Today is the spring equinox in the northern hemisphere and the fall equinox in the southern hemisphere.
What is an equinox?
The vernal equinox occurs when the sun crosses the celestial equator (the imaginary line on the sphere surrounding the earth between the north and south poles). This means that the Sun is exactly between the hemispheres at this time.
When is the vernal equinox in the northern hemisphere?
Equinoxes are hours, not days. This year it is March 20th at 21:24 GMT.
Are day and night the same on the vernal equinox?
You may have heard that day and night are of equal length all over the world, but this is not strictly true. At the equinox, we have about 12 hours of daylight and he has 12 hours of night, but most places on Earth have slightly more daylight.
To understand why, we have to think about sunrise and sunset. Sunrise occurs when the top of the sun is above the eastern horizon, and sunset occurs when the top of the sun sets below the western horizon. The sun rises when only part of it is visible and does not set until it is all gone.
If we define sunrise and sunset as the time when the center of the sun rises above the horizon and then sets, then there are 12 hours of daylight at the equinox. But we don’t.
There are two reasons for this. First, the sun is a disk, not a point of light. This means that the center of the sun may be below the horizon, but sunlight will still reach it. At the equinox, the center of the Sun is visible to him exactly 12 hours a day. After sunset, however, there are a few minutes when the rest of the sun is visible, making the day slightly longer.
Another reason has to do with the refraction of light by the Earth’s atmosphere. For example, at sunset, the apex of the sun remains visible for several minutes after the sun has set. This is because light bends as it travels through the atmosphere. This effect increases the length of the day by 6 minutes longer than her.
When is day equal to night?
The days and nights on the vernal equinox are not of equal length, but there are days when the days and nights are very close to 12 hours each. These days are called equilibrium, but when they occur depends on your latitude.
At latitude 5 degrees north of the equator, isolux occurs on February 24, and at latitude 50 degrees north on March 17. Similarly, at latitude 5 degrees south of the equator, day and night are of approximately equal length on April 14th, and at latitude 60 degrees south on March 22nd.