How are the Zodiac signs determined ?

[Seren_]

This is more an astronomy that astrology question.

I know that that your Zodiac Sign is supposed to be determined by the position of the Sun at your date of birth.

What is not very clear in my mind is how ? Is it the position of the Sun at noon ?

So basically if you are Aquarius it means that from the Earth when looking at the Sun at noon, the Sun is in the Aquarius Constellation (roughly). But since you can't see the stars, it is only infered not actually observed. (I don't doubt that it is correct nonetheless)

For example, Aquarius at noon, means Leo (Aquarius + 6) at midnight in the ecliptical plan. So by observing the Leo constellation you can guess that the newborn is Aquarius.

Wouldn't it be easier if your sign was determined by an observable constellaton the next night ? Is my reasonning correct ?

 

[shadron]

This is more an astronomy that astrology question.

I know that that your Zodiac Sign is supposed to be determined by the position of the Sun at your date of birth.

What is not very clear in my mind is how ? Is it the position of the Sun at noon ?

So basically if you are Aquarius it means that from the Earth when looking at the Sun at noon, the Sun is in the Aquarius Constellation (roughly). But since you can't see the stars, it is only infered not actually observed. (I don't doubt that it is correct nonetheless)

For example, Aquarius at noon, means Leo (Aquarius + 6) at midnight in the ecliptical plan. So by observing the Leo constellation you can guess that the newborn is Aquarius.

As viewed from the earth (at any time of the day or night, have you not the glare or the earth in your view path) the sun would be positioned in a single place against the background of stars, regardless of the time of day. That position jiggles minisculely daily as you move from one side of the earth to the other, but in general the sun majestically traces a single path around the star dome in a year's time. The problem is that precession twists that around one complete cycle every 25,000 years, so, in general, the location shifts one twelfth of the whole circle every 2000 years. The constellations that lie along that circle are the zodiac, and they take up unequal parts of the circle, but astrologers ignore the first problem and average out the second, all miraculously without disurbing their predictions.

Wouldn't it be easier if your sign was determined by an observable constellaton the next night ? Is my reasonning correct ?

You seem to think the sun whirls around the sky in a daily pattern. It does not, not even relatively. If you limit your observation to a single point in time every day, say 10:15AM, in order to filter out that very tiny jiggle caused by your daily motion across the face of earth, the sun will move just a trifle less than one degree across the 360 degree starscape every day.

Since the stars are hard to locate when the sun is up, you could alternatively observe the stars that are directly above you along the line from true north to true south every evening at 10:15PM, and conclude that that ios where the sun will be in 6 months.

There is also some other earth rotational motions that make the exact time of day to make your observations change within about an hour's time; see sundial^WP under "Adjustments to calculate clock time from a sundial reading".

 

[Seren_]

Thanks, it makes sense.

I was totally wrong regarding the rotation of the celestial sphere during 24h, the position of the Sun is indeed constant.

You can easily determine your sign at sunset or sunrise (or around that time) when the sky is dim enough to see the stars.

 

[shadron]

Thanks, it makes sense.

I was totally wrong regarding the rotation of the celestial sphere during 24h, the position of the Sun is indeed constant.

You can easily determine your sign at sunset or sunrise (or around that time) when the sky is dim enough to see the stars.

If that is your desire, yes.

 

[Seren_]

If that is your desire, yes.

Not particularly.

But I always assumed it was related to the position of the Sun at the Zenith, and I wondered for a long time how ancient astrologers determined easily zodiac signs. And as you explained, it is pretty trivial.

 

[Skwinty]

Here are some pointers.

Vernal equinox: The moment when the Sun crosses the celestial equator travelling in a northward direction, on or about March 21. In the northern hemisphere, it marks the first day of spring. The term is also applied to the Sun’s position in the sky at that moment. It is one of two points where the ecliptic and the celestial equator intersect, the other being the autumnal equinox.

Zodiac: The band of constellations through which the Sun passes as it moves around the sky over a year. There are twelve signs of the astrological Zodiac but thirteen constellations in the astronomical Zodiac

Vernal Equinox Point: This stellar position is called the Vernal Equinox Point, the Fiduciary Point [fiduciary means founded on trust], or the First Point in Aries. The latter because when the rules of astrology which apply to this were first formulated, the vernal equinox point was in Aries. It is now in Pisces.

So , if you were born last year on December 26, you wouldn't be a capricorn. so much for astrology.

 

[SlayerofCliffracers]

Easy question.

There 'exists' in the sky something called the eliptic. This is the path that the Sun 'moves' through the sky. The planets are also close to the eliptic, the closer a planet is to the Sun, the closer it is to the eliptic. Outer planets like Pluto are some distance away from the eliptic, while inner planets are close to it.

Now the babylonians divided the eliptic up into 12 signs, each named after a major constellation in that area of the eliptic.

The ancient Astrologers were clever enough to work out the stars didn't dissapear but were on the other side of the earth and were also able to work out that the Sun only outshone the zodiac stars, they were still there.

