Guest guest Posted March 12, 2003 Report Share Posted March 12, 2003 Hi Juan! Thanks for the fine info, and the insights. You are truly the Dean of Astronomy in our ranks. It shows that I read too many astrology books and not enough technical stuff. But, I still say that the " sidereal zodiac, " no matter what the Babylonians used, is now calculated as some delta from the vernal point, and thus defined as such, and not by reference to any fixed star near the ecliptic; nor is it somehow anchored on some fixed point in space. You bring up an excellent point about how epochs are a fixed point in sidereal time -- sidereal time being a fixed-star-to-fixed-star measure. Does practical astronomical observation, however, on a spinning geoid like Earth, require setting up the telescope with reference to the tropical coordinate system so that one can accurately find the object to be viewed? Thanks again, Ed K Oh, on a different issue: I was reading that the ecliptic " pivots " ever so slightly on an axis that is about 23 Virgo/Pisces? Is this true? , " prec2nod " <hylonome@r...> wrote: > Ed wrote: > > > <<Then why, when I go to Astrolog, does the " sidereal " value for Spica > change every year?>> > > > > > There are some clarifications needed. > > > > > Astronomers work with " inertial " or " quasi-inertial " reference frames, > i.e., fiducials points that [almost, or practically] are fixed in > space, i.e., that are independent of the relative motions of the Earth > and the Sun. These fiducials are necessarily sidereal by definition. > Astronomers always work with these sidereal positions, both in the > development of planetary theory (theories of motion of the bodies of > the solar system) and in the " reduction " of observations. > > > > > Observed tropical astronomical positions are always reduced to some > arbitrary " epoch " (e.g., 2000 or 1900.0) which represents the position > of the equinox and equator at some specific point in time used as > fiducial. These " epochs " are all sidereal. > > > > > Earth's orientation in space varies according to (mostly) precession, > nutation, and aberration. These displacements must always be removed > from the observations and the resulting positions compared with > catalogs of reference positions of stars that by definition are > sidereal. This is why you see the right ascension positions in > astronomical references with respect to the J2000 or some other epoch, > i.e., they are always sidereal right ascensions. > > > > > The so-called " apparent " (tropical) geocentric positions given by > astronomical almanacs are a convenience necessary in the process of > removing the Earth's motion from the observed positions. They are used > because they are closer to the observed positions, but these position > are only the first step in the process of reduction of the observed > coordinates. > > > > > Historically, it is known that the Babylonians never used tropical > positions even though they were aware of at least the difference > between the duration of the tropical and the sidereal year (and > therefore most probably of precession). They were never interested in > the tropical reference frame, based on the Earth-Sun relationship, and > referenced everything with respect to relative positions in the sky. > The use of tropical positions in astronomy was a late development, and > came parallel with the development of (abstract) geometric cinematic > models by the Greeks. > > > > > The " synetic " (Fagan/Bradley) vernal point is not based on the > position of Spica. It happened to be empirically very close (about > 0,06'05 " ) to the position that Spica would have in a sidereal zodiac > defined with Spica roughly at 29 degrees Virgo and Aldebaran- Antares > at 15 degrees Taurus-Scorpio (as originally assumed by Fagan from > Babylonian material). The position of the vernal point that exactly > coincides with the sideral position of Spica at 0 Libra by definition > is the so-called " Lahiri " zodiac. > > > > > Astronomically, the apparent position of a star is never used as > reference point, but its so called " catalog " or " epoch " position, > which is sidereal. Since by definition the catalog position is fixed > in space, the " proper motion " of a star is not considered --once its > catalog or epoch position has been established. In other words, proper > motion may be needed to know where a star is at a time different than > the time of its observation, but once this second, calculated position > has been established and is being used as reference, proper motion is > never used. > > > > > Juan Quote Link to comment Share on other sites More sharing options...
Guest guest Posted March 13, 2003 Report Share Posted March 13, 2003 Is there a position in space that doesn't move? That would be a fiducial point! Siderial has a fiducial. Tropical has a fiducial. Now that I am convinced sidereal is right I must defend tropical, audiatur et altera pars. Ed, if you please, distinguish between rhetorical use of " fiducial " and literal. Your web articles are impressive. Congratulations on the Mountain Astrologer publication. I look forward to reading it. (BH) Quote Link to comment Share on other sites More sharing options...
