When do the seasons begin?

When do the seasons begin? Whether a snowstorm is raging on March 21, is there a hard frost, or, on the contrary, a mild thaw has set in - this day in the northern hemisphere is considered the end of winter and the beginning of spring - an astronomical spring. It seems to many that it is completely incomprehensible why the date indicated now, March 21 (in other years - 22), was chosen to serve as the border between winter and spring, although at this time a severe frost may still reign in full force or, on the contrary, it has long been warm weather.

When do the seasons begin?

The fact is that the beginning of an astronomical spring is not determined at all by changeable and unreliable weather signs. The mere fact that the moment of the onset of spring is set the same for all places in a given hemisphere of the Earth should suggest that the features of the weather are not significant here. The weather cannot be the same everywhere on the whole half of the world!

And indeed, when establishing the timing of the onset of the seasons, astronomers are guided by not meteorological, but astronomical phenomena: the height of the midday Sun and the resulting length of the day. This or that weather is already an accompanying circumstance.

The day of March 21 differs from other days of the year in that at this time the border of light and shadow on our planet passes through both geographic poles. Taking the globe in your hands and holding it accordingly turned to the lamp, you will make sure that the border of illumination then follows the line of the earth's meridian, crossing the equator and all parallel circles at right angles. Rotate the globe in this position around the axis, illuminating it with a lamp: each point on the surface of the globe will describe a circle, exactly half of which is immersed in the shadow and exactly half is in the light. This means that at a given moment of the year, the length of the day is equal to the length of the night. The equality of day and night is observed at this time throughout the entire globe from the north to the south pole. And since the day then lasts 12 hours - half a day, the Sun rises everywhere at 6 o'clock and sets at 18 o'clock (of course, local time).

So, this is what distinguishes the date of March 21: day and night are then equal to each other on the entire surface of our planet. The astronomical name for this remarkable moment is the "vernal equinox" - spring equinox because it is not the only equinox in the year. Six months later, on September 23, there is again a moment of equality of day and night - the "autumnal equinox", marking the end of summer and the beginning of autumn. When the vernal equinox is in the northern hemisphere, then on the other side of the equator, in the southern hemisphere, the autumn equinox, and vice versa. On one side of the equator, winter gives way to spring, on the other - summer gives way to autumn. The seasons in the northern hemisphere do not coincide with the seasons in the southern.

Let us also trace how the comparative length of day and night changes during the year. Starting from the autumnal equinox, i.e. from September 23, the light part of the day in the northern hemisphere becomes shorter than the dark one. This continues for a whole six months, during which the days are first shortened - until December 22, and then lengthened until March 21 day becomes equal to night. From that moment on, for the rest of the six months, the day in the northern hemisphere is longer than night. The days lengthen until June 21, after which they decrease, remaining the first three months longer than the night; they will again be equal to night only at the moment of the autumnal equinox (September 23).

These four dates determine the beginning and end of the astronomical seasons. Namely, for all places in the northern hemisphere: March 21 - a day equal to night - the beginning of spring, June 22 - the longest day - the beginning of summer, September 21 - a day equal to night - the beginning of autumn, 22 December 22 - the shortest day - early winter.

On the other side of the equator, in the southern hemisphere of the Earth, autumn coincides with our spring, with our summer - winter, etc.