“By the dawn's early light,” is the correct continuation of “The Star-Spangled Banner.” The phrase describes the first light of morning revealing that the American flag was still flying after a night of bombardment.
Francis Scott Key wrote the lyrics in 1814 after watching the British attack Fort McHenry in Baltimore Harbor during the War of 1812. The sight of the flag at dawn inspired the poem “Defence of Fort M'Henry,” whose words were later fitted to the melody of a popular English song, “To Anacreon in Heaven.”
A common mix-up is assuming the line ends with “ray” or “glow.” Those words fit the imagery, but the canonical lyric is “light.” The song did not become the official U.S. national anthem until 1931, although it had long been used ceremonially. “The Star-Spangled Banner” refers to the large flag Key saw over the fort, now preserved by the Smithsonian’s National Museum of American History.