When dark is really dark

The sky does not switch off.

After the Sun drops below the horizon, the upper atmosphere still catches its light. That lingering light fades through three named stages. Civil twilight ends when the Sun is 6° below the horizon. Nautical twilight ends at 12°. Astronomical twilight ends at 18°. Those angles describe the Sun’s position, not a measured brightness at your observing site.

Our planner uses the last threshold to mark astronomical night. It is a useful starting point for faint stars, nebulae and the Milky Way. It does not mean every earlier minute is wasted: bright planets, the Moon and familiar stars can make twilight an excellent time to begin.

Near the poles, some summer nights never reach astronomical darkness. At other times of year the Sun may stay below the horizon for the whole selected window. A “no crossing” result is often real seasonal geometry, not a broken calculator.

A dark Sun is only half the plan.

Moonlight changes the contrast of the sky. A full Moon can make faint targets harder to pick out even at a rural site. A crescent Moon usually contributes less light, but its height, your target’s direction and atmospheric conditions still matter.

The green window in the planner combines two simple conditions: the Sun is below −18° and the Moon’s center is below the modeled horizon. It does not assign a “sky quality score.” A Moon below the horizon is not the same as a perfect sky, and there are worthwhile objects to observe while the Moon is up.

Try this: choose your location and compare tonight with a date a week later. Watch the Moon curve move relative to the dark shaded region. The useful window may move from evening to the early morning.

Find a better patch of sky.

Distance from direct lighting helps, but choosing where to stand can matter too. A lamp hidden behind a building or a safe line of trees will bother your eyes less than one in your field of view. A low object may be lost in a city’s bright horizon while a higher object is easier to see.

Check access and the return journey during daylight. A site that is dark but unsafe, closed or unfamiliar is not a good observing location. Stay on permitted ground and keep enough light for safe movement. Never sacrifice footing or road awareness to preserve night vision.

Allow about 20–30 minutes for dark adaptation, and avoid unnecessary bright screens. The red-light color option on this site reduces blue/green content, but it does not control your device’s brightness or guarantee protected night vision. Dim the screen yourself and put it away when you can.

Keep a weather-shaped margin.

The planner calculates geometry. It does not know about clouds, haze, wildfire smoke, humidity, fog or turbulence. Thin high cloud can hide faint detail even when a few bright stars remain visible. A forecast is useful, but a look outside before you leave is useful too.

Plan a small, flexible session: one goal, a warm layer, and enough time to settle in. If clouds arrive, a short clear gap can still be rewarding. If no Moon-down dark period appears, switch to Moonwatching rather than treating the whole night as a failure.