Finding true south at solar noon

[Leo.G (Leo)]

I just read a terrific post on here for finding true south. It involves using Stelarium to find the Meridian.


https://www.iceinspace.com.au/forum/...d.php?t=175771


Since I always just guess accurate south (far from accurate) and have for years, I decided I'd do a little research in a hurry (and then the sun went behind a cloud but came out in time, thankfully).
I was actually spot on once, out of hundreds of times (plus some) setting up my mount (only since the mid 80s). Ah, it was a good night.
Searching for data on the exact time of solar noon I came across the following link after deciding guessing wasn't good enough to play with cameras.
https://www.timeanddate.com/astronomy/australia/sydney

This is a link showing Sydney (my noon was 5 minutes later), the data can be changed to find your exact meridian point (upper right hand side search box for "city or place") and the exact time of solar noon for the day (and time zone I'm hoping with DLS).
Searching for "solar noon" on Google gave me many options for solar panels but not much else.


Hopefully this helps others somewhat when the device they had stelarium installed on has long died and I didn't have time to set it up on another portable device before the shown hour approached.


I now have a string line in the yard held with tent pegs, soon, I'll make a steel T-bar to lay on the ground permanently until I get to doing a cement base for a pier I have.
I'm doing this on a sloping lawn, I don't have a solid concrete or paved area to have prepared but a string line has to be better than my guesses using the chimney on the neighbours house as a guide.
I used a longer earth rod (for electrical) to cast a shadow, it won't be precise due to the grass and slope of the yard but it is definitely going to be closer than my crappy eyesight.

[Startrek (Martin)]

Hi Leo
Thanks for posting
That was me, I put it together including procedures about 6 years ago now
The Sun never lies no matter where you are on the planet
Stellarium will give an accurate solar noon time when the Sun traverses past the meridian. You can also google it as well
Hint : The more accurate your set up and markings are the closer to the south meridian line you will be and hence closer to the SCP.
I use fine builders string for setting up the shadow stick plus an Accumaster digital level. A builders bubble level just can’t give you fine accuracy.
One important trick is to ensure your ground is fairly level otherwise you could be many degrees off.
Cheers
Martin

[By.Jove (Jove)]

Surely a plumbob - piece of string with a pointy weight at the bottom - would be better than relying on any level to get a true vertical gnomon ? Then it doesn't matter whether the ground is not level, or uneven. If you need to damp it so doesn't swing in a breeze, place a dish of cooking oil or fine sand under the plumbob so the tip drags in it. Even a rock on a piece of string would do.

There's another method I've used which is useful in summer when shadows are very short around noon. Choose times a couple of hours either side of solar noon eg. solar noon - 2h and solar noon + 2h, and at the appointed times, trace the shadow cast by the vertical. Using school geometry, bisect the angle between these lines to find south. You can repeat this at hourly intervals as well, eg +/- 1h, +/-2h, +/-3h which then gives several measurements over a day.

[Leo.G (Leo)]

Originally Posted by By.Jove

Surely a plumbob - piece of string with a pointy weight at the bottom - would be better than relying on any level to get a true vertical gnomon ? Then it doesn't matter whether the ground is not level, or uneven. If you need to damp it so doesn't swing in a breeze, place a dish of cooking oil or fine sand under the plumbob so the tip drags in it. Even a rock on a piece of string would do.

The shadow on uneven ground does twist/slant, even a plumbbob will give the same as a stick in the mud. It still has to be better than my align with a neighbours chimney which really isn't even close depending on where I'm standing/plonking down my mount.
I am not in the position to have level ground at the moment, however soon I will bury a deep cube of concrete with 16mm threaded rod welded together to mount a pier I have. It is a rental property but it will only need the rod cut off and a little dirt over the concrete if I ever move out or, a big hole and lots of muscle to remove the cube of concrete (housing department house, they won't mind if I don't tell them (like my second garden shed)).


I was in a hurry today because I want to try out a few cameras a very nice gentleman just gave me. Of course now the sky is totally clouded over so likely not tonight. I still have to build up a compact computer because the old laptop I had died and it appears to be the motherboard (I will eventually get another motherboard for the thing if I can find one-something I do know about and can do properly)


Thank you you Martin!
Great thread which I did find after starting this thread (and put a link to it here), I'm sure it will again help many people.
I had Stellarium on my sons iPad 2 but it's since died, coming in to trying to find solar noon with less than 30 minutes to spare wasn't easy. I expected it would be past 1PM with DLS, no bloody idea.

I have a green laser level I could have used for the correct level of the earth rod (electrical earth stake) but I used a post level for today. I will weld up a 40mm x 3 steel flat bar into an accurate T with holes for the legs (feet) of the mount and stake it to a level patch of ground later in the week, I have the steel, the welding equipment, the tent pegs (the mower should pass right over it and I'll paint the steel with rust proof paint), I just need a day without my complaint (I suffer from chronic cluster migraines on a daily basis and have done so for over 50 years) to get to it. That will be faster than the pier for the moment. I also have a spade and a pick to level the ground.

[Merlin66 (Ken)]

SunCalc does it all for you.....
https://www.suncalc.org/#/-36.7798,1...1.19/14:57/1/3

[h0ughy (David)]

its a great way to find n/S with solar noon - i did it this year at qld astrofest with some hecklers chanting for the clouds. it ended up being a super help to setting up. in future i will try to setup without the need for other humans using a long tent pole and some pegs and ropes to hold it up initially set it in the morning and when solar noon comes then peg and rope the shadow on one side of the pole shadow. everything else can be set from there.three ropes on the pole will keep it vertical and perpendicular top thee surface with the aid of a level.

reducing the need for human help is desirable, less people to laugh at you when you stuff up , isn't that right Brendan

[pmrid (Peter)]

Whether you use a solar noon/stringline or some other method, the reality is that these methods will produce a best approximation only when you are setting up your tripod and scope. You may get within a few degrees by these means but you can't do away with fine-tuning your polar alignment because to be useful for imaging, you will want to be within a few seconds rather than minutes. And to get that precise, you will need some form of computer help. Whether it is old-fashioned drift alignment or more new-fangled SharpCap or ASIAIR PA routines.

[Leo.G (Leo)]

Anything is better than my previously long used "there's the neighbour's chimney, that's south" method.
My problem with drift alignment is I don't have a lot of time from when stars appear over the mountains until they drift behind a tree line or other mountains. I've long heard drift is the best available for accuracy but I haven't been able to mess around with it. One day I'll take my gear (a small scope) to a friends farm and try and perfect the method there.
My main imaging to this point has been mainly lunar and planetary when health and weather permit so long exposure and absolute accuracy hasn't been a major issue but I really would like to capture some nebulae images eventually.
For now I'll just stick with being a Lunartic!