Sun & twilight today

For casual observers, the day is neatly split into "day" and "night". For anyone who cares about the sky, the interesting bit is the transition — the roughly two-hour ladder between sunset and true darkness (and again before sunrise). Every one of those steps opens up a different set of objects.

The first stars appear during civil twilight, when the Sun is still only a few degrees below the horizon. By nautical twilight, the horizon is just visible against the sea, which is exactly what mariners historically used to take star sights. By astronomical twilight, the sky is dark enough that only the very faintest deep-sky objects — distant galaxies, the outer arms of the Milky Way — remain hidden by residual glow.

Once the Sun drops past −18°, the sky is officially astronomically dark. This is the state professional observatories, astrophotographers and deep-sky observers care about. At latitudes above roughly 48° in mid-summer the Sun never gets that low, which is why Scandinavia, Scotland and Alaska have "white nights" — a persistent twilight instead of true dark.

The photographer's calendar is different. Golden hour (Sun 0°–6° above the horizon) delivers warm, directional light. Blue hour (Sun 4°–8° below the horizon) is when the sky glows deep cobalt while artificial lights still hold detail — the single most reliable window for cityscapes and architectural work.