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originaltrilogy
20-08-2012, 09:42 AM
If Solar Noon is at 12:24 does that mean at 9:24AM and 3:24PM the shadow will be true east-west?

I need east west line not North-South.

Poita
20-08-2012, 10:30 AM
I'm pretty sure the shadow you want is at 6am and 6pm, not at 9am or 3pm if you know what I mean.

So if your solar noon is 12:24 the shadow of a vertical object wil fall North South (i.e. the shadow on the ground will point south.)
The shadow would be pointing East-West on that same day at 6:24am or 6:24pm or thereabouts.
I think it is how it works?

mithrandir
20-08-2012, 10:49 AM
Poita is more or less right. On any day the sun rises and sets at the same delta azimuth from north. Only on the solstices will they be truly east and west. (Even those statements are not exactly true. The Earth's position changes during the day but the azimuth change is within any margin of error in your measurements.)

A line between points equidistant along the shadow say 4 or 5 hours before and after solar noon would be very close to east-west and avoids objects blocking your horizon. Good enough for most purposes.

Alternatively, make up a 3-4-5 triangle and use that to give you a line at 90 degrees to the solar noon shadow line.

dannat
20-08-2012, 11:15 AM
if solar noon is 12:24, you need an angle 90degrees from that, at 15 deg per hour = 90/15 = 6hrs. 12:24-6 = 6:24. or add 6 to give 18:24.

Due to the problems of twilight i would be using a long stick to mark the solar non point - then use a builders protractor to measure a 90 deg angle form this

Lastly r u in Sydney? solar noon is measured from the east coast (sydney Obs i think)- if you're west of this you need to make a small longitude adjustment - write back where you are if you want me to do this

cathalferris
20-08-2012, 12:07 PM
At or very close to the equinoxes, the sun will rise pretty much due east (within maybe 2 degrees allowing for atmospheric refraction).

No matter where you are in the world, at least from local spring through local autumn between the equinoxes, the sun *will* be able to cast a shadow due east or west. From local autumn through local spring, (for southern latitudes) the sun has to rise between east and south, and sets between west and south, never passing the east-west line so during local winter you can never get a shadow falling true east or west.

It's not as easy to calculate as north-south at local solar noon, as you start getting into fun trigonometry, but any planetarium app for PC or smartphone should allow you to check it out. Look at the alt-az coordinates of the sun, and when the azimuth is at 90 or 270, the shadows will be due west when sun is at azimuth of 90, and due east when the sun is at an azimuth of 270.

gary
20-08-2012, 12:13 PM
Hi Petr,

No it doesn't :)

Since the furthest south the sun ever gets is the Tropic of Capricorn during
the December solstice, it never gets exactly overhead at the zenith at your location
at Bathurst.

Likewise, from what compass bearing it rises and sets throughout the year

For example, at this time of the year it will be rising in north-east and setting in
the north-west. Come December, it will rise in the south-east and set in the south-west.

Around September 22nd/23rd this year it will appear to rise in the East and set in the West.

As has been suggested, the sun will cast a shadow to the south as it crosses
your local meridian. Use a set square to determine East-West from this line.

originaltrilogy
20-08-2012, 12:21 PM
Thank you to everyone.
I now go find a very big protractor!

Barrykgerdes
20-08-2012, 01:14 PM
Use a planetarium program like Stellarium set for your correct latitude and longitude.

Enable azimth grid and use the ctrl - or ctrl = to cycle the time backwards and forwards. In the winter you will see that the sun is never due east or west. At the equinox the sun will rise and set in the east and west. In the summer the sun will rise and set south of the east and west points so it will cast a shadow east west at some time.

That time can be read directly from the display as the time the sun is on the 90 or 270 grid line. At this time the shadow will be East or west.

Barry

Regulus
20-08-2012, 02:05 PM
There are a few compass programs for Android phones that can be set to show either True north or Magnetic north. There is one simply called 'Compass' that can be set for True north.
I haven't tested them for absolute accuracy but they could help you get close.