This study examines the validity of the Tropical Zodiac using three distinct approaches.

First, we explore the concept of the Photoperiodic Zodiac, which combines the seasonal cycles of sunlight with biological, psychological and symbolic effects. This zodiac is based on diurnal and nocturnal arcs. The diurnal arc characterises what classical astrology calls the personal signs, from Aries to Virgo, (days longer than nights in spring and summer). The nocturnal arc characterises the interpersonal signs, from Libra to Pisces (the nights are longer than the days in autumn and winter).

Second, an analysis of astrology book sales, based on a dataset of 313,368 books, shows significant associations between astrological signs and interest in astrology (p < E-6). Further examination reveals a pronounced distinction in sales between personal and interpersonal sun signs (p = 0.009) as well as between sun signs associated with the four seasons. There were also higher sales of books on water signs and lower sales of books on earth signs (p = 0.008). Moreover, there is a remarkable symmetry in the sales differences between opposite signs for each astrological axis (p = 0.007).

Finally, a survey of 70 astrologers asked them to rank the twelve zodiac signs by perceived interest in astrology. The average rankings from the astrologers correlated strongly with the book sales rankings (r = 0.7343, p < 0.0005).
By comparing book sales with the survey, the findings suggest a meaningful alignment between human interest in astrology and particular Sun Signs and elements in the Tropical Zodiac. This pattern also coincides with photoperiodic rhythms in the Northern Hemisphere.