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Space Forum / Amateur Astronomy / June 2006question about inclination of earth's axis.
I'm curious. Suppose I were to be deprived of technology and had to rely only upon my observation, and intuition, how would I know that Earth's axis is inclined at an angle instead of being perpendicular? How did the ancients know that the axis of Earth is not perpendicular, but inclined ? regards, Abhishek. Abhishek wrote: > I'm curious. Suppose I were to be deprived of technology and had to > rely only upon my observation, and intuition, how would I know that > Earth's axis is inclined at an angle instead of being perpendicular? > > How did the ancients know that the axis of Earth is not perpendicular, > but inclined ? The axis of the Earth is clearly not perpendicular to the ecliptic (the apparent orbital plane of the sun), because if it were, we wouldn't have any seasons. The Sun would always cross the sky at the same height, day after day. The amazing thing to me is that the Greeks even uncovered the precession of the equinoxes, and even gave a decent estimate as to how long the period is--about 36,000 years. (The actual value is about 25,800.)  SignatureBrian Tung <brian@isi.edu> The Astronomy Corner at http://astro.isi.edu/ Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/ The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/ My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html [snip] > The Sun would always cross the sky at the same height, day > after day. Hmmm ok. Let's leave seasons out of it for a moment. Correct me if I am wrong here, but isn't this the same conclusion which would be drawn if earth's axis was inclined but of constant inclination /with respect to the sun/? In other words, let me rephrase the question. Suppose earth's inclination with respect to the sun was always constant, so that, say, the northern hemisphere always has summer and the southern always winter. Except for travelling to the opposite hemispehere, how can we deduce that the axis is inclined? [snip] SignatureIoannis > Hmmm ok. Let's leave seasons out of it for a moment. Correct me if I am > wrong here, but isn't this the same conclusion which would be drawn if [quoted text clipped - 6 lines] > travelling to the opposite hemispehere, how can we deduce that the axis is > inclined? The Sun would not be travelling in a great circle. That'd be the first clue. Whether that clue would be interpreted correctly is another matter, of course. SignatureBrian Tung <brian@isi.edu> The Astronomy Corner at http://astro.isi.edu/ Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/ The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/ My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html > > Hmmm ok. Let's leave seasons out of it for a moment. Correct me if I am > > wrong here, but isn't this the same conclusion which would be drawn if [quoted text clipped - 17 lines] > The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/ > My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html No need to reference the axial/Equatorial orientation of the Earth against an object 93 million miles away. Take a look at the Earth from space - http://geophysics.ou.edu/solid_earth/notes/solar_system/slides/earth_from_space.jpg The axial orientation is constant but as the Earth travels in its orbit around the Sun,the orbital orientation (assigned by the terminator) changes longitudinally as well as latitudinaly .We hardly notice it from one day to the next because of the way axial rotation passes through that changing feature but it is there nad generates global climate norms and hemispherical weather pattern (seasons). Lots and lots of productive avenues to be explored by looking at things this way,of course you can always remain with your variable tilting Earth to the Sun/orbital plane - http://www.scienceu.com/observatory/articles/seasons/images/earthyrb.gif Astronomers would not resort to the northern hemisphere tilting towards and away from the sun and pretend that the southern hemisphere does not share the same axis.The 21st century view for global climate norms which can be reduced to hemispherical weather patterns for meteorological purposes is the local change of orbital orientation against fixed axial orientation. Important material ,too important to play around with .Here are the temperature signatures of the change in orbital orientation which assist in the transitionto a more productive astronomical mechanism based on change in orbital orientation - http://www.climateprediction.net/images/sci_images/annual.gif In short - KEEP THINGS LOCAL > [snip] > [quoted text clipped - 11 lines] > travelling to the opposite hemispehere, how can we deduce that the axis is > inclined? In such a case, the Earth's axis would precess not once in 26,000 years but once every year. We would easily see that as a the celestial pole (that point in the northern [for nothern hemisphere observers] sky around which the stars seem to revolve during the night) would wander rapidly in our sky: tonight Polaris might be the pole star, but in half a year Vega would be the pole star. In one full year, Polaris would again be the pole star ... etc etc. Signature---------------------------------------------------------------- Paul