Final answer:
The two different sign systems mentioned include universal hazard signs for safety and the astrology-related Signs of the Zodiac.
Step-by-step explanation:
The two different sign systems from the Things to Know section of the book are likely referring to two systems: one being hazard signs and the other concerning astrology signs, specifically the Signs of the Zodiac. Hazard signs are important for safety and are often recognized universally. For example, a table in the book lists common hazard signs that you should recognize. On the other hand, astrology signs, or the Signs of the Zodiac, are used in astrology to purportedly indicate how the stars might influence a person's life based on their birth month. However, the book notes the discrepancy between the astrological signs assigned to each month in horoscopes and the actual constellations where the Sun is located during those months, which has changed over time due to the precession of the equinoxes.