The March equinox will occur on Earth on March 20. Theoretically, on this day, the duration of the light and dark parts of the day should be the same. But is this the case, and why does this event have a date and an exact time? Let’s find out together!
The March equinox is due on March 20, at 9:01 a.m. GMT (4:01 a.m. EST). According to the name of this phenomenon, it is on this day that the light and dark parts of the day should be the same. But then it is not entirely clear what the exact time has to do with it. After all, if the above definition is correct, then the whole day should be called that.
Historically, this has been the case. As the year moves from the winter solstice to the summer solstice, the daylight hours get longer and the night gets shorter. At some point, the number of light seconds in a day should outnumber the number of dark seconds.
To see this, you just need to carefully monitor the time of sunrise and sunset and wait until the time interval between them is exactly 12 hours. This day will be the equinox. A similar situation should occur in September – we call it the September equinox.
Well, nothing of the sort happens. If you measure the time from dawn to dusk on March 20, it will be 10:10 a.m. GMT (5:10 a.m. EST) in Kyiv. In general, on any given day this period is rarely exactly 12 hours. This is because the increase (decrease) in daylight hours during the day in spring and summer in our latitudes exceeds four minutes.
Length of daylight hours
If you measure the time interval between sunrise and sunset in different places on Earth on the same day of the same year, the duration will be significantly different. The same thing will happen if the length of daylight hours is measured in the same place and on the same day, including March 20, but in different years.
The duration of the light and dark parts of the day is highly dependent on at least three factors. The first is that the astronomical year is not exactly 365 days. To compensate for this fact, there are leap years when February 29 is added.
The second factor is the sphericity of the Earth and the distortion of light rays by the atmosphere. The interval between sunrise and sunset would be exactly 12 hours only if the Earth were a perfect sphere, which it is not. If the observer’s eyes, due to his height or the altitude of the area, are at a higher level than they should be in this case, this allows him to see the Sun before the “official” dawn in this area and after sunset.
This factor would not be so significant if it were not for the gas envelope of our planet. It distorts the Sun’s rays, and this, superimposed on the effect of the height of the observation point, leads to the fact that in certain areas the length of daylight hours can differ by minutes (and even hours in the circumpolar latitudes) from what it would be if the irregularities of the Earth’s surface and atmosphere did not exist.
The third factor is the definition of daylight hours. Its beginning is considered to be the appearance of the upper edge of the Sun above the horizon, and its end is its disappearance behind the horizon. That is, solely due to the diameter of the solar disk, we “win” a few more minutes.
Astronomical equinox
That is why scientists use the astronomical definition of the term “equinox” rather than the traditional one. It is tied to the concept of the celestial sphere, which, as we think, is where all celestial bodies move. There are two important circles on it.
The first is the celestial equator. The stars above our heads make a daily rotation, and only two points remain stationary – the north and south poles of the world. The circle equidistant from both of them is called the celestial equator.
There is also the ecliptic, an imaginary line along which the center of the Sun moves. It’s not our sun that moves around the Earth, but vice versa, but in our case, it’s much more important that this line does not coincide with the celestial equator, but is inclined to it at an angle of 23.5°. Again, this is the angle between the axis of rotation of our planet and the perpendicular to the plane of its orbit.
If two circles lie on the same sphere, have the same diameter, and do not coincide, they must have two intersection points. According to the modern definition, the equinox is the moment when the center of the solar disk is at one of these points. And this moment can be calculated to the minute.
The two points of intersection differ from each other only in that at one point the Sun passes from the southern hemisphere to the northern, and at the other – vice versa. The first of these is called the March equinox. It is now in the constellation Pisces, although the sign of Aries is still used to designate it, which was relevant more than 2000 years ago.
The March equinox is the beginning of the celestial reference frame. From the moment the center of the solar disk is in it, astronomical spring begins. The reason for this choice is that most astronomers still live in the Northern Hemisphere. In the Southern Hemisphere, the opposite is true: starting March 20, daylight hours will begin to decrease, and therefore this equinox will be the September one.
