Equinox shadows trace a straight line from west to east

Posted: March 19, 2023 by oldbrew in solar system dynamics

March equinox arriving.
– – –
Equinox shadows are unique, says EarthSky.

Do you enjoy sundials, and shadows? Did you know that – on the day of an equinox, and only on the day of an equinox – the tip of an upright stick’s shadow follows a straight west-to-east path?

If you track the shadow’s tip (aka its terminus) as it moves across the ground on the day of the equinox, you’ll see it tracing out that straight line, as shown – beginning around 00:20 – in the video below:

Full article here.

  1. P.A.Semi says:

    I’m now wondering, what is a time of “Equinox” this year…
    While Wikipedia lists value from USNO as 2023-03-20 21:25, and explains that it’s when Sun is at 180° celestial coordinates or transferring northward over Earth’s equator,
    but from NASA/JPL ephemerides DE422 (or DE405), which use barycentric coordinates, but oriented according to Earth’s equator, the Sun is at 180° and crossing into positive Y from Earth at 2023-03-21 05:03:00 …

    So did they in USNO and Wikipedia tried to “steal” 7 hours 38 minutes from the Equinox ?
    (which reminds me of “coin clipping”, same as they tried to steal outermost “American” planet Pluto from Solar system and as they tried to steal 300 km from Sun’s radius, which I measured 696.04e6 m +-60km, all Solar FITS files from NASA and Stanford use 696e6 m, but IAU recently “decided” it would be 695.7e6 m…)


  2. saighdear says:

    Now I never knew that! something to do in my retirement…make a Sun Dial, but in view of lack of sunshine these last few days, it may just become a Sign dial ! Yes North is up over yon hill over there, that way! and when the Sun kind of shines directly in that direction , it will be dinner time by the time I get back into the house. Good enough. An interesting article, thank you.

  3. solophonola says:

    Dear Tallbloke,

    It is absolutely incredible that every child in school does not get to understand the geodetic importance of the equinoxes, for the technique you describe is ages old and still forms the basis of land surveying and navigation. But.. the sunrise and sunset shadow will only fall true west-east if the horizon altitude is spot on zero degrees from the observer, and then there’s atmospheric refraction to consider, which is variable depending on terrain and local climate. Finally, if the horizon is a long way distant, then there’s also the curvature of the earth to consider, unless one is a flat-earther!

    Like everything in our world, it’s all a little more complex than your headline would have us believe, this business of defining exact east-west. But it is quite easy to know the direction of these two cardinal points approximately (i.e to within half a degree of exactitude). Then there’s another matter, a cultural problem to face. The megalithic culture of Neolithic times appears to have been able to consistently better this half a degree accuracy, and there are many example that prove that statement. For example, the two largest stones at one of England’s largest diameter stone circles, Long Meg, are set nicely east -west. A line from Stonehenge centre to the centre of Lundy Island, in the entrance to the Bristol channel, has exactly the same latitude at 52.178 degrees north. The Welsh name for Lundy is Ynys Elin which means elbow or right angle. A neat right angle turn then brings a northern line cutting through the Preseli hills, source of the Stonehenge bluestones. The three defined points, Stonehenge, Lundy centre and Carn Wen in Preseli form an accurate 5:12:13 pythagorean right angled triangle. This same exact north-south line is worth noting on the map, for it passes on its bee-line up to the Isle of Man and then Scotland, visiting some notable monuments and landscapes.

    If the Stone Age people could understand how to survey accurately over log distances, why then do archaeologists not recognise this ability, and own what is evidently a megalithic science which formed the basis of our modern sciences.The noted archaeologist Dr Aubrey Burl suggested there was ‘wilful prejudice’ in many archaeology departments. A modern version of a flat earth club.

    Our present model of prehistory is thus incomplete, for there’s a lost legacy here that, for whatever reason, our present culture seemingly wants to avoid looking at. Perhaps that it why there are so many documentaries about Stonehenge and the Stone Age in general that really have little to tell us about the capabilities of the megalith builders.

    Folk need to go on talking and writing about it. Thanks for bringing the subject up.

    Researcher into ancient wisdom, author and presenter.


  4. P.A.Semi says:

    When you construct Sundial, the rod should point to Polaris star…
    When I live at 50°N, it’s inclined 40° from straight zenith direction…

    Then there is “equation of time”, or “Analemma” https://en.wikipedia.org/wiki/Analemma , which means, that solar time differs from civil time by +-18 minutes (or 15 and 18 minutes in either direction), that tracing Sun at civil noon paints shape of 8, not a straight up-down line… It is caused by the elliptic orbit being slower and faster during year, and another difference due to sinusoid movement of Sun having different “horizontal” speed, while the Earth spins at quite constant rate… The orbit subtracts 1 day from year (there are 366.25 “days” per year relative to stars), and the speed, at which it subtracts daily, is different at various parts of year…

    So one may say, that Sundial is not exact by +-18 minutes, or rather that civil time is not exact and true length of day is not constant 24 hours…

    We have a cca 3 meter sundial on the garden with gravel floor circle, cca 2 meter rod, and mom planted flowers at hours… (but now the flowers are mostly hidden in grass weed)


  5. P.A.Semi says:

    Reading Robin Heath’s article https://robinheath.info/royal-sacred-landscape-geometry/
    and relating to my claim of “coin clipping” above…
    So did they actually stole 1/8 of Royal mile while giving it to peasants ?

    Otherwise – metric system with decadic scaling is quite superior to the ancient “mile” systems with strange steps from foot to yard to mile…

    Also, the meter is 1/10e6 of quarter of meridian circle (i.e. distance from pole to equator), it’s the measure defined by Earth size… What is some Earth-based or astronomic measure of a mile, beside being a royal caprice and a primitive medieval remnant ?

    And the most ridiculous on this is, that Americans, who claim to have a Democracy, are still subject to this backwards medieval royal crazy measure system…


  6. oldmanK says:

    The line will be east to west only if it is at the equator. Otherwise atmospheric distortion and the earth’s curvature (refraction) will give some error (minimal, but still enough to give the wrong date/time).
    Eg. At 35deg north, equinox sunrise is about 1/2 degree south of east.

    There are better ways of finding east-west directions, and at most times of the year up to mid-latitude. Ancient methods were more accurate.

  7. P.A.Semi says:

    So after some discussion on Wikipedia Talk page and reading that wiki/Equinox article more carefully, I came to a conclusion, that they’ve stolen those few hours from true Equinox by redefining the “Equinox” by average ecliptic position, defined by EMB (Earth-Moon Barycenter), instead of true vector positions of the celestial bodies Earth and Sun…
    A Nifty Theft, could be said…

  8. P.A.Semi says:

    So it was my fault – the equatorial coordinates in JPL Ephemerides drift from true Equator by cca 20 minutes each year, making one year difference in cca 26000 years…
    So it’s actually the “Precession” who steals 20 minutes from every our year…

  9. oldbrew says:

    Equinox on March 20 means more stunning auroras are coming. Here’s why
    By Rahul Rao published 2 days ago

    Scientists have long tried to understand what ties auroras to the calendar.


    And they’re still trying to understand.

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