This ends with the winter solstice, after which days start to grow longer once again. In the Southern Hemisphere, it’s the reverse.Īfter the autumnal equinox, days become shorter than nights as the Sun continues to rise later and nightfall arrives earlier. What Is an Equinox?ĭuring an equinox, the Sun crosses what we call the “celestial equator”-an imaginary extension of Earth’s equator line into space. The equinox occurs precisely when the Sun’s center passes through this line.įor those in the Northern Hemisphere, when the Sun crosses the equator going from north to south, this marks the autumnal equinox when it crosses from south to north, this marks the vernal equinox. In the Northern Hemisphere, the autumnal equinox occurs in September in the Southern Hemisphere, it occurs in March.
The autumnal equinox is an astronomical event that marks the start of autumn (or “fall”). Due to time zones, the date of the equinox may differ by +/- one calendar day in your location. Other factors influencing the timing of the equinoxes and solstices include variations in the length of a tropical year and in the orbital and daily rotational motion of the Earth, such as the “wobble” in the Earth's axis (precession).Note: Dates listed above are based on Eastern Time ( UTC-5). When this happens, the equinox and solstice dates shift back to the earlier date again. To realign the calendar with the tropical year, a leap day is introduced (nearly) every four years.
Eventually, the accumulated lag becomes so large that it falls on the following date. This means that the timing of the equinoxes and solstices slowly drifts apart from the Gregorian calendar, and the solstice happens about 6 hours later each year. However, our planet takes about 365.242199 days to orbit the Sun. Today's Gregorian calendar has 365 days in a common year and 366 days in a leap year. The date of the equinoxes and solstices varies because a year in our calendar does not exactly match the length of the tropical year-the time it takes the Earth to complete an orbit around the Sun. The equinox in September is the start of fall in the Northern Hemisphere and the beginning of spring south of the equator.Įquinox and solstice dates-years 1-2149 Why Does the Date Vary? The March and September equinoxes mark the beginning of the spring and autumn seasons on Earth, according to one definition. In reality, equinox days don’t have exactly 12 hours of daylight and 12 hours of dark.Įquilux: when day and night are (actually) equalĪstronomical terms & definitions The Equinoxes and the Seasons However, this is literal translation not entirely true. This is the reason it’s called an “equinox,” derived from Latin, meaning “equal night.” In other words, night and day are, in principle, the same length all over the world. On the days of the equinoxes, the Earth’s axis is perpendicular to the Sun’s rays, meaning that all regions on Earth receive about the same number of hours of sunlight. As the Earth travels toward the opposite side of its orbit, which it reaches in December, the Southern Hemisphere gradually receives more sunlight, and the subsolar point travels south.Įarth is tilted as it orbits the Sun, which is why equinoxes and solstices happen. In June, the Northern Hemisphere is tilted toward the Sun, and the subsolar point is north of the equator. The subsolar point moves north and south during the year because the Earth’s axis is tilted at an angle of about 23.4° in relation to the ecliptic, an imaginary plane created by Earth’s path around the Sun. The December solstice marks the southernmost point of its journey. Having reached its northernmost point at the June solstice, it starts moving southward until it crosses the equator on the day of the September equinox.
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