Finding True South - Solar Noon Shadow

[Startrek]

I’ve posted this once or twice over the years ( not sure if it’s a sticky )
Here’s a proven way of finding True South using the Sun’s shadow. It’s been around for centuries…….
It’s particularly handy if you don’t have a permanent set up and you have to plonk your tripod or base ( EQ or AZ ) down in the yard or at a remote site each observing or imaging session.
As long as you have this True South line and the latitude in degrees and mins of your location, then you will end up fairly close to the SCP which is a good starting point for any polar alignment method. I’ve actually “fluked it” a couple of times over the years after setting up the EQ tripod and rig and ended up within 20 to 30 arc mins of the pole ( according to my PA software )

See attached

Martin

[Leo.G]

Martin I have a piece of 1" galvanised water pipe lays across my yard in a small trench after marking wit string and two tent pegs using exactly this method and a T bar I welded up out of 40mm x 3mm flat bar which intersects to give me accurate positioning of the rear legs of my tripod. Only till I eventually get my pier in but it means I can take my astro trac or mount out any night and be close enough for my purposes. The one mention which may or may not show in Martins diagram is you want to do it on level ground otherwise the shadow will twist and be off a little. In my case with a sloping yard I set up some marine ply and levelled it first to get the suns line at precisely solar noo0n for my precise latitude and level the ply for better accuracy.
Your diagram may mention this point, I'm just full of sleeping medication and can't read it at the moment.
Solar noon is not midday and I recently came across a site which allows you to punch in your address and print out a solar noon chart for the entire year, I;ll see if I can find the bookmark tomorrow. and post a link.


Found it:


http://www.solar-noon.com/

[Startrek]

Leo,
The attached procedure notes state level ground on 2 occasions
Notes to be read in conjunction with diagram
Martin

[OzEclipse]

You can also use google earth to find true south:

https://joe-cali.com/astronomy/articles/Google-Earth-Polar-Alignment/index.html

Google Earth has a ruler tool which rules a line and gives a precise bearing from your precise location to a nearby landmark.

The advantage of this method is that if you don't have a landmark to the south, you can take an exact bearing to any landmark in any direction then make an alignment jig to set against your mount and sight to the land mark. It does not rely on being ready at solar noon. Can be used anytime you can see your surrounds. My surrounds are so dark at night that I can't see anything so this has to be done at twilight at the latest. In urban environments, you may be able to use it at night.

In the web article above, I used a large sea wall south of Shanghai that was 64 deg east of north to align 3 equatorial mountings belonging to members of the group.

When chasing objects, especially comets low on the horizon, I often have to set up and polar align during the day. I have also used this method extensively during overseas solar eclipse chasing and I have a wedge for my favourite quiet country lane that gives me a zero degree western horizon for evening comets. The edge of the lane is 27.8 deg west of south and I have a 27.8 degree sighting wedge that I use from that location.

Joe

[Leo.G]

I did mention I was full of sleeping medication and neither my eyes nor spelling were working very well at all. I should have been in bed.

[AstralTraveller]

Leo, I use a pretty similar approach when setting up to visually observe variables. In my case I have three square pavers with a hole drilled in the centre of each into which the tripod legs sit. Obviously even very rough alignment is good enough for that purpose. However I'm thinking of moving to imaging and I'm wondering whether this 'carry and plonk it down' method is good enough to allow imaging for 1 minute at 200mm focal length without trailing. So I'm interested to know what you mean by 'close enough for my purposes'. It might answer my question. [Obviously the mount will have been aligned quite well once and just be carried in and out of the garage.]

Joe, that's a good idea. I can't use it in my backyard as I'm surrounded by trees, my garage and house and the neighbour's granny flat. I have no horizon but in suburbia it works well in keeping light out of the backyard. Don't get me wrong; the yard is well lit by skyglow :mad2: but no direct light most of the time. However it may be useful if I ever the clouds part when there is a comet about.

Martin, one you have the line on the ground how do you align the RA axis along that line? Do you just sight it or is there a less subjective way to make sure they are parallel?

I just did a three-minute proof of concept. I can see the shadow cast by the string of a plumb bob so why not just hang the plumb bob from a tripod? Survey's tripods are made so that a plumb bob can hang exactly below the centre of the tripod, so it can be positioned directly above a benchmark. But if you only want a N-S line that isn't mandatory.

[Startrek]

Martin, one you have the line on the ground how do you align the RA axis along that line? Do you just sight it or is there a less subjective way to make sure they are parallel?


David,
I just use geometry to align the tripod to the true south line then set home position on the mount.

See below and attached

Setting up a Tripod to Align to True Celestial South

NB: This procedure is based on having your True South line already marked on the ground from a previous procedure ( length of true south line marked on ground approximately 500mm long or alternatively 2 fixed points marked on the ground 500mm apart)

1/ Open your tripod fully out and extend all 3 legs out by about 100mm then lock them
2/ Place foot of front tripod leg marked “N” directly on True South line and roughly line up the 2 rear tripod legs equally spaced in respect to the True South line
3/ Level the base platform of your tripod by adjusting the legs in or out as required .Check level of the tripod base in both north/south and east/west directions using a good quality builders aluminium bubble float level.Level the base as accurate as you can then lock the tripod leg adjusters nice and tight.
4/ Extend out your True South Line along the ground just past the intersection of the 2 rear tripod legs using a string line, aluminium angle or a straight edge. The feet of both rear tripod legs must be equidistant from the intersection of the extended True South Line. When adjusting make sure you only move the 2 rear tripod legs left or right whilst keeping the front tripod leg “N” stationary or pivoting but not moving off the True South Line
5/ Once the 2 rear tripod legs are equidistant from the true south line , the tripod is now set up level and pointing to Celestial True South
6/ Ensure you take care in setting up the tripod and measuring accurately to the nearest millimetre.This will assist the accuracy of your polar alignment procedure.

Cheers
Martin

[OzEclipse]

David,

The method works even in a limited space like your backyard. You don't need a distant horizon. You should be able to measure the compass bearing of structural components in your back yard, path, driveway, house walls, shed and reference your telescope to that. I use photoshop to draw two lines, one for the side of the scope, one to eyeball on the structural component. I print that, glue it to a block of foam core then cut accurately with a steel rule and snap-blade knife. If you want to use it repeatedly, cover it with clear contact.

cheers

Joe

[skysurfer]

Simple method, but do you realize that noon is affected by equation of time which changes several minutes over the year ? Any astronomy app will show the true noon time on this day for your location.
Where is is meant for ? You don't have unobstructed view to the pole star in the south ?
If you have an EQ mount with setting circles and a star charting app displaying ephemeris it is much simpler. I can align the scope using a two star alignment using the Sun and Moon or Venus. When the latter two are not available, I use the zenith as 'star' with hour angle of zero and declination equal to your latitude.

It even works on a simple balcony which just fits your scope and tripod.

[leon]

We don't have a Pole Star as such it is very faint Star Light in that direction.

Leon