There are two equinoxes in a year; one at the start of the Spring season and the other at the start of Autumn. Equinox can be regarded somewhat like the opposite of the solstice. The background of word Equinox means ‘equal night’, which is resultant from two Latin words. Equinox is the day or date of the year when the day and night duration is equal.
Equinox is when the Earth is straight, upright on its axis in relation to the Sun. The Earth is exactly perpendicular to the Sun at these two times of the year. There is no axial tilt or 0° tilt of the Earth at these two times. This happens twice a year. Therefore, the rays of the Sun fall straight on the Earth, irrespective of the northern or southern hemisphere.
Whereas, Solstice is when the Sun reaches the highest point, either northwards or southwards of the Earth. That means the Earth’s axial tilt is at a maximum at this time in relation to the Sun. The Earth’s tilt is at its maximum at 23.44°. It means the day or the night is at maximum or minimum time duration of length, depending on the hemisphere; day or night time depending on the north or the south of the Earth’s location.
In Solstice, the longest day is witnessed in the month of June in the northern hemisphere, and in December in the southern hemisphere. Likewise, and it also entails that the shortest night in the northern hemisphere is in June corresponding to the longest night in the southern hemisphere, at the same time.
An example: The longest day on Earth has been experienced in Reykjavik, Iceland. It has recorded the longest day time-sunlight of almost 22 hours (21 hours and 45 minutes), meaning night-dark of only almost 2 hours, on the solstice, on 21st June. Also, Reykjavik has recorded the shortest day of 4 hours and 7 minutes, internet source. It can have a psychological impact on some natives of that area.
In the Autumn and Spring equinox, the Sun rays fall most directly on the imaginary equator line and the adjoining area between the Tropic of Cancer and the Tropic of Capricorn. So the day and the night period will be of equal length.
In Astrology, there is a meaning and interpretation of the equinoxes, i.e., the relation between the Sun and the Earth, or the natives of the Earth. In the ancient times, seasons played a key role. The equinoxes determine the start of a new season, i.e., Spring and Autumn. The Sun called Lord Surya in the Indian Astrology and Hindu religion, and the Sun plays an important role, its placement-positioning in the Janam Kundli, natal-birth chart of any native. The Sun is the source of energy.
The time of Autumn equinox means the time of start of reaping rewards-fruits and celebration. The perpendicular positioning of the Earth to the Sun at the time of equinoxes can have different connotations of each of the native on Earth. Each native has a unique birth chart so the interpretation varies from individual to individual.
The Sun enters the sign of Aries in the Spring equinox, i.e., the 21st of March. Aries is the 1st zodiac sign of the 12. This time generally marks beginning or a new start. At times, Spring equinox time is regarded as the astrological New Year. Many ancient calendars like Vedic, Hindu, Sikh, Buddhist start observance of the New Year at this time. The flora-fauna in the northern hemisphere brighten up with energy after the winters at this time.