Sun's light body on summer solstice
Sun's light body on winter solstice
The Sun wanders between the Tropic of Cancer at latitude 23,4° North and the Tropic of Capricorn at latitude 23,4° South. In the process, the Sun crosses the equator in the middle, at latitude 0°, twice a year. This event is known as the equinoxes. When the Sun approaches the Tropic of Cancer, it crosses the equator on March 21; when it approaches the Tropic of Capricorn, it crosses the equator on September 21. On the equinoxes, the day length and night length are equal everywhere on Earth. Another peculiarity of the equinoxes is the observable motion of the Sun, which is observed linear from any vantage point on Earth. This is due to the fact that the light body of the Sun, being exactly in the middle between the two Foundations of Heaven, is “equalized” - it's neither deflected to the northern foundation nor to the southern, but is in the middle between the two. Therefore, the Sun does not appear to be rotating sideways, but its path appears straight like a string from any vantage point on Earth.
In order to get an idea how the motion of the Sun is observed on the equinoxes, we are going to plot three vantage points along one longitude.
We see how the Sun has just risen for the vantage points 1-3. Naturally, the Sun rises for all three vantage points simultaneously. Picture above shows how the motion of the Sun is perceived by each of the three observers. All three observers see the Sun rise and set in a straight line since on the equinoxes the Sun moves along the straight celestial equator, therefore, its motion is perceived perfectly linear from any point on Earth.