Day will equal night, and shadows will move in a straight line: the moment marking the start of astronomical spring has been announced.

This year, the moment of the spring equinox will occur on March 20 at 12:01 PM Moscow time, as reported by Faina Rubleva, the scientific director of the Moscow Planetarium, during a broadcast on Radio "Komsomolskaya Pravda." This is considered the start of astronomical spring.

- There is a concept of calendar spring, which will start this Saturday, March 1. However, astronomical spring is something different. Day equals night, but this is merely a consequence of the Sun crossing the celestial equator, moving from the southern hemisphere to the northern one. This event leads to day and night being equal, while before that, the night is longer, and the day is shorter, - explained Faina Rubleva.

So, we already have "three springs": there is the calendar spring, the astronomical spring, and also the climatic spring. The beginning of climatic spring is considered to be the days when the average daily temperature rises above zero. For instance, during these days in Moscow, it is warm during the day but very cold at night, so despite the thaw outside the city and the complete absence of snow in the city, climatic spring has not yet arrived.

Are astronomical and climatic springs connected in any way? In the moment, perhaps not, but globally, yes:

- The higher the Sun, the more light and heat it provides, leading to more warming of the Earth, - says Faina Rubleva, - After the equinox, the Sun will spend most of its time above the horizon, making the day longer. For comparison: in Moscow, on the day of the winter solstice, the length of the day is only seven hours.

However, astronomers appreciate the spring equinox not only for the promise of warm days; it is an interesting event in general.

- On this day, the Sun rises exactly in the east and sets exactly in the west, - says Faina Rubleva.

Moreover, it is only on this day that shadows fall in a straight line. One can easily verify this by marking the ends of shadows from any object over a few hours.

The sky above us appears as a dome. The "spherical" appearance of the sky as observed by humans is due to the fact that we live on a spherical Earth, and this fact serves as the best proof that the Earth is round (checkmate, flat-earthers).

Lines and points on the Earth are projected onto the dome of the sky. The North Pole of the Earth corresponds to the North Pole of the sky (near the North Star), and the Earth's equator corresponds to the celestial equator. The plane of the Earth's orbit is represented as the Zodiac, or ecliptic. The ecliptic is a circle in the sky, tilted at 23.5 degrees to the equator (precisely at 23.5 degrees, the Earth's axis is tilted to the plane of the Earth's orbit). This tilt is what causes the seasons.

When the actual Earth is tilted towards the Sun with its northern pole, the northern hemisphere receives more heat, and vice versa. From Earth, we observe that the Sun is positioned north of the celestial equator, meaning it is "visiting" the northern celestial hemisphere. The moment when the actual Sun is directly aligned with the actual Earth’s equator is the equinox.

Dividing the year into astronomical seasons is, of course, conditional. The period from the spring equinox to the summer solstice (around June 20) is considered astronomical spring, from the solstice to the autumn equinox is astronomical summer, followed by autumn, and finally, from the winter solstice to the spring equinox is winter. It is easy to see that this does not perfectly align with the actual seasons. However, the trend of recent years, with warm autumns, supports the case for astronomy. In September, it is very warm, and astronomically, it is still summer, so everything matches up.