Light pollution

Learning goal

You will be able to explain how a city's lights brighten the night sky and why this brightness swallows faint stars and the Milky Way. You will see that dust and moisture in the air scatter city light and spread it across the sky, and that a distant city leaves a dome-shaped glow on the horizon. You will find a dark place from light pollution maps based on satellite measurements, and read a scale that classifies how dark a sky is according to what you can see with your eye. When going to a dark place, you will take into account not only the distance from the city but also the direction you will shoot, the altitude and the clarity of the air. You will know that light pollution is a problem not only for photographers but also for night animals. Seeing how the city's orange glow gives color to a person's face and skin, you will choose the white balance accordingly.

The question

After the bridge frame in the city, your friend goes out on the balcony at midnight. The sky doesn't darken; it stands like a dull cover tinged with orange, and only a handful of bright stars can be picked out. Yet in the frame they shot on the hill last week, a milky, faint path crossed the middle of the sky. The same night, the same sky, but the two are completely different. An observer from the club shows them a colored map on their phone: the city yellow and red, the distant mountains dark blue. Why can't you see the Milky Way from the city?

Context

In the previous lesson you talked about the short moment when the sky and the city lights stand in balance, and the city's lamps of different colors (Lesson G.2). In this track's first lesson you saw that the sensor gathers light over seconds and shows what the eye can't, and that the eye adjusts to the dark in twenty to thirty minutes (Lesson G.1). In the color temperature lesson you also learned to express a light's color as a temperature, and that an orange light corresponds to a low color temperature (Lesson 11.1).

The sensor outdoing the eye in the dark has a condition: the sky itself must be dark. In the city the sky itself is lit: a long exposure gathers not faint stars but this light, and drowns the frame in an orange haze. That is why night sky photography often starts with a journey. This lesson takes up what lights the sky, how to find a dark place and how to measure the darkness of a sky.

Simple explanation

Why is the sky bright?

Skyglow is the part of artificial lights that escapes upward and sideways being scattered by dust, moisture and gas molecules in the air and lighting the night sky. Part of a street lamp's light goes not to the ground but to the sky; thousands of lamps, shop windows and windows together form a bright cover over a whole city. This cover is brighter than the faint stars and the light of the Milky Way; neither the eye nor the camera can pick out the difference between them.

In humid or dusty air, scattering increases and the glow grows; on a cloudy night the undersides of the clouds reflect city light back and the sky brightens even more. A long exposure gathers this glow too: in the city a thirty-second exposure gives a few stars and an orange sky. The moon also lights the sky; a night with a full moon erases faint stars even in a dark place.

Where is a dark place?

A light dome is the brightness of a city's or a town's skyglow that looks dome-shaped above the horizon when seen from afar. Looking from a dark hill, a big city's dome can be seen from tens of kilometers away. That is why, when choosing a dark place, the direction matters as much as the distance from the city: if there is a city on the side of the part of the sky you want to shoot, its dome enters the frame.

Light pollution maps based on satellite measurements show with colors how bright a region is at night: cities yellow and red, dark regions blue or black. Looking on these maps for a point far from cities, high and reachable is a good start. Altitude and the clarity of the air help too: in a high place less of the air is in front of the sky, and in dry, clean air scattering decreases. Stars near the horizon, on the other hand, look fainter everywhere because they sit behind a thicker part of the atmosphere.

How is darkness measured?

The Bortle scale is a scale that classifies how dark a night sky is, from one to nine, according to what can be seen with the eye. It was proposed by the amateur astronomer John Bortle. One marks the darkest sky, places where the Milky Way looks bright and detailed enough to cast a shadow; nine, the sky of a city center where only a few bright stars can be picked out. The Milky Way is often distinct in the dark half of the scale, and faint or invisible in the suburban and city classes.

