align to North

[miguel87]

You can see the curve. The lines for the first and last hour would point in completely different diections

[miguel87]

1 minute ago, Anthonyexmouth said:

1. dont worry about the line of the shadow, its ONLY the tip you need, and that ALWAYS follows a STRAIGHT line. It works within the arctic circle, why woudn't it? 

2. It's VERY accurate if done with a thin/pointed stick. 

It wouldn't because in summer the sun doesnt set, so it would HAVE to draw a circle not a line

[City9Town0]

The end points of the shadow are marked at the start and the end of a ten/fifteen minute period....... The straight line between these points is the East-West line (within a degree of so...)

The error direction is known so a bit of correction can be added with a smidgeon of guesswork. I'll be within 1.5 degrees...

[miguel87]

32 minutes ago, City9Town0 said:

The end points of the shadow are marked at the start and the end of a ten/fifteen minute period....... The straight line between these points is the East-West line (within a degree of so...)

The error direction is known so a bit of correction can be added with a smidgeon of guesswork. I'll be within 1.5 degrees...

No, as you can see from the graph and if you read the section on error calculation, this method could result in an error of over 40 degrees at a high latitude near to summer winter solstice.

Above the artic circle (imagine doing it at 1am till 1.15am in mid summer) it could be 180 degrees out as the sun travels back east from west via north!

Edited April 7, 2020 by miguel87

[miguel87]

If you happen to live not too far north or south and you use this method during the middle of the day it is a reasonable east west guide.

But it is not accurate and certainly not universal. I wouldn't use it to align a telescope for astrophotography!

I dont intend to argue I just know it doesnt work and I dont know how else I can explain it to you.

[City9Town0]

Thank you for your explanation... the points were not new to me but I appreciate your patience and persistence. The thread was about  aligning the HEQ mount. The method is most certainly good enough for that... refinements such as knowledge of errors may even help....

This would get close enough to Polaris to start a polar alignment...

Anyway, I wait until sharpcap can find a few stars and away I go!

I won't go into compass errors due to friction or anomalies in the Earth magnetism if you want to be accurate... ( I do know what accurate means)

This is not helping the OP, to whom I apologise.

End of.

[miguel87]

21 minutes ago, City9Town0 said:

Thank you for your explanation... the points were not new to me but I appreciate your patience and persistence. The thread was about  aligning the HEQ mount. The method is most certainly good enough for that... refinements such as knowledge of errors may even help....

This would get close enough to Polaris to start a polar alignment...

Anyway, I wait until sharpcap can find a few stars and away I go!

I won't go into compass errors due to friction or anomalies in the Earth magnetism if you want to be accurate... ( I do know what accurate means)

This is not helping the OP, to whom I apologise.

End of.

Dont give me end of haha. And dont say that it is still accurate enough to align a mount or find Polaris. If you angle your scope 50 degrees off north you will definitely not be able to find Polaris. 

I can give you a specific location and time and degree of inaccuracy if you like?

I already did that will the artic circle example.

Even if you did it in london on summer solstice at 9pm just as you were setting up to observe, you would be over 40 degrees off!

✌

[miguel87]

The black arrow illustrates the axis of earth's rotation. So as you travel north and this line become increasingly perpendicular to the ground, you can see that the resultant hyperbole exists at an ever shallower angle and casts a steeper curve on the ground.

The relative angles cast on the ground at opposite ends of the curve are vastly different. If you set the north tripod leg of a telescope at a right angle to your supposed east-west line you can see how it would face in drastically different directions.

During the equinox the maximum amount of error (using the method at sunrise or sunset) moves with your latitude, from being 0 degrees out on the equator to 52 degrees out where I live in Swansea and so on further north

If you add in the effect of the seasons these errors would be even greater.

Edited April 7, 2020 by miguel87

[michael8554]

You guys are making it so hard.

Open your planetarium, check it's got your location correctly entered, fast forward to tomorrow.

Click on the Sun

Click on info

Sun transits at 12:09 tomorrow.

Get your stick ready and mark the shadow at 12:09

Simples.

And yes, that worked good enough to permanently install my mount and have enough range to Polar Align.

Michael

[miguel87]

2 minutes ago, michael8554 said:

You guys are making it so hard.

Open your planetarium, check it's got your location correctly entered, fast forward to tomorrow.

Click on the Sun

Click on info

Sun transits at 12:09 tomorrow.

Get your stick ready and mark the shadow at 12:09

Simples.

And yes, that worked good enough to permanently install my mount and have enough range to Polar Align.

Michael

It's currently near the spring equinox, we are at a moderate latitude and you chose the middle of the day. So it will work pretty well tomorrow 👍

That's my whole point. Depends when and where you do it.

What I dispute is that it works whenever the sun is up, all over the planet.

It can be very wrong.

Edited April 7, 2020 by miguel87

[michael8554]

"Solar noon happens at your location when the Earth's rotation brings your local meridian to the side of the planet that faces the Sun. From your perspective, the Sun, after having steadily gained altitude since sunrise, now reaches the top of the arch that its journey describes in the sky every day. At this moment, it appears due south, due north, or in the zenith position exactly above you, depending on your latitude and the time of year."