Guest guest Posted March 13, 2003 Report Share Posted March 13, 2003 Hi Brian, Thanks for the kind compliments. , " brianrhiggins2003 " <brianrhiggins@e...> wrote: > Is there a position in space that doesn't move? I guess that's the rub! All bodies are in motion, but the more distant deep space objects are virtually perfectly stationary due to their immense distance. So, I admit that Juan is correct in his assertion that a sidereal measure must be taken to plot the coordinate systems, and that this ultimately has to alter my conclusions. Yet, the Babylonians had no concept of a " plane of the ecliptic, " (which is a fiducial in and of itself, being the 0* of latitude of the Sun's trek) and this is what both tropical and sidereal zodiacs are measured by, and thus why all sidereal systems are expressed as some delta of the tropical fiducial. Thus, would not any truly sidereal measuement system have to be based on a purely sidereal grid system to be independent of the tropical grid? The galactic plane comes to mind, or the invariable plane, and why not discuss planetary positions in regards to these grid systems if we truly want to divorce ourselves from the Sun-Earth (and thus tropical) grid? I'm not asking this rhetorically, but instead because I would like more clarification. > That would be a fiducial point! Siderial has a fiducial. > Tropical has a fiducial. Now > that I am convinced sidereal is right I must defend tropical, > audiatur et altera pars. Ed, if you please, distinguish between > rhetorical use of " fiducial " and literal. My use of " fiducial " was in the mathematical sense. Very best, Ed K Quote Link to comment Share on other sites More sharing options...
Guest guest Posted March 16, 2003 Report Share Posted March 16, 2003 <<But, I still say that the " sidereal zodiac, " no matter what the Babylonians used, is now calculated as some delta from the vernal point,>> This is just the algorithm, not the definition. How do you think this " delta " is calculated? The ayanamsa is always defined in terms of a fixed epoch. You must always calculate its precessional displacement from that fixed epoch. <<and thus defined as such, and not by reference to any fixed star near the ecliptic;>> If a star is used, it is the position of the star at a certain epoch. Besides, Fagan and his predecessors showed that Aldebaran/Antares and Spica were being used as markers in Babylonia. <<nor is it somehow anchored on some fixed point in space.>> It is always anchored in fixed space by definition. Again: how is the ayanamsa calculated? The answer is: one calculates precessional displacement from a fixed epoch. <<You bring up an excellent point about how epochs are a fixed point in sidereal time -- sidereal time being a fixed-star-to-fixed-star measure.>> " Sidereal " means " without precession " . I explained that you do not use simply a fixed star, but its position at a certain epoch (i.e. fixed) as the fiducial point. The position of epoch is defined in terms of the equinox and the equator, as you are saying, but this is a matter of convention determined by the system of coordinates you use. What matters is that the fiducial is fixed and accurately known. <<Does practical astronomical observation, however, on a spinning geoid like Earth, require setting up the telescope with reference to the tropical coordinate system so that one can accurately find the object to be viewed?>> I am no expert in astrometry, but I assume that normally, astronomers take a photographic plate. The plate is " reduced " geometrically to remove its optical distortions and the nutation, precession, and aberration, and then compared with a reference catalog of stars. Tropical positions are not used at any point here, only the positions referred to the epoch of the star catalog. This is all sidereal. Juan Quote Link to comment Share on other sites More sharing options...
Guest guest Posted March 16, 2003 Report Share Posted March 16, 2003 <<... Yet, the Babylonians had no concept of a " plane of the ecliptic, " (which is a fiducial in and of itself, being the 0* of latitude of the Sun's trek)>> The Babylonians were able to plot the points or " knots " of full moon eclipses, i.e., the ecliptic. This thesis was developed at length by Kristian Peder Moesgaard in " The Full Moon Serpent. A Foundation Stone of Ancient Astronomy? " , in Centaurus, 1980, vol 24, pp.51-96. When one plots 235 consecutive full moon positions (1 metonic cycle=19 years) on a star map one obtains 35 sinusoidal waves that span the zodiac twice... " ... The resulting knots of intersection single out 35 discrete positions of the ecliptic in the neighborhood of which lunar eclipses can occur... " " (p.51) Because the change in longitude as well as latitude between full moons 235 lunations (19 years) apart is very small... " ... the serpent can be dealt with as an almost rigid geometrical structure drifting slowly in relation to the fixed stars. " (p.52) " It is an empirical fact that the full moon serpent exists in the sky. Thanks to the cuneiform text quoted above it is a historical fact that Babylonian astronomers were aware of this and worked with the basic 19 year cycle that creates the serpent. Awareness of the serpent pattern may stem from quite simple observations of