Question about how shadows change throughout the day and year

So I'm trying to make sure I'm thinking correctly about how shadows work as the seasons change vs how they change throughout the day. From what I understand if you put a stick vertically (plumb) & perpendicular in the ground like for a sun dial and then trace the end point of the shadow, the only difference will be the length of the shadow or the size of the arc. The arc will always be facing the same direction but each "ray" will vary in length between the solstices - in the summer they should be shorter thus longer in the winter. We should be able to guess the relative time of year based on the length of the shadow, if it is long enough or you have very accurate measurement device.

So if you go out at the same time every day throughout the year, the shadow from the stick/pole/sundial should always be in the same place but vary in length (excluding issues with daylight savings).

So there is no way that the shadow from stick (like described above) could be off 30-35 degrees at the same exact time of day, from one to another, isn't that correct?

originally posted by: DigginFoTroof
So I'm trying to make sure I'm thinking correctly about how shadows work as the seasons change vs how they change throughout the day. From what I understand if you put a stick vertically (plumb) & perpendicular in the ground like for a sun dial and then trace the end point of the shadow, the only difference will be the length of the shadow or the size of the arc. The arc will always be facing the same direction but each "ray" will vary in length between the solstices - in the summer they should be shorter thus longer in the winter. We should be able to guess the relative time of year based on the length of the shadow, if it is long enough or you have very accurate measurement device.

So if you go out at the same time every day throughout the year, the shadow from the stick/pole/sundial should always be in the same place but vary in length (excluding issues with daylight savings).

So there is no way that the shadow from stick (like described above) could be off 30-35 degrees at the same exact time of day, from one to another, isn't that correct?

It would be if there was no tilt to the Earth's axis. The Earth is tilted on it's axis approx 23 degrees. That tilt results in those lines being separate from the previous lines as the Earth revolves around the sun throughout the year (that is what causes the seasons too).

So, the lines will differ in both length and relative position to each other each day.

ETA: After re-reading your OP, I may have misunderstood your question. Here is a link that describes, what I think, you are asking.
Astronomy with a Stick

a reply to: DigginFoTroof

No, the Earth has a tilt in its axis of 23 degrees (seasonal changes) and also precesses axially (i.e: it has a wobbly oscillation around its axis), as well as rotating and spinning.

The sundial is only approximately the same, with seasonal variances and only on human time frames.

The relative angles at particular times will change due to precessional oscillation with a period of 25,772 years.

This is further complicated by additional variances such as nutation (like precession but caused by gravitational interaction with other bodies like the Sun and Moon) and polar motion (caused by ocean and atmospheric currents and as the molten spinning core of the Earth lags behind crustal movement).

originally posted by: chr0naut
a reply to: DigginFoTroof

No, the Earth has a tilt in its axis of 23 degrees (seasonal changes) and also precesses axially (i.e: it has a wobbly oscillation around its axis), as well as rotating and spinning.

The sundial is only approximately the same and only on human time frames.

The relative angles at particular times will change due to precessional oscillation with a period of 25,772 years.

This is further complicated by additional variances such as nutation (like precession but caused by gravitational interaction with other bodies like the Sun and Moon) and polar motion (caused by ocean and atmospheric currents and as the molten spinning core of the Earth lags behind crustal movement).

Also, didn't the recent plate slip causing the tsunami in Indonesia alter the Earth axis tilt (or did it change the rotation rate)? That would also change the data recorded after that date....wouldn't it?

originally posted by: Krakatoa

originally posted by: chr0naut
a reply to: DigginFoTroof

No, the Earth has a tilt in its axis of 23 degrees (seasonal changes) and also precesses axially (i.e: it has a wobbly oscillation around its axis), as well as rotating and spinning.

The sundial is only approximately the same and only on human time frames.

The relative angles at particular times will change due to precessional oscillation with a period of 25,772 years.

This is further complicated by additional variances such as nutation (like precession but caused by gravitational interaction with other bodies like the Sun and Moon) and polar motion (caused by ocean and atmospheric currents and as the molten spinning core of the Earth lags behind crustal movement).

Also, didn't the recent plate slip causing the tsunami in Indonesia alter the Earth axis tilt (or did it change the rotation rate)? That would also change the data recorded after that date....wouldn't it?

Yes, that would fall under a 'nutational change' which are a little unpredictable due to the number of factors involved and because some of the events (like big quakes) have a pseudorandom occurrence.

a reply to: DigginFoTroof

We should be able to guess the relative time of year based on the length of the shadow, if it is long enough or you have very accurate measurement device.

