Sultry "waist" of the Earth, when the day is equal to night

Equinox is the time when day is equal to night. At the moment when the Sun crosses the equator from south to north - this is from north to south - autumn. At this time, the sun is right above the equator of the earth. The word comes from the Latin equi or equal and the Greek nyx, or the Latin nox, which means equal night.

Equinoxes and Solstices

Equinox Earth

Equinoxes occur at the intersection of the ecliptic and the celestial equator. On September 23, the Sun moves down from the celestial equator and crosses it - the autumn equinox, and on March 21, when it moves up - the spring equinox. These days the axis of the Earth is not tilted to the Sun or far from it, and therefore the circle of illumination cuts all the latitudes in half, which leads to the same duration of this day throughout the Earth.

In both cases, the solar circle of illumination conditionally halves the equator. The equator, an imaginary line with a latitude of 0 degrees, divides the planet into the northern and southern hemispheres. This is the only place where on earth the day is always at night, every day of the year.

Earth is tilted, the maximum angular deviation to the Sun is 23.5 degrees.The day of June 21 with a positive deviation of the Sun relative to the celestial equator is called the summer solstice, and on December 21 with a negative deviation - the day of the winter solstice.

Simply put, the equinox represents the days when the day is equal to nights (March 21 and September 23), the solstice is the shortest (December 21) and the longest days (June 21).

Equinoxes along with the solstices are directly related to the seasons of the year.

Equinox - name is deceptive

Day night are equal

If the axis of the Earth would be perpendicular to its orbital plane, on the whole Earth there was an equal length of day and night. A real sunny day has a different duration, with fluctuations of up to 15 minutes.

There are three reasons for this temporary difference:

  1. The movement of the Earth around the Sun is not an ideal circle, but eccentric.
  2. The apparent movement of the sun is not parallel to the celestial equator.
  3. The phenomenon of precession of the axis of the Earth.

In addition, the Earth performs two different types of motion:

  • around the sun in a fixed orbit, a journey that takes exactly 365.26 days (year);
  • around its axis - forming day and night.

To complete one daily turnover is required, not exactly 24 hours, as we used to count, but 23.93 hours.

The earth is spherical; therefore, half is turned toward the sun, whereas in the other half it is night. The day / night cycle is continuous, with the exception of the earth poles, where it is believed that day and night last for half a year.

In fact, it is not, they are not equal. Just as the commonly accepted opinion regarding the equator, where the day is equal to night all year round, then at the equator one would expect the Sun to rise at 6:00 in the morning and set at 18:00. In fact, at the equator, it rises at 6:03 am in July, at 6:11 in February, 5:53 in May, and 5:40 in late October.

Offset angle

This phenomenon of the actual “equinox” is caused by the fact that the axis of the Earth is tilted at an angle of 23.4 degrees. The "inequality" of day and night is also affected by the phenomenon of refraction. Refraction - the refraction of sunlight through the atmosphere, which visually prolongs the day. Because of this phenomenon, the upper edge of the Sun can be visible, even if it is just below the horizon. For example, it is usually seen in the morning a few minutes before the actual sunrise. This phenomenon is influenced by atmospheric pressure and temperature.

Therefore, the actual duration of the “day” for the North Pole reaches 193 days, and nights for up to 172 days and, respectively, for the South Pole - 172 days lasts day and 193 days night.

Earth Precession Movement

Due to the very slow motion of the heavenly vault (the earth's axis actually moves), the equinox Sun, when the day is equal to night, moves from one sign of the zodiac to another approximately every 2160 years, completing the entire revolution in about 25.920 years. This very slow motion of the firmament is called the precession of the equinoxes. The precession of the equinoxes is the motion of the Earth, which leads to a change in the orientation of the rotation of the axis.

The first assessment of terrestrial precession was made by Hipparchus in 130 BC. er

The axis of the earth precession due to the imposition of such factors:

  • Its shape is not absolutely spherical (it is a flattened spheroid protruding at the equator).
  • The gravitational forces of the Moon and the Sun, acting on the equatorial protrusion, are trying to return it to the ecliptic plane.

Consequences of the precession:

  • displacement of the equinoxes;
  • moving the celestial poles;
  • offset astrological zodiac constellations.

The day and night equinoxes do not have the same length, their date varies depending on the latitude.

Equinox table

The forces of nature are harmonious and consistent. The ancients were very attentive to the seasons and the position of the sun in the sky, because their life depended on timely planting and harvesting.

Our ancestors have long celebrated the days of solstice and equinox as special holidays, when the day is equal to night. Many great Orthodox holidays are still in close proximity to these dates: Christmas (winter solstice) and Easter (spring equinox).

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