the lunation number and the location among the stars for eclipses of the moon. Systematic keeping accountance with such eclipse records during say one century suffices to reveal the existence and positions of the 35 eclipsing knots. To form thereafter the idea of the full serpent means by one stroke of genius to establish sidereal positions for all the intervening full moons that were not eclipsed. " (page 53) The paper goes on for 46 pages! The following is something I wrote in April 2000 in the ACT list (Ed should remember it!): It has always been speculated --and many scholars are convinced-- that the ancients knew about precession long before Hipparchus (d.127 B.C.), and in 1979, Willy Hartner proved that in Babylonia at least in 503 B.C., <<a clear distinction was made between the length of the tropical year and that of the sidereal year>> ('Journal for the Hist. of Astron.' x, 1979, p19). Yet the Babylonian astronomical tradition is built upon sidereal time units, and planetary and star positions are correlated among themselves rather than with the Solstice and equinoctial points of the Sun (see Raymond Mercier, 'Archives Internationales d'Hist. des Sciences', 1976 26, p.200). The Babylonians didn't care about the tropical reference frame, even if they knew about it. The reason for this, according to Mercier, is that their astronomy was based on the observation of the night sky, in contrast to the Greeks, who based it on the Sun. The Babylonians always preferred purely celestial units of time, and plotted phenomena against the visible stars in the sky, while the Greeks used " terrestrial " units, such as the " day " , the passage of the Sun trough the equator, and synodic periods. Greek astronomy <<seems to be primarily concerned to record the precise solar time of events>> (Mercier, p.200) while in Babylonian astronomy and culture <<the sense of time is therefore rather weak>>. The astrology we practice today was born out of the direct experience of the night sky, but it was transformed by the Greeks into an analogical-technological device which we use today. The Greeks used geometric or cinematic models in their description of celestial motions, but there is a total absence of them in Babylonian thinking. According to O. Neugebauer, their <<methods are strictly arithmetical in character, based on numerical sequences>> ('Astronomical Journal', 72-8, 1967, p.965). In concordance with the " night-sky " paradigm, the conceptual universe of the civilization which gave birth to astrology did not include the notions of history, process, and change. Observable occurrences were, in essence <<only the universe of things interacting in time.... a moment of time was apprehended and defined as the sum total of the occurrences and events known to be in temporal conjunction. Moreover, just as the realm of objects is repeatable, in the sense that there exist many examples of the same phenomenon, so too were the moments of time repeatable, if the set of occurrences constituting a particular moment repeated itself>> (J.J. Finkelstein, Journal of Near Eastern Studies 33(2):197-210, 1974). This " cognitive mode of the Mesopotamian intellect " , finds expression in the carefully maintained libraries of omen and divination texts which seem to be the seeds out of which astrology later developed, and belongs to an a-causal and a-historical way of thinking very alien to the Greeks: " The signs on earth just as those in the sky give us signals. " Sky and earth give us signals. " Sky and earth both produce portents " though appearing separately, they are not separate (because) sky and earth are related " A sign that portends evil in the sky is (also) evil on earth " one that portends evil on earth is evil in the sky. " [transl. by Leo Oppenheim, Journal of Near Eastern Studies 33(2):197- 210, 1974] We may assume that there was a gradual shift, during the Hellenistic period, from the sidereal to the tropical " paradigm " , one based on the experience of the night sky and belonging to a culture geared toward the external world in which " time " was not the absolute cumulative quantity that is generally imagined today, and where geometric models of natural phenomena did not exist. The other, characteristic of Greek intellect and inherited by us, is based on the Sun and upon geometric models and projections. Strictly speaking, tropical measurements of time are not " celestial " in the pure sense, they are based on the Sun exclusively, in the solar interactions with the Earth. The fiducial points of a tropical system are established through geometrical projections, they represent " planes " which are derived from solar phenomena, since only the Sun presents the regularity needed to do this: the Moon and the planets all move like a serpent zigzagging through space. The solar motion from equinox to equinox does not provide an adequate reference frame for the absolute time needed by cinematics and celestial mechanics, since it is not " fixed " in space. As a consequence, astronomy before Copernicus regarded <<time as a complex patchwork of incommensurable periods, rather than as a formally continuous mathematical quantity, and the periods in question were expected to have celestial significance.