If you´re going for relative time of year you can use the seasons and the different plants that sprout.

If you want to know the rough time you can use shadow angle and length, I do that all the time. With a compass, you can get it down to minutes.

a reply to: DigginFoTroof

r u serious? think about a tilting planet going around a glowing sphere and that's how shadows behave throughout the seasons.

originally posted by: hombero
a reply to: DigginFoTroof

r u serious? think about a tilting planet going around a glowing sphere and that's how shadows behave throughout the seasons.

Again stop stalking me with your pointless posts. I know you like me, it's obvious. Do you keep a collage of my posts on your wall?

originally posted by: Krakatoa

originally posted by: DigginFoTroof
So I'm trying to make sure I'm thinking correctly about how shadows work as the seasons change vs how they change throughout the day. From what I understand if you put a stick vertically (plumb) & perpendicular in the ground like for a sun dial and then trace the end point of the shadow, the only difference will be the length of the shadow or the size of the arc. The arc will always be facing the same direction but each "ray" will vary in length between the solstices - in the summer they should be shorter thus longer in the winter. We should be able to guess the relative time of year based on the length of the shadow, if it is long enough or you have very accurate measurement device.

So if you go out at the same time every day throughout the year, the shadow from the stick/pole/sundial should always be in the same place but vary in length (excluding issues with daylight savings).

So there is no way that the shadow from stick (like described above) could be off 30-35 degrees at the same exact time of day, from one to another, isn't that correct?

It would be if there was no tilt to the Earth's axis. The Earth is tilted on it's axis approx 23 degrees. That tilt results in those lines being separate from the previous lines as the Earth revolves around the sun throughout the year (that is what causes the seasons too).

So, the lines will differ in both length and relative position to each other each day.

ETA: After re-reading your OP, I may have misunderstood your question. Here is a link that describes, what I think, you are asking.
Astronomy with a Stick

I guess I should have been more specific. I really didn't think I needed to account for 12,500 to 25,000 year differences and how those could effect the position of the shadows. How silly of me to not foresee that (smacks head). And overlooking pole shifts, tectonic plate slippage, yeah another commonality that I should have factored into the daily reading of a sun dial from year to year. ..


I guess I should have said from one year to the next, assuming no asteroid impacts the earth, that a super volcano doesn't ejaculate it's load into orbit, the fact that all the blue whales aren't huddling in some deep water bay somewhere on the planet while all the chickens on all chicken farms are made to jump at the same time I take this reading. I'm sure there are some other possibles I may have missed like the synchronization of all living bovine farting at the exact moment I measure the length of the shadow. Baring all of these above possibilities, the question comes back to basically what I wrote in the first paragraph.

These measurements would be taken on a normal day, let's just pick May 21 (one month before summer solstice - when the sun should be "highest" in the sky and the shadows would be the shortest). So 30 days before (equinox) the shadows would be a little longer and they should be pretty darn close to the same length as July 21 (give or take a day's worth of change).

If the stick cast a 2ft long shadow at noon on the summer equinox it's going to cast a much longer shadow on the Winter equinox, BUT the shadows should align at the same time of day (say 12 noon - discount daylight savings in this example). So if you traced the shadow on both days, the summer equinox noon shadow should fit inside the Winter equinox noon shadow. I'm thinking that this should work for any time of day, that if you traced the outline of the shadow the width should be the same for each "stick shadow" the only difference being the length of the shadow (caused by the tilt/seasons).

That is how/what I understand it to work.

I was trying to find out for certain if I took a reading of this "stick" at the same time every day throughout the year, it should always point in the same spot, the ray should always point in the exact same location - the only difference being the length of the shadow, how far it extends (winter being longer, summer shorter, as explained above).

I hope that can be understood - I do not know if that is correct, but that is what I have come to understand and since I'm not sure, that is why I'm asking. It is kind of important for a project I'm working on because I'm getting some readings of 35-45 degrees off where the shadow is supposed to be - which could mean a difference of mabye 2.5 - 4 hours off (and it is possible I was given the wrong times, so that is why I need to verify this).


Also if you have two identical sticks (as described above), positioned identically (plumb & perpendicular to ground) and they are placed 100 ft apart in an open field, should the shadows they cast be parallel to each other (at least to the eye, maybe there are millionths of a meter or millimeter difference) but for all practicality, they are parallel?