What is Equinox?
An Equinox is an astronomical phenomenon that occurs twice a year, once in the spring and once in the fall, when the Earth's axis is turned neither away from nor towards the Sun. The inclination of the Earth (with respect to the Sun) is 0° at Equinoxes, and the day and night are almost equal in length on Equinox day, i.e. 12 hours.
Equinoxes occur every year on the 20th or 21st of March and the 22nd or 23rd of September, and all days have the same duration of day and night. In the Northern Hemisphere, March Equinox is known as Vernal Equinox and Spring Equinox, while it is known as Autumnal Equinox and Fall Equinox in the Southern Hemisphere. In other words, it is springtime in the United Kingdom, the United States, Canada, Russia, India, and China on March Equinox, while it is autumn in Australia, Argentina, Chile, New Zealand, and South Africa.
Equinoxes are preferably referred to as March Equinox (Northern Equinox) and September Equinox (Southern Equinox).
This only occurs twice a year, on the spring and autumn equinoxes. This means that on these days, the day period is the same (12 hours) at all points on the earth's surface (except right at each pole, where it will be about to change from permanent light to dark, or vice versa). In several parts of the world, especially in the northern hemisphere, the autumnal equinox marks the start of autumn. This is the first day of spring in the southern hemisphere.
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Types of Equinoxes
Equinoxes are classified into two groups. The equinoxes in March and September are each assigned their own names.
Spring Equinox
The Spring Equinox is the first day of spring. The Spring Equinox happens as the sun reaches the equator from the southern to the northern hemisphere, marking the start of the spring season. The North Pole continues to lean back against the sun. The day and night are nearly similar in time. Spring Equinox is near 20 March.
Autumn Equinox
The Autumn Equinox occurs as the sun reaches the equator from the northern to the southern hemisphere, marking the beginning of the autumn season. The North Pole tilts away from the sun. The day and night are nearly similar in time. The autumnal equinox is near September 22nd.
Equinox and Culture
For thousands of years, equinoxes have been a part of many cultures and rituals. Equinoxes, for example, signifies the start of important days such as the new year. Planting and harvesting seasons are determined by the equinoxes. Both equinoxes are designated as national holidays in Japan as a day to honor ancestors. During the equinox, the Hindu temple Angkor Wat in Cambodia is said to be constructed in such a way that the sun rises directly above its central temple. Between 1113 and 1150, the temple was founded. There are many examples of this in various religious and cultural cultures.
Fun Facts
Equinoxes do not always occur on the same day each year.
Saturn has equinoxes as well, but they only happen every 15 years because it takes about 30 years for Saturn to orbit the Sun.
Equinoxes generally occur about 6 hours later each year, with a jump of a day (backward) on leap years.
The Mid-Autumn Festival, also known as the Moon Festival, is held in China to mark the autumnal equinox.
FAQs on Autumnal Equinox
1. What is the autumnal equinox in geography?
The autumnal equinox is a specific event that occurs annually around September 22 or 23 in the Northern Hemisphere. At this moment, the Sun shines directly over the Earth's Equator. This positioning means that the Earth's axis is not tilted towards or away from the Sun, leading to the astronomical start of autumn.
2. What happens to the length of day and night during the autumnal equinox?
During the autumnal equinox, the distribution of sunlight is nearly even across both the Northern and Southern Hemispheres. This results in an almost equal duration of daylight and darkness, with approximately 12 hours of day and 12 hours of night everywhere on Earth. The term 'equinox' itself is derived from Latin, meaning 'equal night'.
3. When will the autumnal equinox occur in 2025?
As per the astronomical calendar for the 2025-26 academic year, the autumnal equinox in the Northern Hemisphere is expected to take place on September 22, 2025. The exact date can vary slightly each year due to the Earth's orbital period.
4. How is the autumnal equinox different from the vernal (spring) equinox?
Both are equinoxes with nearly equal day and night, but they mark opposite seasons. The key difference is:
- The autumnal equinox (around September 22) marks the beginning of autumn in the Northern Hemisphere.
- The vernal equinox (around March 20) marks the beginning of spring in the Northern Hemisphere.
5. What is the difference between an equinox and a solstice?
The primary difference lies in the Earth's tilt relative to the Sun and the resulting length of day:
- Equinox: Occurs when the Sun is directly above the Equator. Day and night are of nearly equal length. This happens twice a year, in March and September.
- Solstice: Occurs when the Sun is at its farthest point north or south of the Equator. It results in the longest day (summer solstice) or the shortest day (winter solstice) of the year. This also happens twice a year, in June and December.
6. Why doesn't the autumnal equinox happen on the same date every year?
The date of the autumnal equinox varies because the Earth's orbit around the Sun takes approximately 365.25 days, not exactly 365. Our Gregorian calendar has 365 days, causing a slight drift of about a quarter of a day each year. The introduction of a leap year every four years helps to correct this and keeps the equinox date within the September 22-24 range.
7. How does the autumnal equinox affect the seasons in the Northern and Southern Hemispheres differently?
The effect is opposite for each hemisphere. For the September equinox:
- In the Northern Hemisphere, it is the autumnal equinox, signalling the start of autumn. After this day, the nights become longer than the days.
- In the Southern Hemisphere, it is the vernal (spring) equinox, signalling the start of spring. After this day, the days become longer than the nights.
8. Does the autumnal equinox mean winter is about to start?
No, the autumnal equinox marks the beginning of autumn, which is the transitional season between summer and winter. The start of astronomical winter is marked by the winter solstice in December, which is characterized by the shortest day and longest night of the year in the Northern Hemisphere.