>> (R. Mercier, quoted above). In other words, time was transformed into distance through patches of space called " cycles " , resulting from the use of geometry to represent natural phenomena. The measurements of time are provided by planetary motions. The key here to me is the word " motions " , which is not the same as " bodies " . Planetary motions, astrologically speaking, are " trajectories " ( " orbits " ) which are really time seen abstractly as distances in space. This is what I feel the coordinates of an astrological chart measure, these one-dimensional distances in a flat space which allow the trajectories to intersect, thus measuring the cycles. Modern astronomy, for this reason, never uses the tropical year or the solar day to calculate planetary positions. Al calculations are carried out in a sidereal reference frame, and only later, at the end of the process, are converted to tropical. The tropical zodiac, as representative of the seasonal cycle is related exclusively to the Sun. When we convert a time-cycle into simultaneous sections of space in the sky, each with different " seasonal qualities " , we are transposing the real experience of time and the physical characteristics of the seasonal cycle into a symbolic matrix in space with which we give qualities to the planets, which have nothing to do with the seasons. The different *seasonal* characteristics, physically speaking, apply exclusively to the Sun. Any other use is symbolical, and comes from a transposition of time into space, which can be done only through abstraction and symbolization. Exactly what happens in the calendar, where discrete units of time become temporal distances or spatial " cells " , and this reflects a change of consciousness from the " real experience out there " to abstraction and pure mathematics. This is what the Greeks did, as opposed to the Babylonians, who could never imagine any of it. Their sidereal zodiacal signs and constellations were geared to the sky and the stars exclusively, to a real experience of the nocturnal sky, and this was inextricably tied to their way of seeing and interpreting the world, their conceptual universe. Juan Quote Link to comment Share on other sites More sharing options...
Guest guest Posted March 16, 2003 Report Share Posted March 16, 2003 , " prec2nod " <hylonome@r...> wrote: > <<But, I still say that the " sidereal zodiac, " no matter what the > Babylonians used, is now calculated as some delta from the vernal > point,>> > > This is just the algorithm, not the definition. How do you think > this " delta " is calculated? The ayanamsa is always defined in terms > of a fixed epoch. You must always calculate its precessional > displacement from that fixed epoch. Hi Juan, I guess that settles it! Thanks for answering the questions. My confusion must come from the fact that all astronomy software for telescopes are geared to the RA/DECL grid, and thus the tropical coordinate system. This is done, apparently, for practical purpose and has no bearing on whether the EQU/ECL node is somehow the base of all stellar calculation. > <<and thus defined as such, and not by reference to any fixed star > near the ecliptic;>> > > If a star is used, it is the position of the star at a certain epoch. > Besides, Fagan and his predecessors showed that Aldebaran/Antares and > Spica were being used as markers in Babylonia. > > > <<nor is it somehow anchored on some fixed point in space.>> > > It is always anchored in fixed space by definition. Well, from *many* fixed spaces averaged out. The vernal point is one anchoring point of convenience. > Again: how is the > ayanamsa calculated? The answer is: one calculates precessional > displacement from a fixed epoch. > > > <<You bring up an excellent point about how epochs are a fixed point > in sidereal time -- sidereal time being a fixed-star-to-fixed-star > measure.>> > > " Sidereal " means " without precession " . I explained that you do not > use simply a fixed star, but its position at a certain epoch (i.e. > fixed) as the fiducial point. The position of epoch is defined in > terms of the equinox and the equator, as you are saying, but this is > a matter of convention determined by the system of coordinates you > use. What matters is that the fiducial is fixed and accurately known. And, it can be be argued that the vernal point is itself " fixed " while all else moves about it? After all, we still use geocentric coordinate systems even though we know that the Sun is the center of the solar sytem. > <<Does practical astronomical observation, however, on a spinning > geoid like Earth, require setting up the telescope with reference to > the tropical coordinate system so that one can accurately find the > object to be viewed?>> > > I am no expert in astrometry, but I assume that normally, astronomers > take a photographic plate. The plate is " reduced " geometrically to > remove its optical distortions and the nutation, precession, and > aberration, and then compared with a reference catalog of stars. > Tropical positions are not used at any point here, only the positions > referred to the epoch of the star catalog. This is all sidereal. Excellent points all around, and quite an education. Thank you so very much!!!!!! Ed K Quote Link to comment Share on other sites More sharing options...
