How Is Dawn And Dusk Measured In Time?

I presume that dawn begins as soon as daylight replaces pitch black outside. But when does dawn end? or is dawn just a moment in time?

It also would appear to me that dusk would terminate when the last bit of daylight disappears and blackness occurs. If this is the case, when would dusk begin?

What brought the whole thing up is that in the Star Spangled Banner, “dawn’s early light” is mentioned. Does this imply that there is a late light within the time interval called dawn?

Serious answers would be appreciated.

Sunrise is the time when the upper limb of the sun reaches the horizon.

Sunset is the time when the upper limb of the sun falls below the horizon.

It is light quite a while before sunrise so the first lightening of the sky could be called dawn. And it is also light for quite a while after sunset so that time the last light disappears could be called dusk. Twilight is the time from sunset until the light disappears.

This site has information about the technical definitions of the various stages of twilight: civil, nautical and astronomical.