Key takeaways
- Real darkness needs the sun to be more than 18 degrees below the horizon. At 60 degrees north, at midsummer, it only gets about 6 or 7 degrees down.
- In southern Finland the sky is never fully dark from about the last week of April to the last week of August. Further north the sun does not set at all for part of the summer.
- A light summer night still offers noctilucent clouds, the moon, bright planets, satellites, narrowband nebulae and blue-hour landscapes.
- Use the bright months to plan, practise and learn your equipment, so that you are ready when the dark returns in late August.
The sky never gets dark in a Finnish summer because the sun never sinks more than about 6 or 7 degrees below the horizon, while astronomers need 18 degrees for true darkness. What you get instead is a long twilight, which Finns call the yötön yö, the nightless night.
Anyone who has spent a midsummer night in Finland knows the strange light. The sun has set, yet the sky glows pale green and rose along the northern horizon until it is time for the sun to climb again. It is beautiful, and it is a very poor sky for photographing faint objects.
Three kinds of twilight
Astronomers divide twilight by how far the centre of the sun lies below the horizon. The stage matters to a photographer, because it decides which targets are possible at all.
| Stage | Sun below the horizon | What the sky is like | What you can photograph |
|---|---|---|---|
| Civil twilight | 0° to 6° | Bright enough to read outdoors; only the brightest planets and stars show | The moon, bright planets, blue-hour landscapes |
| Nautical twilight | 6° to 12° | The horizon is still visible against a coloured sky and the brighter stars appear | Noctilucent clouds, the crescent moon, planets |
| Astronomical twilight | 12° to 18° | The sky looks dark to the eye, but the faintest objects are still washed out | Bright nebulae with narrowband filters, bright clusters |
| Night | More than 18° | Fully dark, apart from the moon and any light pollution | Everything, including galaxies and the Milky Way |
Only when the sun is more than 18 degrees below the horizon does the sky become fully dark, and that is the moment at which most deep sky photographers start work.
How low does the sun go at midnight?
At its lowest point, at local solar midnight, the sun's altitude equals your latitude plus the sun's declination, minus 90 degrees. Declination is the sun's north to south position in the sky, and it swings between about minus 23.4 degrees in midwinter and plus 23.4 degrees at midsummer. At 60 degrees north, which is roughly the latitude of Helsinki, the arithmetic looks like this.
| Date | Sun's declination | Lowest altitude of the sun | The sky at its darkest |
|---|---|---|---|
| 15 April | +9.5° | −20.5° | Fully dark |
| 1 May | +15° | −15° | Astronomical twilight |
| 15 May | +19° | −11° | Nautical twilight |
| 1 June | +22° | −8° | Nautical twilight |
| 21 June | +23.4° | −6.6° | On the border of civil and nautical twilight |
| 15 July | +21.5° | −8.5° | Nautical twilight |
| 1 August | +18° | −12° | On the border of nautical and astronomical twilight |
| 15 August | +14° | −16° | Astronomical twilight |
| 1 September | +8.5° | −21.5° | Fully dark |
By this arithmetic the sky at 60 degrees north is never fully dark from about the last week of April to the last week of August. The figures describe the exact moment of solar midnight and ignore haze and refraction, so treat the dates as a guide rather than a promise.
Further north the picture changes completely. Beyond the Arctic Circle the sun does not set at all for part of the summer, and in the northernmost corner of Finland the midnight sun lasts for more than two months.
The night is not missing in summer. It is only too pale to be useful for anything faint.
AstroNight calculates all of this on your phone for your own date and location. When the sky will not reach full darkness, it tells you so, instead of offering an hour of darkness that does not exist.
What can you photograph on a light summer night?
A bright sky is not the end of the hobby. It simply changes the targets.
| Subject | Why it works | A starting point |
|---|---|---|
| Noctilucent clouds | Thin ice clouds about 80 kilometres up stay lit by the sun long after the ground is in shadow. They appear in the weeks around midsummer at high northern latitudes. | Face north, 1 to 4 seconds at a moderate ISO |
| The moon and bright planets | They are bright enough to stand out against twilight | Short exposures, and a longer lens if you have one |
| Bright emission nebulae | Narrowband filters block most of the sky glow | Short exposures and many frames |
| Blue-hour landscapes | The coloured sky becomes the subject, and a crescent moon or a planet adds a focal point | Tripod, low ISO, 1 to 10 seconds |
| Satellites and the space station | At high latitudes in summer they stay in sunlight while the ground is in twilight | A pass prediction and a wide lens |
Bright emission nebulae are the one deep sky option, and our article on narrowband filters explains how a thin slice of colour can survive a pale sky. The crescent moon and the planets are also good practice subjects for the camera skills described in your first night with a camera and a tripod.
A seasonal planner for southern Finland
| Months | The sky | Good projects |
|---|---|---|
| April | Nights are still dark but shortening fast; true darkness ends in the last week | Galaxies high in the south, late aurora |
| May to July | No astronomical darkness; the northern horizon glows all night | Noctilucent clouds, the moon, planets, satellite passes, planning and practice |
| August | Darkness returns in the second half of the month | The Perseids, the first Milky Way arches over Cygnus |
| September and October | Long dark nights and the equinox aurora season | The Andromeda Galaxy, the Milky Way through Cassiopeia and Cygnus, aurora |
| November to February | The longest nights, and the coldest | Orion, the Pleiades, the Geminids, aurora |
| March | Long nights and another equinox aurora season | The winter Milky Way, aurora |
Making peace with the season
The reward comes later. When the dark returns in August, after weeks without it, the first nights with real stars feel almost like a gift, and by September there are many hours of darkness available. Use the summer to plan, to learn your equipment and to practise on the moon, and you will be ready to make the most of the first deep dark night.
The starry heavens above me and the moral law within me.
Frequently asked questions
Does it ever get completely dark in Finland in summer?
Not in the south for roughly four months. At 60 degrees north the sun stays within about 6 to 12 degrees of the horizon around midsummer, so the sky remains in twilight all night. Further north the sun does not set at all for part of the summer. Full darkness returns in the second half of August in southern Finland.
When does astronomical darkness return in Helsinki?
Our calculation for 60 degrees north puts it in the third week of August, when the sun first sinks more than 18 degrees below the horizon at solar midnight. After that the nights darken quickly, and by September there are many hours of full darkness.
What is the difference between astronomical twilight and night?
In astronomical twilight the sun is 12 to 18 degrees below the horizon. The sky looks dark to the eye, but it is still brighter than a true night, so faint galaxies and nebulae are washed out. Night begins when the sun is more than 18 degrees down.
Can I photograph the Milky Way during the white nights?
Not in any useful way from southern Finland, because its faint structure drowns in twilight. Wait for late August, when the band arches high overhead through Cygnus and Cassiopeia and the sky is dark again.
What are noctilucent clouds?
They are thin clouds of ice crystals in the mesosphere, around 80 kilometres up. They are high enough to catch sunlight when the sun is roughly 6 to 16 degrees below the horizon, and they appear as electric blue ripples low in the north, mostly from late May to mid-August at high northern latitudes.
Sources and further reading
AstroNight is now on the App Store. The free planner and coach for astrophotography works on iPhone and iPad with iOS 17 or later, offline, with no account. Get it on the App Store.