Guest guest Posted March 16, 2003 Report Share Posted March 16, 2003 Hi again, I made this point on ACT going back some time, actually, but it should be noted that even with this level of observation, no specific " plane " of the ecliptic could be plotted, but rather a span of sky that is about 2 degrees in latitude that approximates the ecliptic. Only through observations of solar phenomena (as opposed to lunar) can the true inclination of the ecliptic be measured, and any idea that there is a " plane " be determined, as you state below. The Greeks must have had harsher winters!! I joined ACT in June of 2000, but I'm very happy you reposted this brilliant piece of scholarship and reasoning. It's worth reading over and over. Muchas gracias!! Ed K , " prec2nod " <hylonome@r...> wrote: > <<... Yet, the Babylonians had no concept of a " plane of the > ecliptic, " (which is a fiducial in and of itself, being the 0* of > latitude of the Sun's trek)>> > > The Babylonians were able to plot the points or " knots " of full moon > eclipses, i.e., the ecliptic. This thesis was developed at length by > Kristian Peder Moesgaard in " The Full Moon Serpent. A Foundation > Stone of Ancient Astronomy? " , in Centaurus, 1980, vol 24, pp.51-96. > > When one plots 235 consecutive full moon positions (1 metonic > cycle=19 years) on a star map one obtains 35 sinusoidal waves that > span the zodiac twice... > > " ... The resulting knots of intersection single out 35 discrete > positions of the ecliptic in the neighborhood of which lunar eclipses > can occur... " " (p.51) > > Because the change in longitude as well as latitude between full > moons 235 lunations (19 years) apart is very small... > > " ... the serpent can be dealt with as an almost rigid geometrical > structure drifting slowly in relation to the fixed stars. " (p.52) > > " It is an empirical fact that the full moon serpent exists in the > sky. Thanks to the cuneiform text quoted above it is a historical > fact that Babylonian astronomers were aware of this and worked with > the basic 19 year cycle that creates the serpent. Awareness of the > serpent pattern may stem from quite simple observations of the > lunation number and the location among the stars for eclipses of the > moon. Systematic keeping accountance with such eclipse records during > say one century suffices to reveal the existence and positions of the > 35 eclipsing knots. To form thereafter the idea of the full serpent > means by one stroke of genius to establish sidereal positions for all > the intervening full moons that were not eclipsed. " (page 53) > > The paper goes on for 46 pages! > > The following is something I wrote in April 2000 in the ACT list (Ed > should remember it!): > > It has always been speculated --and many scholars are convinced-- > that the ancients knew about precession long before Hipparchus (d.127 > B.C.), and in 1979, Willy Hartner proved that in Babylonia at least > in 503 B.C., <<a clear distinction was made between the length of the > tropical year and that of the sidereal year>> ('Journal for the Hist. > of Astron.' x, 1979, p19). Yet the Babylonian astronomical tradition > is built upon sidereal time units, and planetary and star positions > are correlated among themselves rather than with the Solstice and > equinoctial points of the Sun (see Raymond Mercier, 'Archives > Internationales d'Hist. des Sciences', 1976 26, p.200). > > The Babylonians didn't care about the tropical reference frame, even > if they knew about it. The reason for this, according to Mercier, is > that their astronomy was based on the observation of the night sky, > in contrast to the Greeks, who based it on the Sun. The Babylonians > always preferred purely celestial units of time, and plotted > phenomena against the visible stars in the sky, while the Greeks > used " terrestrial " units, such as the " day " , the passage of the Sun > trough the equator, and synodic periods. Greek astronomy <<seems to > be primarily concerned to record the precise solar time of events>> > (Mercier, p.200) while in Babylonian astronomy and culture <<the > sense of time is therefore rather weak>>. > > The astrology we practice today was born out of the direct experience > of the night sky, but it was transformed by the Greeks into an > analogical-technological device which we use today. The Greeks used > geometric or cinematic models in their description of celestial > motions, but there is a total absence of them in Babylonian thinking. > According to O. Neugebauer, their <<methods are strictly arithmetical > in character, based on numerical sequences>> ('Astronomical Journal', > 72-8, 1967, p.965). > > In concordance with the " night-sky " paradigm, the conceptual universe > of the civilization which gave birth to astrology did not include the > notions of history, process, and change. Observable occurrences were, > in essence <<only the universe of things interacting in time.... a > moment of time was apprehended and defined as the sum total of the > occurrences and events known to be in temporal conjunction. Moreover, > just as the realm of objects is repeatable, in the sense that there > exist many examples of the same phenomenon, so too were the moments > of time repeatable, if the set of occurrences constituting a > particular moment repeated itself>> (J.J. Finkelstein, Journal of > Near Eastern Studies 33(2):197-210, 1974). > > This " cognitive mode of the Mesopotamian intellect " , finds expression > in the carefully maintained libraries of omen and divination texts > which seem to be the seeds out of which astrology later developed, > and belongs to an a-causal