Schlyter, Grev Turegatan 40, SE-114 38 Stockholm, SWEDEN e-mail: pausch at stockholm dot bostream dot se WWW: http://stjarnhimlen.se/ >In other words, let me rephrase the question. Suppose earth's inclination >with respect to the sun was always constant, so that, say, the northern >hemisphere always has summer and the southern always winter. Except for >travelling to the opposite hemispehere, how can we deduce that the axis is >inclined? If that were true the apparent position of the Earth's pole in the sky would circle around once a year. >Hmmm ok. Let's leave seasons out of it for a moment. Correct me if I am >wrong here, but isn't this the same conclusion which would be drawn if [quoted text clipped - 6 lines] >travelling to the opposite hemispehere, how can we deduce that the axis is >inclined? I don't think that is physically possible. It would require that the Earth's precession be the same as its orbital period. The masses and orbital parameters of the solar system bodies would have to be very different to allow that; I doubt Earth would be supporting any life to observe it. _________________________________________________ Chris L Peterson Cloudbait Observatory http://www.cloudbait.com > I'm curious. Suppose I were to be deprived of technology and had to > rely only upon my observation, and intuition, how would I know that > Earth's axis is inclined at an angle instead of being perpendicular? > > How did the ancients know that the axis of Earth is not perpendicular, > but inclined ? Sun, Moon, and stars wobble. Lots of ancient constructions are built to mark the extremes of the wobble. > Sun, Moon, and stars wobble. Lots of ancient constructions are built to > mark the extremes of the wobble. Or at least imaginative archaeologists seem to think so. Whenever I've seen the actual evidence in these cases it always reminds me a bit of the "evidence" for the pyramids on mars... There is so much room for over interpretation that my bad science alarm bells go off wildly. One thing I know for certain: if you go looking hard enough for anything you are going to find it. And just because you find "it" everywhere you look, provided you look really hard, doesn't mean it's real. Yet that's just the sort of "science" applied in these circumstances. What I'm saying is that without objective evidence most of these claims are less than compelling. They are little more the speculation. Yet they are often cited as fact. SignatureGreg Crinklaw Astronomical Software Developer Cloudcroft, New Mexico, USA (33N, 106W, 2700m) SkyTools: http://www.skyhound.com/cs.html Observing: http://www.skyhound.com/sh/skyhound.html Comets: http://www.skyhound.com/sh/comets.html To reply take out your eye > I'm curious. Suppose I were to be deprived of technology and had to > rely only upon my observation, and intuition, how would I know that > Earth's axis is inclined at an angle instead of being perpendicular? You would probably first have to figure out that the Earth is spherical, rather than flat, and in orbit around the sun, rather than vice-versa. Once that was proven to your satisfaction, you'd notice that at different times of the year, the sun shines down more directly on your hemisphere than at other times. In other words, it reaches a higher maximum angular altitude each day above your horizon during local summer than during winter, the extremes differing by about 45 degrees. During your winter, the sun stays low as it crosses the sky, while in the summer it moves across along a much higher, but parallel, path to that taken by the winter sun. Observation of sunrise and sunset would show that the point on your eastern horizon where the sun rises also changes during the course of the year, by a total amount of approximately 47 degrees. Divide this number by two, and you have the amount by which the Earth's axis is tilted (approximately 23.5 degrees). The above applies to points on earth north and south of the two Tropics; between them things are a little different, but I will save that topic for a later installment. Allison Kirkpatrick wrote: > You would probably first have to figure out that the Earth is > spherical, rather than flat, and in orbit around the sun, rather than > vice-versa. I think the question presumes knowledge that the Earth is spherical (otherwise, it doesn't have an axis). I don't agree that it requires knowing that the Earth revolves around the Sun, except that the question must be rephrased as how one figures out that the Earth's axis is not perpendicular to the Sun's orbital plane. > The above applies to points on earth north and south of the two > Tropics; between them things are a little different, but I will save > that topic for a later installment. Are you sure? How so? SignatureBrian Tung <brian@isi.edu> The Astronomy Corner at http://astro.isi.edu/ Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/ The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/ My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html > Allison Kirkpatrick wrote: >> You would probably first have to figure out that the Earth is [quoted text clipped - 6 lines] > must be rephrased as how one figures out that the Earth's axis is not > perpendicular to the Sun's