Polaris, the compass

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 = Red

Thus, 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.

Ophiuchus, the thirteenth zodiac constellation

This was a blog published this morning in my company's blog so it was first written in English.

It seems that, some days ago, the news was spreading the chaos among horoscope readers and followers saying that an american "investigator" had "discovered" a "new" zodiac constellation making the rest moving from the dates everybody thought they must be. Actually, this horoscope stuff is part of Astrology, a kind of religion that, at present, nothing has in common with the actual science, Astronomy, but the starting "astro". So, I would like to explain everyone interested what were the astronomic bases of Astrology and how this "new discovery" affect our horoscopes.

I have already explained yesterday that, at present, almost everyone accept that actually is Earth who is orbiting around the Sun and, while is turning around its own axis, make us, seeing everything from Earth surface, feel like the Sun, the Moon and the stars are turning around us. However, the ancient Greeks didn't know that. They, as many other ancient civilizations, believed that actually the earth was fixed and surrounded by a sphere, the celestial sphere, in which surface the stars was painted as shiny dots. These "dots" are in fixed positions on the celestial sphere surface so, when the celestial sphere is moving, all the stars keep the same position one respect each other. That effect leaded our ancestors to think that the stars were really grouped in so called constellations and, joining dots like a child game it was, they saw in these constellations things, animals or characters from the nature or their believes and folk. Each culture, each civilization, found in the sky different forms, different characters, different constellations, but all of them believed that they were groups of stars that were really close. Some of them, like the zodiac constellations, are very known today, at least their names.

Between the celestial sphere and Earth they thought the planets (from ancient Greek "planetai", meaning wandering star) was, stars which position was not fixed in the celestial sphere. To the Greeks, even the Sun and the Moon, since they were moving from the other stars, were planets. They also thought that, if the constellations were characters from their mythology, the planets (stars with enough power to move by themself) must be Gods. The name of the Sun and the Moon has evolved depends of the language but didn't happened the same with the rest of planets. In that way, they found 7 planets: Sun, Moon, Mercury, Venus, Mars, Jupiter and Saturn and they gave these names to the days of a week, as so we do today. To show this I must to mix English with Spanish (a Latin root language that I know very well) so sorry if you get a little lost.

Monday (in English) is the Moon Day.

Tuesday (Martes in Spanish) is the Mars Day.

Wednesday (Miercoles in Spanish) is the Mercury Day.

Thursday (Jueves in Spanish) is the Jupiter Day.

Friday (Viernes in Spanish) is the Venus Day.

Saturday (we come back again to English) is the Saturn Day.

Sunday (English again) is, as we can directly read, the Sun Day.

Actually, I don't know where your planet names came from, I suppose I must investigate a little about it for next time.

But we know that Earth and the rest of planets (we'll forget the Moon from this point) are orbiting the Sun in nearly circular orbits. If we still were thinking the celestial sphere exists but, in this case, with the centre in the Sun, we still were seeing the stars as fixed points in the night sky. Because we are moving around the Sun, the celestial sphere must be static so we can see different stars and constellations in summer than in winter. May be this is a mess for many so I'll try to explain it slowly.

"When are you going to talk about Horoscope?" Be patient, we are close.

Think in a cross like the image below. In the centre of the cross we have the Sun and at the end of each line we have the seasons: winter, spring, summer and autumn. If you are in summer and you try to see through the Sun to the celestial sphere at the other side (don't do it, you'll be blind!) you can't see any star because the bright of the Sun are hiding then at daylight, but they are there indeed. At night, you are looking to the outside of the cross, seeing the summer stars and constellations. When you are in winter and try to look to the summer constellations the Sun is in your way, blocking your line of sight.

