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Thread: Direction of setting sun at solstice

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    Direction of setting sun at solstice

    I have just witnessed the setting of the sun at solstice here in Tenerife, which is 'exactly' in line with the edge of a pyramid. Yes, believe it or not, a pyramid. There is no archeological evidence to suggest that the pyramid is more than 200 years old, but of course there are those, doubtless connected with tourism, who would claim a much earlier date.

    Now I know that the date of solstice will change, but I can't work out whether with time the actual direction of the setting sun at solstice will change, and if so, by how much. Those who suggest that the aligment is coincidental would have to explain another curiosity that there is another aligment, namely three clearly marked points 'exactly' in the direction of the rising sun at winter solstice, in almost the opposite direction. Again, I can't work out whether there is a latitude at which these two directions are exactly 180 degrees apart. The situation is complicated by the fact that the sun sets at the summer solstice behind a mountain range with an elevation of around 15 degrees, because one hour later I saw the sun set again, in the sea, on the other side of the island.

    My question is really whether this direction of setting sun changes sufficiently with time to be evidence for the age of the pyramid, assuming that there is a connection between it and the orientation of the pyramid.

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    You would need additional information to determine it. No matter what the shift, as long as the Earth is tilted, and it spins, there will be a solstice, and it will be due East.
    The most likely way would be to know the position of the stars at the solstice to determine the Earth's location at each of the tims.

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    Quote Originally Posted by NEOWatcher View Post
    as long as the Earth is tilted, and it spins, there will be a solstice, and it will be due East.
    Now I am really confused. Are you perhaps confusing solstice with equinox?

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    Quote Originally Posted by Perikles View Post
    Now I am really confused. Are you perhaps confusing solstice with equinox?
    Oops, sorry, yes.
    Same issue though, just scratch the tilt of the axis. It can help determine it.

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    What exactly is the viewpoint that's in line with the pyramid?

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    Quote Originally Posted by Perikles View Post
    Again, I can't work out whether there is a latitude at which these two directions [sunset at summer solstice and sunrise at winter solstice] are exactly 180 degrees apart.
    Wouldn't they be 180 degrees apart at the equator (assuming a flat horizon)? Sunset on the summer solstice would be 23.5 deg. N of W while sunrise on the winter solstice would be 23.5 deg. S of E. I'm not completely sure that azimuth equals declination (roughly) at the equator, but it should be close. The equation of time and other perturbations might also affect this, but I'm not sure and I'm simplifying this right now.

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    Quote Originally Posted by MAPNUT View Post
    What exactly is the viewpoint that's in line with the pyramid?
    When you stand very close to the wall of the pyramid and look along it, you can see that the sun sets at the point on the horizon which is the extrapolation of the wall. The walls are at an angle but I could not determine whether it is the same angle as that of the setting sun. Was that your question?

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    Quote Originally Posted by Tobin Dax View Post
    Wouldn't they be 180 degrees apart at the equator (assuming a flat horizon)? Sunset on the summer solstice would be 23.5 deg. N of W while sunrise on the winter solstice would be 23.5 deg. S of E. I'm not completely sure that azimuth equals declination (roughly) at the equator, but it should be close. The equation of time and other perturbations might also affect this, but I'm not sure and I'm simplifying this right now.
    Yes, I suppose that is the case on the equator. The pyramid has a latitude of 16 deg 24', so considering the elevation of the point of sunset it sort of makes sense.

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    Quote Originally Posted by Perikles View Post
    I have just witnessed the setting of the sun at solstice here in Tenerife, which is 'exactly' in line with the edge of a pyramid. Yes, believe it or not, a pyramid. There is no archeological evidence to suggest that the pyramid is more than 200 years old, but of course there are those, doubtless connected with tourism, who would claim a much earlier date.

    Now I know that the date of solstice will change, but I can't work out whether with time the actual direction of the setting sun at solstice will change, and if so, by how much. Those who suggest that the aligment is coincidental would have to explain another curiosity that there is another aligment, namely three clearly marked points 'exactly' in the direction of the rising sun at winter solstice, in almost the opposite direction. Again, I can't work out whether there is a latitude at which these two directions are exactly 180 degrees apart. The situation is complicated by the fact that the sun sets at the summer solstice behind a mountain range with an elevation of around 15 degrees, because one hour later I saw the sun set again, in the sea, on the other side of the island.

    My question is really whether this direction of setting sun changes sufficiently with time to be evidence for the age of the pyramid, assuming that there is a connection between it and the orientation of the pyramid.
    The obliquity of the Earth's spin axis varies between about 22.1 and 24.5 degrees with a period of some 40,000 years, with its current value about 23.5 degrees.1 Thus, the azimuth of your June solstice sunset point will vary a degree or so on either side of its current location with the same period. It would not have changed enough in the past 200 years to be noticeable visually.

