On Tuesday (20) at 21:08 Brazilian time, the September equinox occurred—an astronomical event that marks the change of seasons on the planet. At this moment, the Sun crosses the Earth's equator, heading towards the Southern Hemisphere.
Because of this, spring begins in the south, while autumn arrives in the north. This phenomenon is caused by the tilt of the Earth's axis and affects the climate, the length of daylight, and various natural processes around the world.
The word 'equinox' comes from the Latin words 'aequus' (equal) and 'nox' (night), indicating the idea of nights having the same duration as days. However, despite this concept of balance between day and night, the equinox does not guarantee exactly 12 hours of light and 12 hours of darkness.
In practice, the period of illumination is usually slightly longer. This difference is related to the characteristics of the Sun itself and how its light interacts with the Earth's atmosphere. One factor is that astronomy considers the start of the day when the upper edge of the Sun appears above the horizon, and the end of the day only when its last part disappears. Since the count is based on the edges of the solar disk, not its center, additional minutes of illumination are summed up during the day.
The date when day and night have strictly equal durations is called an equinox and occurs at a different time. According to Josine Nascimento, an astronomer at the National Observatory (ON) and manager of the Division of Communication and Science Popularization (DICOP), days and nights become equal on dates close to the equinoxes, depending on the observer's latitude.
After the equinox, solar illumination in the Southern Hemisphere begins to gradually increase. Josine explains: 'From this moment, the Sun rises earlier and sets later because the vernal equinox is the path to summer.' She adds: 'This continues until, approaching the summer solstice, we get the longest day and the shortest night.'
Another important factor contributing to the increase in illumination duration is atmospheric refraction. The atmosphere acts like a lens, bending sunlight and making the Sun appear higher than it actually is. This accelerates sunrise and delays sunset, extending the daylight period. Astronomer Marcelo Zurita, president of the Paraíba Astronomical Association (APA), member of the Brazilian Astronomical Society (SAB), technical director of the Brazilian Meteor Observation Network (Bramon), and columnist for Olhar Digital, reveals: 'The refraction effect adds approximately 4 minutes and 40 seconds to the day's duration in regions near the equator,' noting that this difference is even greater for higher latitudes.
This effect varies depending on air temperature, pressure, and humidity, which explains the small differences in time recorded in different regions. According to the science education platform EarthSky.org, during the equinox, the Sun rises exactly in the east and sets exactly in the west almost everywhere on Earth, except at the poles. This happens because it is aligned with the celestial equator, an imaginary line above the Earth's equator.
An interesting point is that during this season, sunrise and sunset occur faster. The transition time from light to dark is shorter due to the angle of the Sun's trajectory relative to the horizon.
Changes are also noticeable in nature. In the Southern Hemisphere, days begin to get longer, temperatures rise, and vegetation regains new life. Animals begin their breeding and migration cycles, following the increase in sunlight.
Although the calendar has 365 days, the actual time required for the Earth to complete one orbit around the Sun is approximately 365 days and 6 hours. Since these four astronomical events marking the seasons depend on the planet's precise position in space, equinoxes and solstices occur about 5 hours and 48 minutes later each normal year.
To prevent the seasons from slowly shifting in the calendar, the Gregorian calendar uses leap years. Adding one day every four years, February 29th, serves precisely to compensate for this daily surplus and maintain the equinoxes on the same dates.
Since rounding to six hours is not perfect, this small difference accumulates and leads to an extra day approximately every hundred years. For long-term correction of this surplus, the calendar abolishes the leap year in most centuries. For this reason, years such as 1700, 1800, and 1900 did not have an extra day, while the year 2000 retained the extra day as an exception to the rule.
According to Zurita, historically, the calculation of the equinox never depended on measuring the duration of day and night, as humanity only began to track time with accurate clocks in the 18th century. Previously, this event was marked purely visually on the horizon. 'For ancient cultures, the equinox was marked by the Sun passing midway between the winter and summer solstices,' explains Zurita. Old observers tracked the Sun's position on the horizon: solstices occurred when it was farthest north or south, and the equinox was fixed when the rising or setting point occurred exactly halfway.
Determining the exact date of the solstices was a practical difficulty because during this period the Sun's position on the horizon hardly changes. This apparent immobility gave rise to the very concept of 'solstice,' meaning 'standing Sun.' Conversely, the shift from solar equinox to equinox is rapid and noticeable. Thanks to this observational clarity, the event gained immense significance for ancient peoples. 'It passes through this point only twice a year, so equinoxes were always valued by all cultures as markers of seasonal change,' emphasizes Zurita. This visual simplicity made the phenomenon an essential foundation for organizing calendars and agricultural cycles.