Blue hour explained: the twilight window after sunset

Blue hour is the slice of twilight when the sun has gone (or is about to appear) but the sky still glows a deep, even cobalt. For city photographers it is the sweet spot of the whole day: sky brightness and artificial lights come into balance, and a long exposure can hold detail in both. Like golden hour, it is a photography term rather than a scientific one — and being precise about what you mean matters when you plan around it.

Definition — and an honest note that it varies

Our tables and calculator treat blue hour as the period when the geometric centre of the sun is between 6° and 4° below the horizon. That convention matches the elevation bands in PhotoPills’ magic-hours article. Wikipedia’s blue hour entry is careful to say no official definition exists and that the blue colour is most pronounced with the sun roughly 4–8° below the horizon. Practically: our band is the early, brightest part of that range, chosen because it is the portion where city lights and sky still expose together comfortably. If an app tells you blue hour runs to −8°, it isn’t wrong — it’s just using a wider band, and the light at the deep end is dimmer and bluer than most subjects need.

Why the sky turns blue (and not black)

After sunset the ground is in shadow, but the upper atmosphere is still sunlit: the sun illuminates the air from below the horizon at a very shallow angle. Air molecules scatter short wavelengths much more strongly than long ones — the same Rayleigh scattering that makes the daytime sky blue — but now the light traverses an extremely long path and the warmer wavelengths are absorbed and scattered away before reaching your eye. What comes down is a soft, diffuse blue with no hard shadows, fading smoothly toward astronomical darkness as the sun sinks past −18°.

That smooth fade is what makes blue hour forgiving: exposure changes gradually, and a sequence of frames shot a minute apart blends into a time-lapse with almost no jumps. It is also why the window is short near the equator — the sun’s path is steep there, so it drops through the −6° to −4° band quickly.

How long is blue hour, really?

The strict −6° to −4° band is short — often shorter than photographers expect. Measured evening windows from our engine (2027 solstice dates):

In practice most shooters keep working well past the −6° line into nautical twilight, which is why Wikipedia’s “4–8° below the horizon” description feels closer to lived experience — the wider band roughly doubles these durations at mid-latitudes. Our calculator shows both the strict band and the nautical twilight boundary so you can choose your own cutoff. And at very high latitudes in summer, blue hour effectively never ends, sliding straight into the next dawn — see equator vs high-latitude light.

Shooting the window well

Arrive before sunset and set up during golden hour — the best blue-hour shoots are really one continuous session across both bands. Mount the camera on a tripod, expose for the highlights in the sky, and let the city lights sit a stop or two above black. As the sky dims, re-expose every few minutes; the moment sky and street look equally bright in your histogram is the classic “balance point” most cityscape portfolios are built around. Full workflow, from scouting to the last frame, is in planning a blue-hour cityscape shoot.

Related reading

• Golden hour explained — the band just before blue hour begins
• Planning a blue-hour cityscape shoot — the full field workflow
• How sunrise times are calculated — our method and validation