Steve Adams

Oct. 23 is a holiday in Thailand marking the 1910 passing of King Chulalongkorn, but in Boston its significance is mainly limited to land-use circles, as a factor in how high skyscrapers can be built.

It’s the date that developers are required to use when calculating the length and duration of the shadows that their towers throw on surrounding neighborhoods.

The choice of a date in late October might seem a bit arbitrary, but the Oct. 23 designation reflects the importance of the sun’s warming glow in the spring and fall. According to the Boston Redevelopment Authority, the date was chosen because it’s at the tail end of what is traditionally considered the outdoor season, while there’s still sunlight in the late afternoon.

Or to put it in gloomier terms, this is one of the last days of the year that people can be expected to enjoy the outdoors in Boston.

-STEVE ADAMS