Good enough ?

Michael

[miguel87]

Just now, michael8554 said:

"Solar noon happens at your location when the Earth's rotation brings your local meridian to the side of the planet that faces the Sun. From your perspective, the Sun, after having steadily gained altitude since sunrise, now reaches the top of the arch that its journey describes in the sky every day. At this moment, it appears due south, due north, or in the zenith position exactly above you, depending on your latitude and the time of year."

Good enough ?

Michael

I think you are having a different conversation to us.

We are talking about how accurate and universal the stick-tip east to west shadow line is.

 

[michael8554]

Excuse me? In your reply to my post you disputed that it would work whenever the sun is up, all over the planet.

I never mentioned the East-West shadow line, as it's not necessary to spend time doing it.

Michael

[miguel87]

You just came into the conversation and told us that at noon the sun casts a shadow which points south. Yeah, we all know that.

It's just completely off topic.

I'm not disputing what you say at all, it's just not what we are discussions. We are not trying to find a method. We are discussing the usefulness of a particular method.

[miguel87]

If at any time of day anywhere in the world, you trace the shadow of the tip of an upright stick. Mark two points a period of time apart say 30 mins or so, draw a line between the points.

Does that line point accurately east and west and therefore be used to find true north?

[michael8554]

28 minutes ago, miguel87 said:

You just came into the conversation and told us that at noon the sun casts a shadow which points south. Yeah, we all know that.

It's just completely off topic.

I'm not disputing what you say at all, it's just not what we are discussions. We are not trying to find a method. We are discussing the usefulness of a particular method.

Anthony suggested a method of finding true north.

It went off-topic with your meanderings into why it wouldn't work.

You had the chance to say that the sun was south at local noon, but you didn't.

Michael

[Merlin66]

The shadow technique needs to be done around noon.

Suncalc gives you a direct measure of your (north-south) meridian. Just input your location.

https://www.suncalc.org/#/-38.1667,144.7167,3/2020.04.08/08:30/1/0

 

 

 

[miguel87]

6 minutes ago, michael8554 said:

Anthony suggested a method of finding true north.

It went off-topic with your meanderings into why it wouldn't work.

You had the chance to say that the sun was south at local noon, but you didn't.

Michael

I think If I told him the sun was south at noon he probably would have taken it as an insult.

Somebody said that they knew a very easy and very accurate way of doing it.

I explained that It wouldn’t work.

another person also backed the idea and I showed some evidence for my opinion.

The original poster was looking for an accurate method.

What did you want me to? Lie and say yeah it will work fine?

Just because they persisted in saying it was accurate isn’t really my fault. Surely I have the right disagree?

 

In fact I pretty much did say it works at noon because I said it depends what time you do it and could be wrong at sunrise and sunset??

Edited April 7, 2020 by miguel87

[miguel87]

3 hours ago, Anthonyexmouth said:

it works all days, any day. as long as the sun shines. 

im not talking about the direction of the shadow, just the very tip of the shadow. that tip travels in a due east direction as the day progresses. 

This.

It’s not true, and that’s literally all I said.  But multiple times they kept persisting that it is accurate to around 1 and a half degrees.

[MarkAR]

Here's another one.

Draw a circle as close to the diameter of the spread of the tripod legs as you can, also mark the centre spot. Marker on the end of a piece of string and something tied the other end for the centre point.

Lay a straight edge across the centre point with phone compass pointing north. Mark where it crosses the circle a north and south.

Put the centre pointer on the north mark and draw an arc right across the circle. Move centre pointer to next mark and repeat until you have a six pointed daisy wheel.

Place tripod on the 3 most appropriate points.

Get kids to colour in the flower.

 

[JamesF]

I'm inclined to agree with Miguel that the "joining two shadows of the tip of a stick" method can't work in the general case.

However, I also suspect that the general case may not be particularly relevant.  For example, never having tried it myself, I'd guess that it's only a practical method to use when the shadow is relatively short and that will perhaps be reasonably close to noon.  Attempting it half an hour after sunrise say, would mean the shadow of the tip of the stick was so far away as to make the method completely impractical.  For the same reason it won't be very practical if the Sun doesn't get sufficiently high in the sky.  It is perhaps plausible therefore that under conditions when it is practical to perform this method that the arc described by the shadow of the tip of the stick is sufficiently indistinguishable from a straight line as to be good enough to give an adequately accurate approximation to the east-west line required.

I also find myself wondering whether, were one to draw a circle centred on the base of the stick that passed through the first position of the shadow and wait for the shadow to cross the circle again, that joining the two points where the arc of the shadow and the circle intersected would definitely give a true east-west straight line (again, assuming it was even a practical method to attempt in the local conditions).

It's far too late for me to want to do the maths to attempt to work out the errors involved though

James

[Davey-T]

In the past I've used a long spirit level across the rear tripod legs with a wooden set square pointing north and rotated the rear legs to point the set square north with a compass, you could of course just draw lines on the ground this way and then stand the tripod on them if its a solid surface, no Sun needed as you can't rely on it shining when you need it 😂

Dave

[Freddie]

Test underway in back garden!