    If you have a "true" horizon, such as at sea with your viewpoint close to the surface, the June solstice sunset point and the December solstice sunrise point will be approximately 180 degrees apart, with some distortion due to atmospheric refraction. At your location, with the mountains raising the sunset point, that point's azimuth will be shifted to the south, so the two points will be several degrees off of an exact 180.

    At the equator, the solstice sunrise and sunset points are displaced 23.5 degrees from due east and west. At your latitude this displacement is about 27 degrees. I estimated it by plotting the circles on a globe, since my spherical trig is really rusty.

    1 http://en.wikipedia.org/wiki/Axial_tilt
    Wiki not infallible, but this one looks OK for this purpose.

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    Quote Originally Posted by Perikles View Post
    Yes, I suppose that is the case on the equator. The pyramid has a latitude of 16 deg 24', so considering the elevation of the point of sunset it sort of makes sense.
    My bold for reference. Did you mean longitude? My globe shows Tenerife about 28 degrees north latitude.

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    Quote Originally Posted by Perikles View Post
    My question is really whether this direction of setting sun changes sufficiently with time to be evidence for the age of the pyramid, assuming that there is a connection between it and the orientation of the pyramid.
    The Medicine Wheel National Historic Landmark has been found to align with several key astronomical events. For the oldest sections of the Wheel, however, which was built over hundreds of years, the Earth's precession had to be taken into account.

    While attending Rocky Mountain Star Stare 2010, one of the members, a local professor of astronomy, gave a fascinating presentation on the wheel during his archeoastronomy lecture, and included more than 50 slides. It's estimated period of construction ranges between A.D. 1200 and A.D. 1700.

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    I've got a copy of Gerald Hawkin's book, Stonehenge Decoded, wherein he details a remarkable number of 'alignments' associated with that monument. For a while I believed that this complex array of stones had been set up to mark innumerable different celestial events.

    Nowadays the predominant theory seems to be that Stonehenge is aligned with just one event- the Midwinter solstice (not the midsummer one all the latterday pagans and druids go on about). All the other 'alignments' occur simply because Stonehenge is a geometrical figure, probably laid out using string and pegs beforehand. By coincidence many of the stones line up roughly with various astronomical events- but it would be difficult to avoid chance alignments of some kind.

    The solstice seems to be the only definite alignment at many ancient sites- whether midsummer or midwinter is difficult to tell, as they are 180 degrees away from each other.
    Last edited by eburacum45; 2010-Jun-24 at 02:04 AM.

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    Quote Originally Posted by Hornblower View Post
    My bold for reference. Did you mean longitude? My globe shows Tenerife about 28 degrees north latitude.
    Ooops sorry, yes, I misread the Google coordinates. Latitude is 28 degrees 19'. Thanks for your input, by the way. The lines towards midsummer setting and midwinter rising are not quite parallel, as you stated.

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    Perikles,

    The human mind is very good at seeing shapes and alignments in random objects. No doubt this ability was valuable when we needed to to see the wolf in the thicket, but today it causes all sorts of false associations. In the UK the matter of 'leylines' has long been a controversy; ancient monuments, churches (as markes of other ancient monuments), paths are seen as being 'in line' and thus must be markers of ancient power and spirituality.

    In fact, if you put random points on a sheet of paper, you will be able to draw lines between many of them so that three or moe lie on a straight line. See this article in the Wiki: http://en.wikipedia.org/wiki/Leylines

    Also, a pyramid has four sides. They point in four directions. It would be remarkable if at some time of the year, at least one side would not line up with the sunset.

    John

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    As a related side-note: at the Solstice, the Sun sets due North if you are just South of the Arctic Circle. ... And then it rises in almost the same place a few seconds later. So a Henge there would have an interesting set of monolith placements.
    Forming opinions as we speak

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    Quote Originally Posted by JohnD View Post
    Also, a pyramid has four sides. They point in four directions. It would be remarkable if at some time of the year, at least one side would not line up with the sunset.

    John
    I hear what you say about seeing patterns where there aren't any - the constellations are a prime example. The four sides of a pyramid will also point to something at some time. Here however the alignment with the solstice, in both directions (along one wall, and the other a set of 3 markers) is notable. At what point do you say that this is more than coincidence? I'm not trying to find connections here, other than possibly two hundred years ago the pyramid was built deliberately with that alignment by someone with a sense of humour or another agenda.

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    Quote Originally Posted by antoniseb View Post
    As a related side-note: at the Solstice, the Sun sets due North if you are just South of the Arctic Circle. ... And then it rises in almost the same place a few seconds later. So a Henge there would have an interesting set of monolith placements.
    Thank you! All this talk of 180 degrees had me puzzled.