However the earth as it spins around the Sun wobbles, the eliptic shifts very slowly. So as Skeptics are very keen on telling people, the first 2/3rds of every sign (at the moment) is in the sky the next sign. So there is a high chance that most Tauros for instance are visually Aries.

In Indian Astrology that is exactly how it works. Not so in Western Astrology.

The thing is that the precession doesn't really change the actual physical alignment of objects, because precession is a shift in the earth orbital axis, quite seperate to the actual motion of the planets and stars themselves.

Hopefully the reason that the Astrological signs were fixed is because of this, not because they suddenly got lazy.

The actual physical alignment of the stars to earth with whatever other body is (almost) the same as it was 1400 years ago in the age of Aries.

 

[DickK]

http://en.wikipedia.org/wiki/Ecliptic

 

[nathan]

Easy question.

Pity your answer is so bogus[*]

There 'exists' in the sky something called the eliptic.

If there is, it's irrelevant to astrology. (You've repeated that spelling too consistently for it to be a typo.)

This is the path that the Sun 'moves' through the sky.

no it's not. You've just described the ecliptic. That is a plane in which the sun appears to orbit Earth. (Or alternatively, the plane in which the Earth orbits the sun, it's the same thing)

The planets are also close to the eliptic,

The orbits of the planets are not inclined very much to the ecliptic.

the closer a planet is to the Sun, the closer it is to the eliptic.

Wrong.

Outer planets like Pluto are some distance away from the eliptic, while inner planets are close to it.

Wrong.

Hopefully the reason that the Astrological signs were fixed is because of this, not because they suddenly got lazy.

Perhaps it's because you can't tell the difference?

[*] Not intended to imply deliberate misleading.

 

[SlayerofCliffracers]

If there is, it's irrelevant to astrology. (You've repeated that spelling too consistently for it to be a typo.)

How is the basis of the signs in Astrology 'irrelevant'.

no it's not. You've just described the ecliptic. That is a plane in which the sun appears to orbit Earth. (Or alternatively, the plane in which the Earth orbits the sun, it's the same thing)

Yes it's the ecliptic I am talking about.

The orbits of the planets are not inclined very much to the ecliptic.

Totally wrong. All the inner planets orbit relatively close to the ecliptic. The further a planet is away from the Sun, the further it's orbit tends to be from the Sun in the sky.

Wrong.

You evidently don't know what you are even talking about. I found that out using the astronomical program Alycone Ephemeris.

http://www.alcyone-ephemeris.info/

Wrong.

You evidently don't know what you are even talking about again.

Perhaps it's because you can't tell the difference?

[*] Not intended to imply deliberate misleading.

I have really had enough of this forum.

 

[BillC]

What do you mean by "orbit close to the ecliptic"? Do you mean with low inclination to the ecliptic plane?

 

[SlayerofCliffracers]

What do you mean by "orbit close to the ecliptic"? Do you mean with low inclination to the ecliptic plane?

It think so. However it would appear that this is just a general rule I find out.

Certainly the inner planets (Mercury to Saturn) follow this rule. Neptune and Uranus appear not to be following it at the moment, this is perhaps because they are crossing the ecliptic at the moment I think, but they are so slow moving that Alycone refuses to give me their entire orbit. Or maybe they don't follow it at all.

Pluto is definately a long way as is Eris from the ecliptic. This is what I was thinking of.

It's certainly pretty complicated though.

 

[BillC]

It think so. However it would appear that this is just a general rule I find out.

Certainly the inner planets (Mercury to Saturn) follow this rule. Neptune and Uranus appear not to be following it at the moment, this is perhaps because they are crossing the ecliptic at the moment I think, but they are so slow moving that Alycone refuses to give me their entire orbit. Or maybe they don't follow it at all.

Pluto is definately a long way as is Eris from the ecliptic. This is what I was thinking of.

It's certainly pretty complicated though.

Your general rule is wrong. The planets' inclination to the ecliptic is essentially fixed: it is irrelevant at what point in their orbit they are in. Here are the 8 planets orbital inclinations, courtesy of Wolfram Alpha:

1 | Mercury | 7.005
2 | Venus | 3.395
3 | Saturn | 2.484
4 | Mars | 1.851
5 | Neptune | 1.769
6 | Jupiter | 1.305
7 | Uranus | 0.770
8 | Earth | 0.00005 | (in degrees)

The inner planets (Mercury, Venus and Mars) possess amongst the highest inclinations, not the lowest, as you suggest.

Pluto and Eris are not planets. Their high inclination suggests in part a different source for their origin, which played a part in the debate as to the definition of planet.

I believe you may be thinking of elongation, but that is nothing to do with inclination to the ecliptic.

 

[shadron]

First, realize that in a gravitational 2-body situation the plane of the orbiting body necessarily always passes through the gravitational center of the system. Always - a direct result of the theory of gravitation acting within celestial mechanics. That situation can be perturbed at times and places by other bodies in the system, but it is mainly true at all times.

