sun positions as calculated by gensky

Hi list...

I just compared the sun positions as given by gensky for Singapore with
those calculated online using a form found under

http://aa.usno.navy.mil/data/docs/AltAz.php

I use the following parameters:

SINGAPORE
E 103deg48', N 1deg17'
Altitude and Azimuth of the Sun
Jun 21, 2008
Zone: 8h East of Greenwich

gives me for 1.30pm
            altitude azimuth (north->east)
13:30 67.1 346.1

Running gensky like:

gensky 6 21 5.30GMT +s -a 1.25 -o -103.75

results in a command line:

# gensky 6 21 5.30GMT +s -a 1.25 -o -103.75
# Local solar time: 12.19
# Solar altitude and azimuth: 67.6 173.0
# Ground ambient level: 16.1

Where azimuth is given (welcome to radiance world :wink: given in south to west direction.

So I have a difference of .5 degree for elevation and 6.9 degrees for azimuth.

Do I get something wrong here about time zone calculation?

TIA&CU, Lars.

Lars,

From the gensky manpage:
-m mer The site standard meridian is mer degrees west of Greenwich. (Use negative angle for east.) This is used in the calculation of solar time. Be sure to give the correct longitude also! If a time zone or solar time is given directly, then this option has no effect.
So it looks like solar time is not determined when you specify a time zone, e.g. GMT. I never use the time zone settings myself, preferring to use the site meridian. For your example:
gensky 6 21 13:30 +s -a 1.25 -o -103.75 -m -120
giving:
# Solar altitude and azimuth: 67.1 166.0
Which is much closer to where the Navy would have the sun (probably very nearly identical with the difference due to rounding).
-John

···

-----------------------------------------------
Dr. John Mardaljevic
Senior Research Fellow
Institute of Energy and Sustainable Development
De Montfort University
The Gateway
Leicester
LE1 9BH, UK
+44 (0) 116 257 7972
+44 (0) 116 257 7981 (fax)

[email protected]
http://www.iesd.dmu.ac.uk/~jm

Hi Lars,

gensky 6 21 5.30GMT +s -a 1.25 -o -103.75

results in a command line:

# gensky 6 21 5.30GMT +s -a 1.25 -o -103.75
# Local solar time: 12.19
# Solar altitude and azimuth: 67.6 173.0
# Ground ambient level: 16.1

You are entering the time with a decimal point, so it is in fact 6h
18min...!
I tried the following and I think it works (just like John was saying):

# gensky 6 21 5:30GMT +s -a 1.25 -o -103.75
# Local solar time: 12.39
# Solar altitude and azimuth: 67.1 166.0
# Ground ambient level: 16.0

HTH,
Santiago

···

____________________________________________________________
Electronic mail messages entering and leaving Arup business
systems are scanned for acceptability of content and viruses

You are entering the time with a decimal point, so it is in fact 6h
18min...!

John, Santiago, thank you!!! So it was just a typo, now I can even give
time in GMT again. Lars.

You are entering the time with a decimal point

Duh! Thanks for spotting that Santiago. Mind you, I'm still a little flummoxed: a difference of 12mins would give (I believe) a displacement in azimuth of, at most, 3deg. So, why is it 6deg?
jm/duke 44% gensky 6 21 5.30GMT +s -a 1.25 -o -103.75 | grep azi
gensky: warning: -6.9 hours btwn. standard meridian and longitude
# Solar altitude and azimuth: 67.6 173.0
jm/duke 45% gensky 6 21 5.50GMT +s -a 1.25 -o -103.75 | grep azi
gensky: warning: -6.9 hours btwn. standard meridian and longitude
# Solar altitude and azimuth: 67.1 166.0
-John

···

-----------------------------------------------
Dr. John Mardaljevic
Senior Research Fellow
Institute of Energy and Sustainable Development
De Montfort University
The Gateway
Leicester
LE1 9BH, UK
+44 (0) 116 257 7972
+44 (0) 116 257 7981 (fax)

[email protected]
http://www.iesd.dmu.ac.uk/~jm

flummoxed: a difference of 12mins would give (I believe) a
displacement in azimuth of, at most, 3deg. So, why is it 6deg?

We are in Singapore, on the equator...

CU Lars.

Time to update my mid-latitude-centric world view!

Thanks, Lars.

-John

PS. The annoying part is that I now recall this observation when I did a study a few years ago set in ... Singapore!

Time to update my mid-latitude-centric world view!

Be assured that this drives me crazy again and again when I get the sun
where I would never expect it to be :slight_smile:

PS. The annoying part is that I now recall this observation when I
did a study a few years ago set in ... Singapore!

Now I am getting curious :slight_smile:

By the way, is there a chance to add some timezones to gensky?
Singapore's is not covered yet.

CU Lars.

Now I am getting curious :slight_smile:

It harks from the bad old days of working on theoretical/speculative projects to demonstrate new approaches. Description here:
http://www.iesd.dmu.ac.uk/~jm/pdfs/p504_slides.pdf

It turns out that the actual roof of Changi airport looks very different from the CAD rendering on which I (very loosely) based my parametric model.
http://upload.wikimedia.org/wikipedia/en/c/cc/Changit3checkin.jpg

Whatever. It was a (fun) academic exercise.

-John