Equinoxes happen two times in a year when day and night are of equal length. ... In the Earth's Northern Hemisphere, for example, the vernal equinox cues the beginning of spring around March 21 and the autumnal equinox indicates the beginning of fall around September 23.

The Equinox

An equinox is an astronomical event that occurs twice a year when the Sun is directly above the Earth's equator, and day and night are of equal length. The word "equinox" is derived from Latin, meaning "equal night." During an equinox, the tilt of the Earth's axis is neither inclined towards nor away from the Sun, resulting in equal distribution of sunlight across the Earth.

Frequency of Equinoxes

The correct answer to the question is option 'C' - an equinox happens twice a year. Let's explore why this is the case:

1. March Equinox: The March equinox, also known as the vernal equinox, occurs around March 20th or 21st every year. This marks the beginning of spring in the northern hemisphere and autumn in the southern hemisphere. On this day, the Sun is directly above the Earth's equator, and the lengths of day and night are nearly equal all over the world.

2. September Equinox: The September equinox, also known as the autumnal equinox, occurs around September 22nd or 23rd each year. This marks the beginning of autumn in the northern hemisphere and spring in the southern hemisphere. Similar to the March equinox, on this day, the Sun is again directly above the Earth's equator, resulting in equal day and night lengths.

Reasons for Equinoxes

The equinoxes occur due to the Earth's axial tilt, which is approximately 23.5 degrees relative to its orbit around the Sun. This tilt causes the Sun's rays to fall more directly on one hemisphere during summer and less directly during winter. However, during the equinoxes, the tilt of the Earth's axis is such that both hemispheres receive an equal amount of sunlight.

Additional Information

It is important to note that there are two other astronomical events related to the equinoxes:

1. Solstices: The solstices, which occur around June 21st and December 21st each year, mark the beginning of summer and winter, respectively. During these events, either the North Pole or the South Pole is tilted towards or away from the Sun, resulting in the longest and shortest days of the year.

2. Precise Timing: The exact timing of equinoxes may vary slightly each year due to the Earth's elliptical orbit and other astronomical factors. However, the equinoxes generally occur within a day or two of the same dates each year.

In conclusion, an equinox happens twice a year, in March and September, when the Sun is directly above the Earth's equator, resulting in equal day and night lengths.

An Equinox happen twice in a year on 21st March and 23rd September.
it change due to revolution and rotation.
the answer of this question is on page number20
last paragraph and next paragraph in our ncert book