Overview

• The September equinox will occur at 18:19 UTC, the instant the Sun crosses the celestial equator.
• The astronomical start falls on Monday even as many in Argentina mark the Day of Spring on Sunday the 21st.
• The equinox brings nearly equal day and night worldwide and signals spring in the Southern Hemisphere and autumn in the Northern Hemisphere.
• The difference between the 365.2422-day tropical year and the 365.2425-day Gregorian year moves the equinox between September 21 and 23, realigning on cycles of about 400 years.
• From that moment, Argentina sees progressively longer days and typically milder conditions as the season advances.