and a-historical way of thinking very > alien to the Greeks: > > " The signs on earth just as those in the sky give us signals. > " Sky and earth give us signals. > " Sky and earth both produce portents > " though appearing separately, they are not separate (because) sky and > earth are related > " A sign that portends evil in the sky is (also) evil on earth > " one that portends evil on earth is evil in the sky. " > > [transl. by Leo Oppenheim, Journal of Near Eastern Studies 33 (2):197- > 210, 1974] > > We may assume that there was a gradual shift, during the Hellenistic > period, from the sidereal to the tropical " paradigm " , one based on > the experience of the night sky and belonging to a culture geared > toward the external world in which " time " was not the absolute > cumulative quantity that is generally imagined today, and where > geometric models of natural phenomena did not exist. The other, > characteristic of Greek intellect and inherited by us, is based on > the Sun and upon geometric models and projections. > > Strictly speaking, tropical measurements of time are not " celestial " > in the pure sense, they are based on the Sun exclusively, in the > solar interactions with the Earth. The fiducial points of a tropical > system are established through geometrical projections, they > represent " planes " which are derived from solar phenomena, since only > the Sun presents the regularity needed to do this: the Moon and the > planets all move like a serpent zigzagging through space. > > The solar motion from equinox to equinox does not provide an adequate > reference frame for the absolute time needed by cinematics and > celestial mechanics, since it is not " fixed " in space. As a > consequence, astronomy before Copernicus regarded <<time as a complex > patchwork of incommensurable periods, rather than as a formally > continuous mathematical quantity, and the periods in question were > expected to have celestial significance.>> (R. Mercier, quoted > above). In other words, time was transformed into distance through > patches of space called " cycles " , resulting from the use of geometry > to represent natural phenomena. > > The measurements of time are provided by planetary motions. The key > here to me is the word " motions " , which is not the same as " bodies " . > Planetary motions, astrologically speaking, are " trajectories " > ( " orbits " ) which are really time seen abstractly as distances in > space. This is what I feel the coordinates of an astrological chart > measure, these one-dimensional distances in a flat space which allow > the trajectories to intersect, thus measuring the cycles. > > Modern astronomy, for this reason, never uses the tropical year or > the solar day to calculate planetary positions. Al calculations are > carried out in a sidereal reference frame, and only later, at the end > of the process, are converted to tropical. > > The tropical zodiac, as representative of the seasonal cycle is > related exclusively to the Sun. When we convert a time-cycle into > simultaneous sections of space in the sky, each with > different " seasonal qualities " , we are transposing the real > experience of time and the physical characteristics of the seasonal > cycle into a symbolic matrix in space with which we give qualities to > the planets, which have nothing to do with the seasons. > > The different *seasonal* characteristics, physically speaking, apply > exclusively to the Sun. Any other use is symbolical, and comes from a > transposition of time into space, which can be done only through > abstraction and symbolization. Exactly what happens in the calendar, > where discrete units of time become temporal distances or > spatial " cells " , and this reflects a change of consciousness from > the " real experience out there " to abstraction and pure mathematics. > > This is what the Greeks did, as opposed to the Babylonians, who could > never imagine any of it. Their sidereal zodiacal signs and > constellations were geared to the sky and the stars exclusively, to a > real experience of the nocturnal sky, and this was inextricably tied > to their way of seeing and interpreting the world, their conceptual > universe. > > Juan Quote Link to comment Share on other sites More sharing options...
Guest guest Posted March 17, 2003 Report Share Posted March 17, 2003 <<My confusion must come from the fact that all astronomy software for telescopes are geared to the RA/DECL grid, and thus the tropical coordinate system. This is done, apparently, for practical purpose and has no bearing on whether the EQU/ECL node is somehow the base of all stellar calculation.>> I imagine that aligning the telescope to the instantaneous equator (tropical " of date " instead of " of epoch " ) allows you to locate objects easily in the sky and also to move the telescope so that it can follow the earth's rotation, which is essential when you are taking long-exposure photographs of the night sky. Also, the observed azimuth, altitude, and hour angle (distance from the meridian), which is converted to right ascension and declination (I think this is fundamental in celestial navigation), are tropical. So you are probably right that the use of tropical positions is " for practical reasons " . But the determination of the precise positions, necessary for the computation of orbits and long-range ephemerides, must be done later through a process of reduction of the observations done in a sidereal (quasi-inertial, i.e., fixed) reference frame. Juan Quote Link to comment Share on other sites More sharing options...
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