Time Format Affecting Local Solar Time and Solar Position in gendaylit vs. gensky

radiance-general
1

Hello all,

I have a question regarding how the time format affects local solar time and solar position in gendaylit. It seems that different time representations for non-integer hours lead to variations in the calculated solar time and solar position. This behavior appears to be different from gensky.

For example, when simulating for Tokyo, Japan on September 15th, the header generated by gendaylit shows different local solar time and solar position depending on the time format used. However, gensky does not exhibit this difference.

#gendaylit 9 15 11:30 -m -135 -o -139.76 -a 35.69 -W 378 363 -O 1
#Local solar time: 11.39
#Solar altitude and azimuth: 55.5 -16.2

#gendaylit 9 15 11:30JST -m -135 -o -139.76 -a 35.69 -W 378 363 -O 1
#Local solar time: 11.39
#Solar altitude and azimuth: 55.5 -16.2

#gendaylit 9 15 11:30JST -o -139.76 -a 35.69 -W 378 363 -O 1
#Local solar time: 28.39
#Solar altitude and azimuth: -18.0 -106.5

#gendaylit 9 15 11.5 -m -135 -o -139.76 -a 35.69 -W 378 363 -O 1
#Local solar time: 11.89
#Solar altitude and azimuth: 56.5 -2.9

#gensky 9 15 11:30 -m -135 -o -139.76 -a 35.69
#Local solar time: 11.89
#Solar altitude and azimuth: 56.5 -2.9

#gensky 9 15 11:30JST -o -139.76 -a 35.69
#Local solar time: 11.89
#Solar altitude and azimuth: 56.5 -2.9

#gensky 9 15 11.5 -m -135 -o -139.76 -a 35.69
#Local solar time: 11.89
#Solar altitude and azimuth: 56.5 -2.9

I suspect that gendaylit requires the time to be specified in decimal format (e.g., 11.5 for 11:30) rather than the HH:MM format to produce accurate results.
Could someone confirm if this is the correct approach, or clarify why this difference occurs?

Any clarification or guidance would be greatly appreciated.

Thank you for your time and help!

Best regards

2

I’m not sure if gendaylit accepts HH:MM or time zones as part of its time specification. These are features that I added to gensky early on, but it wasn’t added to gendaylit, which was developed and is managed by @Jan_Wienold and others. I’m not sure how much effort it would be to add these features, but I just checked, and it’s not in the currently distributed src/gen/gendaylit.c code.

Best,
-Greg

3

Hi,Greg

Thank you very much for your helpful response. Your explanation clarified the correct time format for gendaylit.

While reviewing related resources, I noticed that the tutorial Daylighting Simulations with Radiance using Matrix-based Methods (available at this link) uses the HH:MM format for specifying time in gendaylit. Based on your explanation, this might be an error. I thought it would be helpful to bring this to your attention.

Thank you again for your time and support!

Best regards,
Fujita

4

Hi Fujita,

Thank you for alerting us to this error. I will see if @Taoning_Wang has the original tutorial source document by @sarith or can edit the PDF directly.

Cheers,
-Greg