The ecliptic is a circle drawn on the background of stars defined by projecting the orbit of the Earth outwards from the Sun's center (this can also be visualized as a plane that passes through the sun center and the earth's orbit in all places). If you also draw upon the background the orbits of the various planets are projected outwards from the center of the sun, these orbit paths will not exactly trace the ecliptic; they will rise and dip around the ecliptic by small amounts. The angle that they make to the ecliptic at the places where they cross it is known as the inclination^WP of the orbit. The lower the inclination the closer the orbit matches the ecliptic.

Mercury: 7°
Venus: 3°
Earth: 0°, of course.
Mars: 2°
Jupiter: 3°
Saturn: 2.5°
Uranus: 1°
Neptune: 2°
Pluto: 17°, which says much about it's likely origin.
Eris: 44°

The moon has an solar inclination of a tad over 5°. The earth is tilted some 23° off the ecliptic, so the moon has an Earth-centered inclination of between 18 and 28°.

Six numbers, sufficiently accurately determined, define the orbit of a minor body around a primary. With those six numbers, an astronomer can calculate the location of the body at any moment in time and find it with a telescope from any spot on Earth, absent any third body perturbation. This is what is meant by the absolute determinism of Newtonian mechanics. The inclination of the body's orbit to the ecliptic is one of those numbers. The observation of a body in three locations of its orbit (say on three different nights) is theoretically sufficient to determine those six numbers.

Finally, the inclination of the orbits are why the planets and the sun and moon come close together at conjunctions rather than actually eclipsing (or transiting) each other all the time. Only rarely do they meet at the point on the ecliptic where they cross, and thus rarely eclipse each other.

 

[SlayerofCliffracers]

Standing in awe of Shadrons Astronomical knowledge.

 

[nathan]

Standing in awe of Shadrons Astronomical knowledge.

I await your apology.

 

[H'ethetheth]

First, realize that in a gravitational 2-body situation the plane of the orbiting body necessarily always passes through the gravitational center of the system. Always - a direct result of the theory of gravitation acting within celestial mechanics. That situation can be perturbed at times and places by other bodies in the system, but it is mainly true at all times.

The ecliptic is a circle drawn on the background of stars defined by projecting the orbit of the Earth outwards from the Sun's center (this can also be visualized as a plane that passes through the sun center and the earth's orbit in all places). If you also draw upon the background the orbits of the various planets are projected outwards from the center of the sun, these orbit paths will not exactly trace the ecliptic; they will rise and dip around the ecliptic by small amounts. The angle that they make to the ecliptic at the places where they cross it is known as the inclination^WP of the orbit. The lower the inclination the closer the orbit matches the ecliptic.

Mercury: 7°
Venus: 3°
Earth: 0°, of course.
Mars: 2°
Jupiter: 3°
Saturn: 2.5°
Uranus: 1°
Neptune: 2°
Pluto: 17°, which says much about it's likely origin.
Eris: 44°

The moon has an solar inclination of a tad over 5°. The earth is tilted some 23° off the ecliptic, so the moon has an Earth-centered inclination of between 18 and 28°.

Six numbers, sufficiently accurately determined, define the orbit of a minor body around a primary. With those six numbers, an astronomer can calculate the location of the body at any moment in time and find it with a telescope from any spot on Earth, absent any third body perturbation. This is what is meant by the absolute determinism of Newtonian mechanics. The inclination of the body's orbit to the ecliptic is one of those numbers. The observation of a body in three locations of its orbit (say on three different nights) is theoretically sufficient to determine those six numbers.

Finally, the inclination of the orbits are why the planets and the sun and moon come close together at conjunctions rather than actually eclipsing (or transiting) each other all the time. Only rarely do they meet at the point on the ecliptic where they cross, and thus rarely eclipse each other.

I'm not sure if this detailed and interesting reply is to the point though. It seems to be, because apparently SlayerofCliffracers is satisfied with it, but I'm not sure he's wrong (about this specific issue). I took his comment to mean that the angular distance from the ecliptica as perceived from earth is bigger for the outer planets. That is, the orbital inclination multiplied by the orbital radius, and then seen from earth. I'd have to draw myself a diagram and plug in the numbers to be able to say if that angle is greater or not.

In fact, I don't think I will be able to do anything useful until I've done just that.

ETA: If I didn't screw up, SlayerofCliffracers is also wrong on that issue. I get an angle of 3.7° for Jupiter, and 7.0° and 4.7° for Venus and Mercury respectively.

 

[Foolmewunz]

How are the Zodiac signs determined ?

Well, when a mommy woo and a daddy woo love each other very much,.....


(snipped to avoid saccharine overdose)




And nine months later a little Libra with Gemini rising and moon in Scorpio is born.

 

[SlayerofCliffracers]

I await your apology.

I have to apologise for being wrong?

 

[nathan]

I have to apologise for being wrong?

No, you claimed I didn't know what I was talking about, and Shadron posted the data that showed that I did (before I got to post it).

You can apologize at large for making incorrect claims if you want too.