What happens to stargazing when the sun barely sets — and what you can still see.
If you've ever stayed up late on a June night in the UK and noticed the sky never goes truly black, you're not imagining it. At latitudes above about 50°N — which includes all of Britain, Scandinavia, southern Canada, and northern Europe — the sun doesn't drop far enough below the horizon in summer to produce true astronomical darkness.
Astronomical darkness begins when the sun is 18° or more below the horizon. At that depth, the sun's scattered light no longer contributes to the sky background, and the sky is as dark as it gets. In midwinter, this happens within about 90 minutes of sunset. But as the summer solstice approaches, the sun's path flattens out, and it skims just below the horizon rather than plunging deeply.
In London (51.5°N), true astronomical darkness disappears for about three weeks either side of the solstice — roughly late May to mid-July. In Edinburgh (55.9°N), it's gone for nearly two months. In Tromsø (69.7°N), the sun doesn't set at all for eight weeks. Even at the latitude of Paris (48.9°N), the shortest nights have barely two hours of full darkness.
Astronomers divide twilight into three phases based on how far below the horizon the sun sits. Each phase determines what's visible in the sky.
**Civil twilight** (sun 0° to 6° below horizon) is the brightest phase. The sky is still light enough to read outdoors without artificial light. Only the very brightest objects — Venus, Jupiter, and sometimes the crescent moon — are visible. Civil twilight is when most people notice the sky "getting dark," but it's far too bright for any serious stargazing.
**Nautical twilight** (sun 6° to 12° below) is noticeably darker. The horizon is still defined, and bright stars become visible. Navigators historically used this phase because they could see both the horizon and reference stars simultaneously — hence the name. At this depth, you can observe bright planets, prominent constellations (Orion, the Plough, Cassiopeia), and the brightest deep-sky objects like the Orion Nebula.
**Astronomical twilight** (sun 12° to 18° below) is the final transition. The sky appears dark to the casual eye, but a faint residual glow from the sun remains. Sensitive instruments and dark-adapted eyes can still detect the difference between astronomical twilight and full darkness. Once the sun passes 18° below, astronomical darkness begins and the sky is as dark as it gets.
In the UK in midsummer, the sun may only reach 12° to 16° below the horizon at its lowest point — meaning the sky never gets past nautical or shallow astronomical twilight. The entire night is spent in this in-between state.
The loss of astronomical darkness doesn't mean stargazing stops. It means the faintest objects — dim galaxies, faint nebulae, the outer reaches of the Milky Way — are washed out by the residual sky brightness. But plenty of targets remain excellent throughout the summer.
**Planets** are largely unaffected by twilight. Venus, Jupiter, Saturn, and Mars are bright enough to punch through a twilight sky easily. In fact, summer is often the best time to observe planets because they're higher in the sky and atmospheric seeing (turbulence) tends to be better in warmer air. Saturn's rings, Jupiter's cloud bands and moons, and Mars's surface features are all at their best.
**The moon** is completely unaffected. Lunar observing — craters, mountain ranges, shadow play along the terminator — works just as well in twilight as in full darkness. Many experienced observers actually prefer lunar work in summer because the shorter nights don't interfere.
**Bright deep-sky objects** are still within reach. The Orion Nebula (M42) in winter, or the Ring Nebula (M57) and Albireo (the double star in Cygnus) in summer, remain visible from Bortle 4–5 sites even without full darkness. Star clusters like the Pleiades and the Double Cluster in Perseus also hold up well.
**ISS and satellite passes** are actually at their best in summer. The ISS and other low-orbit satellites are only visible when they're in sunlight while your sky is dark — and the long twilight of summer means this geometry persists for hours, sometimes producing multiple passes per night. From the UK in June, you can see the ISS several times between 10pm and 4am.
**Aurora** can still be seen against the twilight sky at high latitudes during strong geomagnetic storms. The aurora produces its own light, so it doesn't need full darkness — it just needs the sky to be dark enough for the green and red glow to stand out. During major events (Kp 7+), aurora has been observed from the UK even in June.
Rather than showing no data for weeks during summer, Clear Skys uses a twilight scoring system. The app checks how far below the horizon the sun actually gets at its lowest point and applies a sky-brightness penalty on a sliding scale.
When the sun reaches −16° to −18° (deep nautical twilight), the penalty is just 5 points — you can still see down to roughly magnitude 5.5–6.0, which is enough for many deep-sky objects. At −14° to −16° (mid nautical), the penalty is 12 points. At −12° to −14° (shallow nautical), it's 20 points. At −9° to −12° (deep civil), only bright objects are realistic and the penalty is 30 points.
If the sun never drops below −9°, the night is unscored — conditions are too bright for meaningful stargazing beyond planets. For the UK (50–59°N), the sun reaches at least −12° on every night of the year, so every UK night gets a score even at the solstice.
The observing window shifts from the astronomical darkness window to the civil twilight window (sun below −6°), which is always longer. This means you still get hourly scoring, a best viewing window, and a verdict — the score is just calibrated for the brighter sky. When you see a score of 72 on a June night, it means weather conditions are good but the sky brightness limits what's visible.
Read the full scoring methodology →The return of astronomical darkness depends on your latitude. For London, full darkness returns in the second or third week of July. For Edinburgh, it's late July or early August. For Tromsø, it's mid-August. The transition is gradual — you'll notice nights getting noticeably darker from late July onward, with each week bringing a few more minutes of true darkness.
August is when the UK stargazing season truly ramps up again. Nights are still warm enough for comfortable observing, the Milky Way is high overhead in the southern sky, and the Perseid meteor shower peaks around August 12. September through March brings progressively longer darkness windows, with the winter solstice offering 12+ hours of astronomical darkness even from southern England.
The cycle is predictable. If you bookmark the Clear Skys forecast for your location, you'll see the darkness window expand week by week from late July onward — a reassuring countdown to proper stargazing weather.
Even in summer, there's always something worth seeing. Check your location's forecast — Clear Skys tells you exactly what the sky depth is tonight, what's visible, and whether it's worth heading out:
Check tonight's stargazing conditions for any location worldwide.
Search your location →The sun doesn't drop far enough below the horizon. True astronomical darkness requires the sun to be 18° below, but from the UK in June the sun only reaches about 12–16° below at its lowest. The residual scattered sunlight keeps the sky in perpetual twilight all night.
Yes — planets, the moon, bright stars, ISS passes, and aurora during storms are all visible throughout summer. What you lose is the faintest objects: dim galaxies, faint nebulae, and the detailed structure of the Milky Way. Clear Skys scores twilight nights with a brightness penalty so you know exactly what to expect.
Astronomical twilight is the phase when the sun is 12° to 18° below the horizon. The sky appears dark to the casual eye but a faint residual glow remains. Once the sun passes 18° below, astronomical darkness begins and the sky is as dark as it gets. In the UK, this full darkness disappears for several weeks around the summer solstice.
For London, full astronomical darkness returns in mid-July. For Edinburgh, late July. For northern Scotland, early August. From late July onward, each week brings several more minutes of true darkness, and by September the UK has 6+ hours of full darkness per night.