New Delhi: This year, the March equinox, or the astronomical beginning of the spring season in the Northern Hemisphere, falls on March 20. Earth has two types of seasons, namely meteorological and astronomical. The tilt of Earth’s axis with respect to the Sun-Earth plane, and the planet’s position during its orbit around the Sun are responsible for the “astronomical” seasons.
“Meteorological seasons” are based on seasonal temperature variations modified by fluctuations in the amount of solar radiation received by Earth’s surface over the course of a year. Each year, the meteorological season of spring begins on March 1 and ends on May 31.
Earth’s tilt on its axis and the ceaseless motion the planet has while orbiting the Sun result in the equinoxes and solstices.
An equinox is one of the two times in the year when the Sun is exactly above the Earth’s equator, and day and night are of equal length. It is an event in which the planet’s subsolar point passes through its Earth’s equator. The subsolar point on a planet is the point at which the Sun is perceived to be directly overhead, and where the Sun’s rays strike the planet exactly perpendicular to the surface.
There are two equinoxes in a year, namely March and September equinoxes. March equinox is known as Spring equinox or the vernal equinox in the Northern Hemisphere. Meanwhile, seasons are reversed in the Southern Hemisphere, where it will be autumn.
This year, the simultaneous seasons will occur on March 20, at 15:33 UTC (9:03 pm IST), according to NASA.
Here are 10 things you need to know about the first day of the astronomical spring season in the Northern Hemisphere.
1. On the March Equinox, the Sun will pass directly above the equator, bringing nearly equal amounts of day and night on all parts of Earth. An equinox results in about 12 hours of daylight and 12 hours of night, at the equator.
2. However, the day and the night are not exactly equal on the equinox. Most regions on Earth get to see more daylight than night time, on the equinoxes. There are two reasons for this. One of them is how sunrise and sunset are defined. The other reason is the atmospheric refraction of sunlight. Refraction of sunlight by Earth’s atmosphere is another reason why the day is longer than 12 hours on an equinox.
Since sunrise and sunset are defined as the exact moment the upper edge of the Sun’s disk touches the eastern and the western horizon, respectively, the time it takes for the Sun to fully set makes the day just a bit longer than the night on the equinoxes, according to timeanddate.com.
The Sun’s upper edge is visible from Earth several minutes before the edge actually reaches the horizon due to atmospheric refraction or the bending of light. Also, at sunset, one can see the Sun for several minutes after it has dipped under the horizon, causing every day on Earth, including the days of the equinoxes, to be at least six minutes longer than it would have been without atmospheric refraction.
3. The March Equinox is the first equinox of the year. The second equinox, called the September equinox, takes place on or around September 22 every year. It marks the spring equinox in the Southern Hemisphere, and the fall equinox in the Northern Hemisphere.
4. The March equinox will bring earlier sunrises, later sunsets, softer winds, and budding plants, in the Northern Hemisphere. The Southern Hemisphere will experience a reversed season. There will be later sunrises, earlier sunsets, chillier winds, and dry, falling leaves.
5. People living in the Northern Hemisphere can watch the Sun as it sets just a bit farther north on the horizon until the June Solstice.
The June Solstice is the event when the Sun reverses directions, moving back to the south.
6. After the March equinox, the Northern Hemisphere begins to tilt more towards the Sun, resulting in increasing daylight hours and warmer temperatures.
7. The word equinox has originated from two Latin words – aequus and nox, which mean equal and night respectively.
8. On the March Equinox, the Sun crosses the celestial equator, heading northward as seen from Earth. Celestial equator is the great circle in which the plane of the terrestrial Equator intersects the celestial sphere. First Point of Aries is the point where the Sun crosses the celestial equator northwards.
9. The March equinox does not fall on the same date every year. The March Equinox has only occurred twice on March 21 – in 2003 and 2007. During the last decades of the century, the March equinox is more likely to occur on March 19.
10. The March equinox marks a time of transition and new beginnings in many cultures. The date of the March equinox plays a huge role in calculating the dates for when to celebrate Jewish Passover and Christian Easter, according to timeanddate.com. The Jewish holiday Passover commemorates the Hebrews’ liberation from slavery in Egypt. Easter is one of the principal holidays in Christianity, which marks the Resurrection of Jesus Christ.
The Iranian New Year, or Nowruz, which has been celebrated for over 3,000 years, occurs on the March equinox. Meanwhile, some people in Japan remember their ancestors by celebrating Higan during the March equinox and the September equinox. Higan, meaning “other shore”, is a Buddhist holiday which celebrates the spiritual move from the world of suffering to the world of enlightenment.