
I would like to apologize to all the inhabitants of the South which passes through this blog because, from now on, I'll concentrate my lessons mainly in the northern hemisphere, since it is the one I know better.
Returning to the geocentric model (Earth at the center of the Universe). We have said earlier that the celestial sphere rotates around the Earth (remember that this is totally false, it is only a model) making all the stars turn at a time. However, what is right in the axis of this sphere does not move even if the sphere turns. Thus, if we had a point in the sky that marks the North Pole of the celestial sphere and what we look for a whole night we would realize that while all the stars appear to revolve around him, that point still.
Well, this "point" is a star called Polaris (or simply North Star). Is not really fair at the North Pole of the sphere but is so, so close you can be considered as it is. Then go with an image representing the North Pole of the celestial sphere and you can see highlighted Polaris and the constellation to which it belongs, Ursa Minor.
To the left of the image may be a number of details about the star. Since this notation we will still find useful in future entries I will stop and explain it a little. For now, we just stop at the first two lines. In the first of these may be the traditional name that the star has, if any, which in this case is Polaris, between brackets, his real name, and after the dash, the number in the Hipparcos Catalog (I will not stop to explain this now, but not all stars are in this Catalog)
The stars, even those with no traditional name, are named according to their apparent magnitude and the constellation to which they belong. The apparent magnitude is the brightness of a star that seems to have seen with the naked eye. All celestial bodies have apparent magnitude associated with them, including the Sun and the Moon. How to define the magnitude is a bit complicated to explain here but we just left with the idea that the smaller the size, the greater the apparent brightness of the star. For example, the Sun has a magnitude of -26, Sirius (the brightest star in the night sky) has a magnitude of -2 and, as you can see, Polaris has a magnitude of about 2.
The value in parentheses after the letters B-V refers to the color. According to their values, the star would be the following colors:
<0.15 = Blue
-0.15 / -0.05 = White Teal
-0.05 / +0.2 = White
+0.2 / +0.5 = White Beige
+0.5 / +0.9 = Yellow
+0.9 / +1.5 = Orange
> +1.5 = RedThus, Polaris is a star of yellow color.
Well, the stars of a constellation, including those not part of his original drawing, but they just are in the region of the celestial sphere that is defined by the constellation, are ordered from lowest to highest magnitude (from highest to lower brightness). However, instead of calling to Polaris, the brightest star in the constellation Ursa Minor, 1-UMi (Ursa Minor), it is done using the Greek alphabet and becomes α-UMi.
Therefore, and unlike many people think, Polaris is NOT the brightest star in the sky, so Sirius is much brighter than it. However, as we have seen, is a star that can be viewed throughout the year and, moreover, is fixed in the evening sky for being in the North pole of the celestial sphere. Of course, that as the Earth's axis coincides with the axis of the alleged celestial sphere, that this star is in the North Pole means that the north pole of the Earth itself points to the star, making it incredibly useful in guiding and especially in navigation, because the star always mark the terrestrial North Pole.
Also, contrary to what many people believe, Polaris does NOT belong to Ursa Major, but her younger sister, making it, at first, a star hard to find, since Ursa Minor is not so easily visible and recognizable as Ursa Major. How to find it I'll leave to the next entry, because I do not want entries get too long.
I'll just let me add one more detail about the usefulness of knowing place Polaris, represented by the following image. If we messure (for example, with a protractor and a plumb) the angle the form theline unites us with the pole star and the line unites us to the horizon will give us, directly, the latitude at which we find ourselves. If we were at the North Pole would have to Polaris just above our heads, at 90 degrees to the horizon, while being in Equator Polaris is just over the horizon, forming an angle of 0 ° with it.
As I said, Polaris is the most most useful star and, therefore, one of the the most popular of the northern hemisphere and in many cases, the path to follow to find it serves as a basis for finding the other constellations which has close to it and that we will see in subsequent entries. The impressive figure of Draco (the Dragon) and the fascinating history of Cepheus and Cassiopeia, kings of Ethiopia, will soon be revealed.
No comments:
Post a Comment