Detailed explanation: what is "dayspring"?

The word used to be familiar to many English people from the Bible passage known as the Benedictus which is used in Morning Prayer in the Book of Common Prayer. It is from Luke 1 verses 68-79. Verse 78 reads Through the tender mercy of our God, whereby the dayspring from on high hath visited us. In modern Bible translations the word is given as dawn.

The derivation is fairly self-evident; the beginning of the daylight, as a spring of water may be the beginning of a river.

http://www.finedictionary.com/dayspring.html

The 1382 Wycliffe Bible translates Job 38:12 as:

Whether aftir thi rising thou comaundedist to the morutid, and hast shewid to the dai spring his place?

"dai spring" means "dawn" in this verse.

Exact meaning, with a link to the meaning in a dictionary.

The OED (which I think is pay-walled, so I can't post the link) defines the word as a synonym of "dawn" and "daybreak", with the addition that its later use is chiefly poetic or figurative.

Information about where dayspring came from and where it was first used.

The first attestation is in the 1382 Wycliffite Bible as a translation of Latin aurorae in Job 38:12:

"Whether […] thou […] hast shewid to the dai spring his place"

The word itself is a compound word consisting of the nouns day and spring. The latter is derived from the verb to spring, which has one of its meanings listed in the OED: "Of dawn, morning, daylight, etc.: to appear, to become perceptible; to appear over the horizon or in the sky". The earliest attestation of spring in this sense is from the beginning of the 14th century:

"Al þe day & al þe niȝt, Til hit sprang dai liȝt." (from "King Horn", c1300).

dayspring is a compound word, composed of day and spring.

day in this case is our common understanding of that portion of a 24-hour period in which there is sunlight.

spring is used in an archaic sense of "origin", "beginning", or "source". (It's actually from a Germanic word that means an upwelling of water, such as a well or stream.)

From a more technical perspective, "sunrise" is an approximate synonym. I highly encourage you to watch a sunrise, to get a sense of the changes that occur from the period of complete darkness until the sun is fully visible.

When we were dating, my (now) wife and I watched a sunrise from a boardwalk on the Atlantic Ocean coast of the United States. We arrived about a half hour prior to sunrise. It was quite dark, except for the street lamps behind us. Ahead was pitch black with the sound of ocean waves breaking.

After a few moments, the horizon began to reveal itself - the boundary between sea and sky. While the sea remained relatively dark, the sky shifted through shades of gray, blue, yellow, and then white. Streaks (rays*) of light reflected through the wispy clouds. Eventually, a deep orange sliver of the sun peeked above the horizon. Along with the sea and the land, we were immediately bathed in light. The light grew and grew as the sun climbed, until it was fully revealed.

In my mind, then, the sunrise is that entire process, while dayspring consists of that instant where light suddenly breaks across everything around you.

* Regarding light rays, these are actually formed when clouds partially block sunlight, at different levels of dispersion. In some places the light will be more completely blocked, while in others it streams through relatively unhindered. This results in a "ray" or beam effect, even though from a purely physical perspective it's somewhat of an illusion.