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Old 30-06-2022, 21:23   #1
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Where on earth is Aries?

This might be a somewhat esoteric post. All searches I did on the forum for “Aries” came up with windvane steering or a feed back to electronic navigation.

I have enjoyed practicing celestial navigation for many years and thus far have been quite happy to adopt the Aries GHA figures from the Almanac. Lately I’ve been pondering exactly where those GHA numbers are calculated to.

I know about the history of why we use Aries (aka First Point of Aries) as a reference point for the stars. Yet as much as I have researched ad nauseam, I have not seen (or perhaps understood) an explanation of where Aries “is” as it travels its’ daily 360° circuit of the globe (Yes, I get that it’s the earth rotating not Aries, but the consequence is the same).

Can anyone enlighten me? If “Aries” is a fictitious point in the sky, so be it. But why then use a fictitious point when we could use an observable point, such as the star Sirius or Antares as a reference point for all other stars?

I may be misunderstanding something here (won’t be the first time or the last!). Can anyone enlighten me?
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Old 30-06-2022, 21:58   #2
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Re: Where on earth is Aries?

Aries isn’t fictitious and it can be measured. It’s just the name that needs explaining. The symbol (♈︎) as used in astronomy is a reference to the vernal equinox, and the sun’s position at that time.

Aries, the reference meridian, is the meridian of the sun at the exact moment that it crosses the celestial equator on the vernal equinox. Thus it can be measured directly every year in March. GHA angles are all derived from that position of the sun.

Way back when that got figured out in Greece (maybe not for the first time, but at least for the reference used by Western navigators) the vernal equinox occurred when the sun was in Aries. With precession we’re now in Pisces headed for Aquarius, but the name has stuck. Given that the sun has been in Pisces for more than 2000 years (it was barely still in Aries when this got sorted) it really seems we should call it the Pisces meridian.

That’s how I understand it, I am by no means an astronomer or master navigator.

https://usm.maine.edu/planet/why-ver...ly-pisces-date
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Old 30-06-2022, 22:07   #3
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Re: Where on earth is Aries?

'First point of aires' was a thing back in antiquity.
Since then things - stars and stuff - have been precessing westwards at one degree every 72 years ( I pinched that from here https://en.wikipedia.org/wiki/First_Point_of_Aries ).

So while it once was in the corner of the constellation of Aires it is just nowhere it particular on the celestial sphere but remains ' one of the two points on the celestial sphere at which the celestial equator crosses the ecliptic' ie the one at the vernal equinox.

I now know more about the first point of Aires than I ever did before.
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Old 30-06-2022, 22:40   #4
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Re: Where on earth is Aries?

Another bit of trivia. The closest readily discenable star ("currently") to the First Point of Aries is Lambda Piscium (λ Piscium)
https://theskylive.com/sky/stars/lambda-piscium-star
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Old 30-06-2022, 22:51   #5
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Re: Where on earth is Aries?

Quote:
Originally Posted by HeywoodJ View Post
Aries isn’t fictitious and it can be measured. It’s just the name that needs explaining. The symbol (♈︎) as used in astronomy is a reference to the vernal equinox, and the sun’s position at that time.

Aries, the reference meridian, is the meridian of the sun at the exact moment that it crosses the celestial equator on the vernal equinox. Thus it can be measured directly every year in March. GHA angles are all derived from that position of the sun.

Way back when that got figured out in Greece (maybe not for the first time, but at least for the reference used by Western navigators) the vernal equinox occurred when the sun was in Aries. With precession we’re now in Pisces headed for Aquarius, but the name has stuck. Given that the sun has been in Pisces for more than 2000 years (it was barely still in Aries when this got sorted) it really seems we should call it the Pisces meridian.

That’s how I understand it, I am by no means an astronomer or master navigator.

https://usm.maine.edu/planet/why-ver...ly-pisces-date
I get that, but what I obviously don't get is that the vernal equinox occurs at a precise GHA. Therefore, by my weird logic, the GHA of Aries would be a fixed number, which it is clearly not. So is the GHA at that precise point in time effectively transferred to the celestial sphere and then we measure GHA from that? Just seems odd that we'd do that (if indeed we do) when we've got some tangible points to reference from.
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Old 30-06-2022, 23:32   #6
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Re: Where on earth is Aries?

