Why 15° degrees for the default declination?
The default declination value is chosen such that it results in a chart that varies the most from day to day, which means it must be close to the equator. It should also end in ´5´ so that the RA grid labels appear as legible as possible. 15° just fits right in, any larger and the view gets too close to the NCP and tends to vary less than at the equator every month.
