I am currently reading the manpages for the gensky command. The description states: “By default, the time is interpreted as local standard time on a 24-hour clock”.
I am wondering, if I do not specify the time zone designation, does the “local standard time” imply that a specific time zone will be chosen automatically? If so, which time zone will be chosen and how does it determine the local standard time for user?
If the time preceeded with a ‘+’ sign, it is interpreted as local solar time (sundial time), in which time neither -o nor -m are needed.
If the time is not preceeded with a ‘+’ sign, it is taken to be clock/mean/standard time, and both -o and -m are needed. There is no built-in smartness that would allow gensky to determine the TZ based on lat/lon.
Run
$ gensky -defaults
+s # Sunny sky with sun
-g 0.200000 # Ground plane reflectance
-t 2.450000 # Atmospheric turbidity
-a 37.815214 # Site latitude (degrees)
-o 122.040010 # Site longitude (degrees)
-m 120.000000 # Standard meridian (degrees)
… to see what the default settings are. The default location is Berkeley, CA.
If -m and -o are the same, you get local mean time, which is the mean time (man made or clock time), but without the correction for time zones.
This is like going back to the early 1800s, before TZs were introduced: Each town observed its own local mean time. Travelling east or west on those newfangled railways, one would have needed to correct ones pocket watch every so and so often by setting it to the time displayed by the local station clock.
Could I ask another question? Based on your answer, " If the time is not preceded with a ‘+’ sign, it is taken to be clock/mean/standard time, and both -o and -m are needed." If I used the following code:
gensky 11 26 14:00 +s -a -27.47 -o -153.03
How does Radiance recognize the time? Is 14:00 the time of the site ( -a -27.47 -o -153.03)?
or Should I need give the TZ after the time according to the time zone (TZ) of the size?
Thank you so much for your time and help. I appreciate it.
Thank you so much for your reply and detailed explanation. It has helped me gain a deeper understanding of the gensky command. I truly appreciate your assistance.
You’re welcome, Hao. I made a small typo in my previous reply: it should read ‘-m -150’, not -m 150’. Sorry about that. I am editing the previous reply.
No problem. What’s more important is understanding how the gensky command works and the function of -m option.
By the way, for the gensky command, is there any difference between using the TZ (e.g., YST/YDT) provided in the man page and defining the TZ with the -m option?
The -m option and the TZ specifications provide the same information, and follow the table given in the gensky man page. You need to use the standard abbreviations to get correct solar time, which includes any daylight-savings offset for some (called European Summer Time in the EU). For example, the following two commands give the same sky:
gensky 11 26 15:11EET -o -23.58 -a 46.77
gensky 11 26 15:11 -m -30 -o -23.58 -a 46.77
If you did the first command without EET, you would get get the unfortunate default value of 120 for the standard meridian as Axel points out.
Incidentally, you may specify decimal hours instead of hours and minutes if you prefer, and get a more precise solar position by including a specific year with the -y option. Unlike most Radiance tools, all options follow the main arguments in a gensky command line.
Thanks a lot for the detailed reply to my question - I truly appreciate it. Your explanation, along with Axel’s, provides a very clear understanding of the solar time definition in the gensky command. I hope this will also be helpful to other newcomers to Radiance.
Thanks again for your time and patience in addressing my question.