Quote:
Originally Posted by Wideocean7 View Post
I get that, but what I obviously don't get is that the vernal equinox occurs at a precise GHA. Therefore, by my weird logic, the GHA of Aries would be a fixed number, which it is clearly not. So is the GHA at that precise point in time effectively transferred to the celestial sphere and then we measure GHA from that? Just seems odd that we'd do that (if indeed we do) when we've got some tangible points to reference from.
You've got the cart before the horse. A system had to be chosen. As the time that the vernal equinox occurs could be measured even in ancient times, was used to define the celestial "prime meridian", the zero point that you measure the angles of the stars from. At this time, the vernal equinox, you look at where the sun appears in the celestial sphere and that is the zero point. Aries just happened to be in the vicinity at that time in history.

From: Earthsky

To summarize, the First Point of Aries marks the location of the sun – in front of the backdrop stars of the zodiac – on the March equinox.

From:
https://astronavigationdemystified.c...oint-of-aries/

Just as the Greenwich meridian has been arbitrarily chosen as the zero point for measuring longitude on the surface of the Earth, the first point of Aries has been chosen as the zero point in the celestial sphere. It is the point at which the Sun crosses the celestial equator moving from south to north along the ecliptic (at the vernal Equinox in other words). This point is known as the ‘First Point of Aries’ because in 150 B.C. when Ptolemy first mapped the constellations, Aries lay in that position. However, although still named the ‘first point of Aries’, due to precession, the vernal equinox now lays in the constellation Pisces.
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Old 30-06-2022, 23:49   #7
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Re: Where on earth is Aries?

Quote:
Originally Posted by Wideocean7 View Post
I get that, but what I obviously don't get is that the vernal equinox occurs at a precise GHA. Therefore, by my weird logic, the GHA of Aries would be a fixed number, which it is clearly not. So is the GHA at that precise point in time effectively transferred to the celestial sphere and then we measure GHA from that? Just seems odd that we'd do that (if indeed we do) when we've got some tangible points to reference from.
GHA is derived from a tangible point and a specific time.

The First Point of Aries is the tangible point where the Celestial Equator and the Ecliptic meet . At the vernal equinox, the sun is at that point. That is the tangible point and time used to establish the zero point of GHA (GREENWICH hour angle). At any other time of year, every point in the cellestial sphere (including the First Point of Aries) has a different relationship to the arbitrary Greenwich. meridian.

What other tangible point(s) do we have (other than the First Point of Libra at the autumnal equinox ) ?
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Old 01-07-2022, 00:06   #8
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Re: Where on earth is Aries?

Quote:
Originally Posted by Wideocean7 View Post
I get that, but what I obviously don't get is that the vernal equinox occurs at a precise GHA. Therefore, by my weird logic, the GHA of Aries would be a fixed number, which it is clearly not. So is the GHA at that precise point in time effectively transferred to the celestial sphere and then we measure GHA from that? Just seems odd that we'd do that (if indeed we do) when we've got some tangible points to reference from.
My brain is beginning to hurt --

First Point of Aires moves only slowly but if we kept it at a fixed point the sidereal hour angle of all the stars would keep moving and the table of the navigational stars at the side of the almanac page would keep on having to be adjusted. changed.

So it is done the way it is for simplicity's sake.
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Old 01-07-2022, 00:39   #9
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Re: Where on earth is Aries?

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Originally Posted by cal40john View Post
You've got the cart before the horse. A system had to be chosen. As the time that the vernal equinox occurs could be measured even in ancient times, was used to define the celestial "prime meridian", the zero point that you measure the angles of the stars from. At this time, the vernal equinox, you look at where the sun appears in the celestial sphere and that is the zero point. Aries just happened to be in the vicinity at that time in history.

From: Earthsky

To summarize, the First Point of Aries marks the location of the sun – in front of the backdrop stars of the zodiac – on the March equinox.

From:
https://astronavigationdemystified.c...oint-of-aries/

Just as the Greenwich meridian has been arbitrarily chosen as the zero point for measuring longitude on the surface of the Earth, the first point of Aries has been chosen as the zero point in the celestial sphere. It is the point at which the Sun crosses the celestial equator moving from south to north along the ecliptic (at the vernal Equinox in other words). This point is known as the ‘First Point of Aries’ because in 150 B.C. when Ptolemy first mapped the constellations, Aries lay in that position. However, although still named the ‘first point of Aries’, due to precession, the vernal equinox now lays in the constellation Pisces.
So let me see if I understand you correctly and please forgive me if I sound obtuse - I'm not. If Aries' GHA is calculated from the meridian of the sun at the precise moment of the vernal equinox, that must mean that this meridian is fixed at that point in time in the celestial sphere does it not? Therefore as the earth rotates, the GHA between that "fixed" meridian and Greenwich changes (as it does).

