Astronomically, there is but one epoch which may be determined with precision; that is the instant of transit. A heavenly body, be it the Sun, the Moon, a planet, or a star, is said to be "in transit" when it is at the longitude of the observer, which is to say, when the heavenly body is at its highest point in the sky. Thus, the Sun is in transit at high noon.
Sunset and sunrise vary with the local terrain. The moment of sunrise or sunset may be measured with accuracy only by an observer on the ocean who has an unobstructed view of the horizon; even then, refraction phenomena render the measurement somewhat imprecise.
By definition, evening begins with the setting of the sun, and the sun begins setting at high noon. Morning begins with the rising of the sun; thus, morning begins twelve hours after evening, at midnight. A day is comprised of an evening followed by a morning.
The Lord God, who is characterized by light, began his work of creation with the command, "Let there be light." Likewise, the work of each day of creation began at high noon, the evening preceding the morning.
