Q: Is a sunset defined in astronomy as the moment when the trailing edge of the Sun's disk disappears below the horizon? ¶
Q: Is a sunset observed? ¶
Q: Is a sunset distinct from twilight, which has three phases, the first being civil twilight, which begins once the Sun has disappeared below the horizon, and continues until it descends to 6 degrees below the horizon? ¶
A: Yes, the second phase is nautical twilight, between 6 and 12 degrees below the horizon; and the third is astronomical twilight, which is the period when the Sun is between 12 and 18 degrees below the horizon.
Q: Is a sunset calculated from the leading and trailing edges of the Sun, respectively, and not the center, the duration of a day time is slightly longer than night time? ¶
A: Yes, Further, because the light from the Sun is refracted as it passes through the Earth's atmosphere, the Sun is still visible after it is geometrically below the horizon.
Q: Are sunsets driven by the axial tilt of Earth? ¶
A: Yes, and daily rotation of the Earth, the planet's movement in its annual elliptical orbit around the Sun, and the Earth and Moon's paired revolutions around each other.