Season Cross

So, coming back to the geocentric model (the Earth in the centre) it actually looks like the Sun is moving not only around Earth but also from the background of stars, following a path along the year called ecliptic. Because the orbits of all the planets are all of them, more or less, in the same plane as our one, from our point of view, they follow a path in the celestial sphere in the same area that the Sun does. This area, a band that fills about 8 degrees above and below the ecliptic (distances on the surface of a sphere is usually measured by degrees) is called the zodiac band. The word "zodiac" comes from ancient Greek, again, and means "wheel of animals". That was because most of the zodiac constellations are animals, like the crab, the lion, the bull or the goat.

In theory, and it was originally, Astrology used the positions of the Sun and the planets in reference to the background of stars, the zodiac constellations, to predict the future. So, if you are Aquarius that is because, in the moment of your birth, "the Sun was in Aquarius", which means that, from our point of view, if we looked to the Sun and it didn't blind us, the constellation that we'd see behind it would be Aquarius.

But, actually, the astrologists forgot to look at the sky a long time ago. They started to use tables and charts to know where the planets must be instead looking where they really are. The rules they are using have been out of date for more than 2000 years, and so they still are today. Moreover, it's very probable that the modern astrologists are using a different system than their babylonic colleagues, who first developed the astrology, mixing and integrating on their system the changes that other people were discovering in the sky but still without looking at it, just changing their charts.

For example, astronomers and astrologists set the beginning of the year on the same moment, the vernal equinox (the moment that the day and the night last the same and give the beginning of the spring in the north hemisphere), at about the 21st of March. But astrologists say that, in this day, the Sun is entering in Aries, and that's why you are Aries if you've born between about 21st of March and 21st of April. Nevertheless, currently the Sun is very inside Pisces constellation on that day making all the zodiac constellations to move about a month backward, what means that you must be the previous constellation you though you were. For example, if you were born on 25th of April, astrologists say that you are Taurus but astronomers say you must be Aries.

"Where do this difference come from?" The third movement of the Earth, the precession, produces this difference. It's not the moment to explain how it works but one the effects of this movement is that the position of the Sun at the moment of the vernal equinox is moving respect the background of stars. Every year, the Sun is in a slightly different position moving in the opposite direction at the order of the zodiac constellations, from Aries to Pisces and, in a future, from Pisces to Aquarius. But the astrologists are fixing the position of the Sun every year in the same place, the border between Pisces and Aries, since more than 2000 years ago. They, of course, are totally wrong.

“OK, that's why the zodiac constellations has been moved but, what's about the thirteenth constellation, Ophiuchus?” This is astronomically a zodiac constellation, and it was actually known by the Greeks, but it is not for astrology. The ecliptic (remember, the path the Sun draw on the celestial sphere) crosses Ophiuchus between Scorpio and Sagittarius. Also, for astronomy, the zodiac constellations have different sizes and the Sun is not spending the same time on each one. Nevertheless, astrology divides the path of the Sun in 12 regions of exactly the same size. Each region has the name of a sign (they are not constellations, they are signs) that, originally, corresponded with the constellation the Sun was expending more time in that period of time, leaving Ophiuchus out of the wheel.

Ophiuchus

As I said before, they forgot to look at the sky a long time ago and didn’t notice that, at present, the signs don’t correspond with their original constellations at all. They [the astrologist] insist in the astrology don’t follow the same pattern that the astronomy but, at the same time, they are trying to make people believe they predict the future looking at the sky. You are free to believe in what you want but, in my opinion, they must lose the starting “astro” a long time ago.

"But then, this american investigator, has he discovered something new?" No, he just said what was known for centuries. The issue was the press has made from this a very productive "new" story.

A couple of additional small differences between astronomy and astrology are:

• Exists another astronomic zodiac constellation, Cetus, the Whale, in which the Sun spends about one day.
• The names of two of the stars of Libra are “northern claw of the scorpion” and “southern claw of the scorpion” in arabic. This fact leads to think that Libra was probably added by the Romans or even introduced by modern astrology 2000 years ago as a way to made more equitable the zodiac distribution. If we take Libra as part of Scorpio and count Ophiuchus as a zodiac constellation, Scorpio would last about a month and a half and Ophiuchus only two weeks.