Equinoctial sunrise 3000 BC is inaccurate

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    Originally from ticket #14665.

    Using CSAP for archaeological research I noted that equinoctial sunrise in 3000 BC did not appear to be due East although the present day position seems to be correct. This has caused serious concerns about how accurate is the depiction of the night sky in the distant past. I would very much welcome your comments on this.

    Hi Malcolm,

    Please read this Kb article:

    https://support.simulationcurriculum.com/entries/20723932-Archeoastronomy-Mathematical-Accuracy-Starry-Night

    In short, you should not use Starry Night as your only source of data for archeoaastronomy. Cross references with NASA and other reputable astronomy sources.

    Hope this helps to answer your query.

    Keiron Smith
    Starry Night & Sky Safari Software Support Team

    Keiron,

    Many thanks for your reply explaining the problems involved in accurately recreating the night skies of the distant past

    I did however run another check on sunrise positions this time using, just for fun, the Stonehenge panorama with a date of 2000 BC. Oddly, the June and December positions were correct but sunrise at the equinoxes in March and September were in two quite separate places instead of the same one.

    Thus the movement of sunrise along the eastern horizon (and the equivalent sunsets) must be inaccurate. This would seem to be a fairly straightforward issue. Is there any chance of a fix?

    With best wishes

    Malcolm Taylor

    Hi Malcolm,

    You are welcome to submit your issue with supporting documentation that includes the following:

    1. A step by step description of every action taken in SN to achieve your specific result.
    2. Screenshots of the result achieved - including all significant data such as location, time, date, etc.

    We have to be able to reproduce your result exactly as you have reproduced it.

    Keiron Smith
    Starry Night & Sky Safari Software Support Team

    Hi Keiron

    Thanks for your email. Herewith the steps taken using CSAP which seem to produce inaccurate equinoctial sunrises.

    1. Set Home Location - Stonehenge, England. Lat. 51 degrees 4 mins N, Long. 1 degree 48 min W.
    2. Select Horizon Panorama - Grass (for an unobstructed view).
    3. Save as Home Location.
    4. Click on Home on toolbar.
    5. Change date to March 21 2500 BC.
    6. Select E orientation from toolbar.
    7. Click on Sunrise from toolbar.

    Sunrise appears to be approximately ESE instead of due E.

    8. Change date to September 21 2500 BC.
    9. Click on Sunrise from toolbar.

    Sunrise appears to be approximately ENE instead of due E.

    I have included a couple of images to illustrate the above. I'm not sure how to attach them so I hope they reach you.

    Best wishes

    Malcolm Taylor

    Keiron, I think I have managed to attach the images properly. The first one is March 21 2500 BC and the second is September 21.

    On Wednesday, 16 April 2014, 10:16, Malcolm Taylor <tayloms@yahoo.co.uk> wrote:

    Hi Keiron

    Thanks for your email. Herewith the steps taken using CSAP which seem to produce inaccurate equinoctial sunrises.

    1. Set Home Location - Stonehenge, England. Lat. 51 degrees 4 mins N, Long. 1 degree 48 min W.
    2. Select Horizon Panorama - Grass (for an unobstructed view).
    3. Save as Home Location.
    4. Click on Home on toolbar.
    5. Change date to March 21 2500 BC.
    6. Select E orientation from toolbar.
    7. Click on Sunrise from toolbar.

    Sunrise appears to be approximately ESE instead of due E.

    8. Change date to September 21 2500 BC.
    9. Click on Sunrise from toolbar.

    Sunrise appears to be approximately ENE instead of due E.

    I have included a couple of images to illustrate the above. I'm not sure how to attach them so I hope they
    reach you.

    Best wishes

    Malcolm Taylor

    Hi Thomas,

    Thanks for all the info and screenshots.

    Here is what our resident expert has said about your finding. Everything in Starry Night is as it is supposed to be.

    "Basically, the sun rises due east in, and sets due west in, the spring and fall equinoxes - when both hemispheres are literally equally - in the winter solstice it sets as far to the southeast and in the summer solstice as far northeast as it can get. This is due to the earth’s tilt.

    http://physics.weber.edu/schroeder/ua/SunAndSeasons.html

    Does this help?

    Keiron Smith
    Starry Night & Sky Safari Software Support Team

    Thanks for the reply, Keiron, and the attachment, but I'm afraid your resident expert isn't correct. Everything in Starry Night CSAP is not quite as it should be. I would ask you to try a very simple experiment.

     Using Stonehenge as Home Location and current time, simply change the date to March 21 and select Sunrise. The sun rises due east. Change the date to September 21 and again the sun rises due east. This is correct.

    Now change the year to 2500 BC and try the March and September dates once more. The sun rises in two very different locations either side of due east. This is clearly in error and of course has a knock on effect.

    Hope this makes my concerns clear.

    Yrs

    Malcolm Taylor

    Hi Malcolm,

    "Now change the year to 2500 BC and try the March and September dates once more. The sun rises in two very different locations either side of due east. This is clearly in error and of course has a knock on effect."

    This is due to precession.

    Please read here:

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

    Part 2: Effects.

    ok?

    Keiron Smith
    Starry Night & Sky Safari Software Support Team

    Hi Keiron,

    This is turning into a bit of a saga. I would respectfully point out that precession merely changes the background of stars against which the sun rises.

    As regards the place where the sun rises or sets from an Earth viewpoint, I would refer you to the still extremely accurate marking of the December solstice by what remains of the great trilithon at Stonehenge. And the (perhaps less intended) June solstice by the Heelstone. Sunrise and sunset positions do indeed change but over a period of say 5000 years the amount is extremely small.