orbital plane. The spherical Earth result was obtained by the ancient Greeks observing that the Earth's shadow crossing the moon during lunar eclipses was always observed to have a round edge, no matter what time of night the eclipse was observed to occur. They deduced that the only shape for the Earth that could give that result is more or less spherical. The Earth orbiting the Sun bit wasn't realy settled until Kepler worked out that that the Earth's apparent motion as viewed from a fixed point on Mars' orbit, looked the same as any other planet. I.e the ratio of speed varied in inverse accordance with the ratio of distance from the Sun just as he had worked out for the other planets and also that this apparent orbit was centered between the equant and the Sun. Again just like for the other planets. Of course this isn't proof that the Earth orbits the Sun but is pretty good supporting evidence. >> The above applies to points on earth north and south of the two >> Tropics; between them things are a little different, but I will save >> that topic for a later installment. > > Are you sure? How so? I would like to know too. Klazmon. > The Earth orbiting the Sun bit wasn't realy settled until Kepler worked > out > that that the Earth's apparent motion as viewed from a fixed point on > Mars' orbit...etc. Also the aberration of star light, discovered by Bradley ca. 1725. This was the first convincing observational evidence of the revolution of the Earth about the Sun. SignatureCurtis Croulet Temecula, California 33°27'59"N, 117°05'53"W >> The Earth orbiting the Sun bit wasn't realy settled until Kepler worked >> out [quoted text clipped - 4 lines] > was the first convincing observational evidence of the revolution of the > Earth about the Sun. Yes but I would say that Bradley's discovery was the first direct evidence. Kepler's evidence is indirect, though I still think it is convincing. At least to me. Believe it or not some don't even accept the abberration of starlight as convincing: http://en.wikipedia.org/wiki/Modern_geocentrism It is always possible to come up with an ad hoc explanation to dismiss any observation and there are a ready supply of kooks who do just that. Klazmon > > The Earth orbiting the Sun bit wasn't realy settled until Kepler worked > > out [quoted text clipped - 8 lines] > Temecula, California > 33°27'59"N, 117°05'53"W No sense in directing either of you to page 86,like your mathematical kin you love to re-write the history of discovery because nobody objects - http://mitpress.mit.edu/journals/pdf/POSC_13_1_74_0.pdf Ole Roemer's insight is extremely intricate and a natural extension of Keplerian reasoning and methods. People who do not wish to insult themselves can approach Kepler's representation of the plotted motion of Mars against the stellar background and its orbital comparison against the Earth's annual orbit will find it incredibly rewarding. The empricists,which includes you,got the wrong handle on that representation and believe that it is geocentric.It is not and Kepler,who actually constructed the exquisite diagram says it himself that it represents orbital comparisons - "Copernicus, by attributing a single annual motion to the earth, entirely rids the planets of these extremely intricate coils [spiris], leading the individual planets into their respective orbits [orbitas],quite bare and very nearly circular. In the period of time shown in the diagram, Mars traverses one and the same orbit as many times as the 'garlands' [corollas] you see looped towards the centre, with one extra, making nine times, while at the same time the Earth repeats its circle sixteen times " Astronomia Nova 1609 Bradley just drove the final nail in the coffin of astronomy that began with Flamsteed,Newton built on and Bradley finished off.A person who gets joy from what Kepler is describing through the 'Panis Quadragesimalis' representation has every right to consider himself an astronomer,those who ignore it or can't grasp it are mere mathematicians. >I'm curious. Suppose I were to be deprived of technology and had to >rely only upon my observation, and intuition, how would I know that >Earth's axis is inclined at an angle instead of being perpendicular? > >How did the ancients know that the axis of Earth is not perpendicular, >but inclined ? By looking at the apparent path of the Sun in the sky. The sun is high at noon in Summer, low at noon in Winter and midway during the Spring and Autumn. It is just as if the orbit of the Sun was in a plane inclined at 23-1/2 degrees to the plane of the Earth's equator (in the geocentric planetary systems of the day). Dreyer's History of Astronomy from Thales to Kepler has good coverage of Greek astronomy. > >I'm curious. Suppose I were to be deprived of technology and had to > >rely only upon my observation, and intuition, how would I know that [quoted text clipped - 11 lines] > Dreyer's History of Astronomy from Thales to Kepler has good coverage > of Greek astronomy. I watched a good documentary on climate chaos on the BBC last night,good coverage by David with the appropriate concern for increasing CO2/Temperature signatures and their effects. Asking people to change their behavior is a worthy sentiment however who is going to change you