    My coordinates are 45 25′ 15″ N, 75 41′ 24″ W . I'm nowhere near the arctic circle, but the day today is about 18 hours long and from sunset to sunrise the separation is maybe 90 degrees. Sunset for me is NW but I haven't seen sunrise in a bit. I'm thinking it should be NE.

    I was thinking about this the other day. The only time sunset and sunrise should be exactly 180 degrees apart is on both equinoxes. I figure it applies to the entire world, save the poles.

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    Quote Originally Posted by ShinAce View Post
    Thank you! All this talk of 180 degrees had me puzzled.
    ...
    I was thinking about this the other day. The only time sunset and sunrise should be exactly 180 degrees apart is on both equinoxes. I figure it applies to the entire world, save the poles.
    The question was about two different times of year: summer solstice sunset and winter solstice sunrise.

    Grant Hutchison

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    Quote Originally Posted by grant hutchison View Post
    The question was about two different times of year: summer solstice sunset and winter solstice sunrise.

    Grant Hutchison
    Two different times of year and two different times of day. I had to double-check this when I was writing my answer. The 180-degree situation wouldn't work otherwise.

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    Quote Originally Posted by grant hutchison View Post
    The question was about two different times of year: summer solstice sunset and winter solstice sunrise.

    Grant Hutchison
    Thanks - I was beginning to wonder about how I expressed myself.

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    With a level horizon and no refraction, the summers solstice sunset and winter solstice sunrise will be pretty close to exactly 180 degrees apart, from any latitude below the polar circles. There will be small deviations from perfection because the sun is unlikely to be precisely at the solstice point at the time of set and rise, and because the Earth's orbit is not perfectly circular; and tiny deviations because of the slow change in axial inclination.
    You should be able to convince yourself of this by taking an imaginary sky sphere, drawing the equator and tropics on it to mark the equinox and solstice lines, and then slicing it through its centre with an inclined plane to represent the local horizon. The intersection points between solstice lines and horizon plane are symmetrically placed about the centre point, for any plane that crosses the solstice lines. You can verify this by inspection, symmetry arguments (the part of the sphere above the plane is identical to the part below the plane, except rotated 180 degrees), or by some spherical trig.

    Grant Hutchison

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    Thank you very much for that, I shall think about it more carefully. Presumably in my particular case the fact that the lines are not quite parallel is attributable to the elevation of the horizon for the setting sun at midsummer. (I don't suppose there are many places on land where both the rising sun at midwinter and the setting sun at midsummer are at zero elevation.) At any rate, this almost 180 degree effect is independent of latitude, thus making nonsense of any claims that these pyramids were built at a particular latitude to take advantage of this phenomenon. So the alleged almost parallel alignment is not surprising. Interesting, thanks.

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    Well, it appears that Grant is correct. I should have been able to figure that out myself, based on some animations I've used this summer.

    If you're having trouble visualizing what Grant is saying, look at this interactive flash animation. The check-boxes can show the solstice paths of the sun and slider bar will change the latitude of the observer. You can click-and-drag the celestial sphere around to change your view.

    Using that, if I look straight down from zenith, I can see that the paths of the sun above the horizon on the solstices would be a complete circle if you folded the horizon in half along the E-W line. That means that the two points in question will always be 180 degrees apart for anyone between the Arctic and Antarctic Circles.

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    Quote Originally Posted by Tobin Dax View Post
    If you're having trouble visualizing what Grant is saying, look at this interactive flash animation. The check-boxes can show the solstice paths of the sun and slider bar will change the latitude of the observer. You can click-and-drag the celestial sphere around to change your view.
    I am usually quite good at visualizing astronomical movements, but I was struggling with this one. It was not obvious to me that given that the angular range of sunrise during the year varies considerably with latitude, the two directions in question are still 180 degrees apart. That is a very useful link to all kinds of animations, thanks.

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    Newgrange in Ireland is aligned with the midwinter solstice sunrise; Maeshowe with the midwinter solsice sunset. Stonehenge is less definite, and could be aligned with either the midwinter sunset or the midsummer sunrise. Latterday pagans tend to go for the midsummer sunrise, as it is much nicer time of year (usually) to hold a festival; but the definite and unambiguous associations of Maeshowe and Newgrange seem to establish the importance of midwinter in prehistoric times.

    Julian Richards points out that the midwinter solstice has considerable agriculrural significance, the midsummer solstice less so; so maybe the midwinter alignment is the primary one.

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    Quote Originally Posted by eburacum45 View Post
    Newgrange in Ireland is aligned with the midwinter solstice sunrise; Maeshowe with the midwinter solsice sunset. Stonehenge is less definite, and could be aligned with either the midwinter sunset or the midsummer sunrise.
    At Karnak in Egypt, the building is oriented so the dawn sun enters directly into a dark room on midwinter day each year to mark the turn of the seasons.

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