    The summer solstice sunrise at Stonehenge in 2500 BC is a fraction less than 50 degrees azimuth. The winter solstice is a fraction over 130 degrees. Over a year the sun travels through an angle of 80 degrees. Halfway along in either
    direction of course it has to rise in the same place: due East.

    In this context the phenomenon of precession is irrelevant.

    In 2500 BC the equinoxes were effectively the same as now: March 21 and September 21. Starry Night places them about April 4 and October 19.

    I sincerely hope we are making some progress. I do appreciate your responses and of course none of this is criticism but simply an attempt to make Starry Night as accurate as possible.

    Regards

    Malcolm Taylor

    Hi Malcom,

    There are a few reasons why seasonal dates can vary from year to year.

    1) A year is not an even number of days and neither are the seasons.
    To try and achieve a value as close as possible to the exact length of
    the year, our Gregorian Calendar was constructed to give a close
    approximation to the tropical year which is the actual length of time
    it takes for the Earth to complete one orbit around the Sun. It
    eliminates leap days in century years not evenly divisible by 400,
    such as 1700, 1800, and 2100, and millennium years that are divisible by
    4000, such as 8000 and 12000. This explains your April date for 2500 BC
    (Julian date 808399.07263, roughly March 11 without gregorian calendar -
    SN displays it as April 11).

    2) Another reason is that the Earth's elliptical orbit is changing its
    orientation relative to the Sun (it skews), which causes the Earth's
    axis to constantly point in a different direction, called precession.
    Since the seasons are defined as beginning at strict 90-degree
    intervals, these positional changes affect the time Earth reaches each
    90-degree location in its orbit around the Sun.

    3) The pull of gravity from the other planets also affects the
    location of the Earth in its orbit.

    Regards,
    Pedro Braganca
    Education Director

    Hi Pedro,

    Many thanks for your reply. I'm afraid the problem does not quite appear to be resolved. Perhaps I could l address some of the points you make.

    1. With respect, I think that the Gregorian/Julian calendar issue is a little bit of a red herring. We are merely referring to convenient labels. If by some quirk of calendrical extrapolation the vernal equinox in 2500 BC were to fall on the day we describe as April 11, then the solstice dates would also have changed. This does not appear to be the case: the label attached to the summer solstice for example is June 21. This is of fundamental importance.

    2. Having checked once again using the Stonehenge location for 2500 BC, the vernal equinox appears to be about April 10, and the autumnal equinox about October 13. This gives roughly even periods between equinoxes via both the summer and winter solstices. (Professor Alexander Thom states that the movement of the perihelion made them exactly equal in 4040 BC but today there is a difference of about 7 days.) So far so good. However the time taken from summer solstice to equinox is 114 days, and from equinox to winter solstice 70 days when they should both be somewhere in the low 90s.

    3. The issue of precession. As you know the angle of obliquity of the ecliptic is currently 23.45 degrees. In 2500 BC it was 24 degrees. The precessional effect on the position of sunrise is about 0.2 degrees every thousand years. Thus in the last four and a half thousand years the sun has 'moved' by less than one degree. The layout of Stonehenge confirms this.

    In summary, this means that in 2500 BC the sun should follow a path almost identical to its current one which doesn't appear to be the case on CSAP.

    If there are any errors of fact or logic in the above, I will readily accept any corrections, but for the moment I'm afraid my opinion remains unchanged. I would sincerely welcome your comments.

    With best wishes,

    Malcolm Taylor.

    Hi Pedro

    A month has elapsed since my last email re. the seemingly inaccurate depiction of the equinoctial sun for dates around 2500 BC.

    I took on board the points you made and tried to deal with each one of them and assess how they might affect the problem.

    I hope you were able to read my response. It does seem that the issue is by no means resolved and I would very much appreciate any further feedback you can give. Is it simply a slight error in the software programming?

    Regards

    Malcolm Taylor

    Thomas,

    I've tried to follow this thread to figure out exactly what we're disagreeing about.

    It seems as though at some point, you agree that the Equinox in 2500 BC is in fact on (or around) April 11 (and not on March 21 which I believe was your original assumption). On that day (in Stonehenge) the Sun, as rendered by Starry Night, correctly rises due East.

    The reason for the difference in dates is mostly due to a change in the way that leap years are handled... The Julian Calendar (which Starry Night uses pre-1582) applies a leap year every four years, period. Indeed the reason the Gregorian calendar was created was to re-align the equinox with March 21, so we should expect that earlier times in history won't match that label.

    I guess at this point there's some disagreement then about the Solstice dates? I apologize for having you repeat yourself, but it's essential that I understand exactly what you're expecting to see, vs. what we're actually displaying, so that I can determine if there really is an error somewhere.

    Regards,

    Dave

    Technical Director - Simulation Curriculum Corp.

    Dave,

    Many thanks for your email. (I'm Malcolm by the way not Thomas.)

    You have provided a single piece of information which clarifies everything, namely that Starry Night uses only the Julian calendar pre-1582 which I was not aware of. Thus equinoxes and solstices ALL occur roughly 20 days 'later'. I wish this had been made clearer - perhaps somewhere in the user's guide.

    And talk of precession and the gravitational effects of other planets I'm afraid merely served to cloud the issue.

    (I must add however that it does complicate archaeoastronomical research a little and I would much prefer the use of the Gregorian calendar throughout with perhaps an app for calculating the Julian date in historical times when needed.)

    Nevertheless, thanks again for sorting things out.

    Malcolm Taylor

    Comments

    • Avatar
      Glastonbury Magic

      Does the same calendrical consideration apply for the SkySafari 4 Pro Mac software?

      I would be interested to know what parameters to enter (Calendar system, etc) in order to view the equinoxes and the solstices for 3,000 BC.

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