direction in thinking when assigning the astronomical causes.You just described a variable tilting Earth to the Sun using hemispherical descriptions based on meteorological weather patterns (seasons).You used the tilt of the Earth to do the job that most intelligent people would know emerges from orbital changes in the Earth's position. Do you think ,as entire ecosystem's collapse that those who are suffering from climate imbalances that they will congratulate you for bringing precession into a discussion on annual climate norms,not because of anything good but because you can.To describe meteorological weather patterns using astronomical causes was fine in the 17th century but not in the 21st.The human contribution to climate change is too great to ignore and without the ability to distinguish between climate norms due to the astronomical cycles and the relationship between the motions and orientations of the Earth and the signatures from human activity then what is the point ?. Assigning change in orbital orientation to the Earth against fixed axial orientation is no big shock to genuine people ,to see people assign an axial/equatorial tilt to the Sun would be shocking if it were not so depressing.God forbid any of you ever acquire anything like a conscience in this matter,the disappointment would be enough to supersede anything like climate concerns.It means people can no longer think. >I'm curious. Suppose I were to be deprived of technology and had to >rely only upon my observation, and intuition, how would I know that >Earth's axis is inclined at an angle instead of being perpendicular? Apart from the absence of seasons, if the earth's axis was perpendicular to the ecliptic, the angle between the sun and the pole star (assuming there was one) would always be 90 degrees. -- Richard > >I'm curious. Suppose I were to be deprived of technology and had to > >rely only upon my observation, and intuition, how would I know that [quoted text clipped - 5 lines] > > -- Richard I absolutely detest trivia because I well know how it becomes a nuisance rather than a tool but it is approriate here . If the distance from the Earth to the Sun is 1 inch,the distance from Earth to polaris,using the same comparison, is over 40 miles . (431 * 62 240 AU).Try triangulating that one !. A reasonable and responsible astronomer would approach the matter differently.The comparative size of Venus and Earth and the transit of Venus provides a more realistic idea of the size of our parent star - http://www.oldstarlight.com/Venus%20Transit%206-8-4.jpg It is almost impossible to impress on another the energy output of our Sun across a distance of 93 million miles except to acknowledge that radiation from the enormous celestial object is almost constant,half the Earth receives that energy and the other half is in its own orbital shadow - http://geophysics.ou.edu/solid_earth/notes/solar_system/slides/earth_from_space.jpg It takes men to consider things in correct geometric perspective,the enormity of the Sun's energy output replaces referencing the Earth axial orientation against the Sun never mind to Polaris .Assigning changing orbital orientation and the behavior of axial rrotation passing through that change is a fresh and productive way to approach this material. >If the distance from the Earth to the Sun is 1 inch,the distance from >Earth to polaris,using the same comparison, is over 40 miles . (431 * >62 240 AU).Try triangulating that one !. It's approximately 40 miles from here to Glasgow. My nose is approximately one inch long. I have no trouble at all determining the angle between my nose and the direction of Glasgow. -- Richard > >If the distance from the Earth to the Sun is 1 inch,the distance from > >Earth to polaris,using the same comparison, is over 40 miles . (431 * [quoted text clipped - 5 lines] > > -- Richard The preface to the great 1543 Western astronomical work- 'De Revolutionibus' is the old Greek maxim - 'Let no one untrained in geometry enter here.' Neither the Ptolemaic astronomers nor the great Copernican astronomers retained the stellar background when drawing their conclusions on planetary motion but the later empiricists did which is why 500 years later I have to explain the ins and outs of the heliocentric system from plotted motions to creatures who have had the intuitive faculty beaten out of them. Copernicus,in chapter 11 of De Revolutionisbus,explains hemispherical weather patterns but this needs a complete overhaul in an era where the global climate picture supersedes his description based on axial tilt. http://webexhibits.org/calendars/year-text-Copernicus.html The feebleminded or those untutored in geometry should not approach the text just as the preface requested.It probably rules on almost everyone here insofar as I have yet to see an objection raised to the awful Newtonian mutation of Copernicus' work. Sorry to be so late with the reply. I just want to thank everyone for their inputs. It's helped (+: Thank you all. regards, Abhishek. > I'm curious. Suppose I were to be deprived of technology and had to > rely only upon my observation, and intuition, how would I know that [quoted text clipped - 5 lines] > regards, > Abhishek. |
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