I'm obviously having trouble getting my head around this. You mentioned the Greenwich Meridian being an arbitrary choice, yet it is tangible and physical. All celestial bodies have their own GHA and dec which are also "physical" locations to the extent that they have projected geographical positions. Yet Aries seems to be locked in to an indiscernible position in the sky defined by a moment in the calendar (the vernal equinox).

Or in my British vernacular is this complete bo??ocks?!
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Old 01-07-2022, 01:16   #10
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Re: Where on earth is Aries?

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Originally Posted by Wideocean7 View Post
You mentioned the Greenwich Meridian being an arbitrary choice, yet it is tangible and physical.
Its location is tangible with respect to the earth. It is not fxed with respect to any other celestial body. It moves with respect to them because of the earth's rotation and movement around the sun.
Quote:
All celestial bodies have their own GHA and dec which are also "physical" locations to the extent that they have projected geographical positions.
They are not fixed"geographical" locations. That would mean that they are always in the same direction from a given point on the Earth. Clearly that is not so.

You are conflating GHA with SHA (Sidereal Hour Angle) which is their position relative to the fixed point in the celestial sphere called the "First Point of Aries"

A body's GHA changes constantly at the same rate that the GHA of Aries and the Sun do (they all change by 360° over a 24 hour period.)


Quote:
Yet Aries seems to be locked in to an indiscernible position in the sky defined by a moment in the calendar (the vernal equinox).
No it is perfectly discernible and is the same regardless of any calendar. It's one of the two points where the Celestial Equator and the Ecliptic meet (It's just a little bit down and left of the star Lamba Piscium). Because of orbital geometry, that is where the sun must be at at the vernal equinox and that made it easy for the ancients to identify as a suitable origin for their calculations. (The vernal equinox is a physical event at a specific point in time during the earth's movement round the sun as a result of the Earth's inclination. It is only a "calendar" event because our calendar is based on that same sun's orbit. )
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Old 01-07-2022, 06:18   #11
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Re: Where on earth is Aries?

I’ll take a stab. The stars are “fixed” in the sky. But they really aren’t. Because of precession and wobble they move around just a little bit. By measuring the location of the “moving” sun against the “fixed” stars at a known point in the celestial dance you are in effect measuring the precession and wobble of the Earth, and thereby get the very minor changes in star position that result from that. You are in effect asking "at the equinox, how much has the sun moved against the backdrop of the stars". The answer is very, but it has moved. This question is really a proxy for "how much has the Earth tilted/wobbled to cause the sun to appear to move against the backdrop of the stars."

The equinox is a readily definable and easily measured event, and so makes a convenient time to take that measurement. The full cycle takes ~26000 years, so the changes within a single lifetime are about one degree, or a little less than one minute of arc per year. That minute of arc can represent a mile, so needs to be accounted for in your position calculations.
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Old 01-07-2022, 18:53   #12
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Re: Where on earth is Aries?

Just remember that you're standing on a planet that's evolving
And revolving at nine hundred miles an hour
That's orbiting at nineteen miles a second, so it's reckoned
A sun that is the source of all our power

The sun and you and me and all the stars that we can see
Are moving at a million miles a day
In an outer spiral arm, at forty thousand miles an hour
Of the galaxy we call the 'milky way' --Monty Python, Galaxy Song
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Old 01-07-2022, 19:14   #13
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Re: Where on earth is Aries?

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Originally Posted by michaelratinter View Post
Just remember that you're standing on a planet that's evolving
And revolving at nine hundred miles an hour
That's orbiting at nineteen miles a second, so it's reckoned
A sun that is the source of all our power

The sun and you and me and all the stars that we can see
Are moving at a million miles a day
In an outer spiral arm, at forty thousand miles an hour
Of the galaxy we call the 'milky way' --Monty Python, Galaxy Song
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Old 01-07-2022, 20:19   #14
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Re: Where on earth is Aries?

Quote:
Originally Posted by michaelratinter View Post
Just remember that you're standing on a planet that's evolving
And revolving at nine hundred miles an hour
That's orbiting at nineteen miles a second, so it's reckoned
A sun that is the source of all our power

The sun and you and me and all the stars that we can see
Are moving at a million miles a day
In an outer spiral arm, at forty thousand miles an hour
Of the galaxy we call the 'milky way' --Monty Python, Galaxy Song
...
And pray that there's intelligent life somewhere out in space,
'Cause there's bugger all down here on Earth!
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Old 01-07-2022, 21:16   #15
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Re: Where on earth is Aries?

Quote:
Originally Posted by StuM View Post
...
And pray that there's intelligent life somewhere out in space,
'Cause there's bugger all down here on Earth!
Oh so very true..
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