This scale tells a photographer in advance what a place will give at night. Light pollution isn't only the photographer's problem either: many creatures, such as birds migrating at night, insects and turtle hatchlings coming out onto the shore, are affected by artificial light. That is why not turning on your light in a dark place unless it's needed is also a matter of care. Under a dark sky a person looks different too: on a hill at the city's edge the orange glow gives the face a warm color, while setting the white balance for the sky can pull the skin toward gray or blue. So that the skin, whether light or dark, stays in its own tone, taking the color setting not from the sky but from the source lighting the face, or leaving the person as a silhouette, is a conscious decision.

Visual evidence

Your friend opens the light pollution map the observer showed them. The city is bright red; the foothills of the mountains to the north dark blue. On the map, seventy kilometers from the city, they find a high plateau close to the road; to the south, on the side where the Milky Way will rise, there is no town.

First they make a half-minute exposure on a hill at the city's edge: an orange sky, a few stars. Then they shoot the same frame on the plateau: the sky deep blue, thousands of stars, a faint band of light rising from the horizon. To the north, on the city's side, a faint orange dome shows above the horizon.

Selin from the club had stood on the hill at the city's edge, in front of the orange glow; the glow striking her face had pulled her dark skin toward orange. On the plateau they choose to photograph her as a silhouette against the sky; in one frame they also light her face with a small, warm lamp and make the color setting for that lamp's light. Her skin is in its own warm brown.

Common mistakes

Summary

The Milky Way can't be seen from the city because skyglow, formed by artificial lights scattering in the air, is brighter than the faint stars, and a long exposure gathers this glow too; distant cities leave light domes on the horizon. A dark place is found from satellite-based light pollution maps, taking direction, altitude and the clarity of the air into account, and the darkness of the sky is classified with the Bortle scale; the color the glow gives a face is managed by choosing the white balance for the face or with a conscious silhouette.

Mini assignment

In this assignment you will look for a dark sky in your own region.

1. On a light pollution map, look at where you live and its surroundings; choose a point far from cities, high and safely reachable, and note the cities in the direction you want to shoot.
2. With the same settings, shoot a thirty-second frame first where you live, then at the dark point you chose, and compare the color of the sky and the number of stars.
3. Add a person to one of the frames; decide whether you will make the color setting for the source lighting them or leave them as a silhouette.

Check yourself

• How different are the sky color and the number of stars in the two frames?
• Is there a light dome on the horizon at the dark point, and in which direction?
• Is the skin of the person in the frame the color of the glow, or their own tone?

Glossary

Mini quiz

1. What is skyglow?

• A) The light of the moon
• B) The stars' own brightness
• C) Artificial lights scattering in the air and lighting the night sky
• D) The heat of the camera's sensor

2. Why does direction matter when choosing a dark place?

• A) Because the light dome of a city on the side of the sky to be shot can enter the frame
• B) Because the wind always blows from the same direction
• C) Because stars are visible only in the north
• D) Direction doesn't matter

3. What does one show on the Bortle scale?

• A) A city center
• B) The darkest sky
• C) A night with a full moon
• D) A cloudy night

4. What is done to protect the skin of a face in front of the orange glow?

• A) The white balance is set for the sky
• B) The ISO is raised to the highest
• C) The exposure is pulled down
• D) The color setting is made for the source lighting the face, or a conscious silhouette is chosen

Answers

1. C — The light lamps send upward scatters in the air and forms a cover that lights the whole sky.
2. A — A city's glow shows as a dome on the horizon and lights a frame facing that side.
3. B — The scale starts at one with the darkest sky and reaches a city center at nine.
4. D — A white balance chosen for the face separates the skin from the orange of the glow, while a silhouette leaves the color knowingly.

Summary card

• Skyglow swallows faint stars; a long exposure gathers it too.
• Cities leave light domes on the horizon; check the direction you will shoot.
• Find a far, high and clear place from a light pollution map.
• Bortle scale: one the darkest, nine a city center.
• Don't turn on your light needlessly in the dark; for a face in orange glow, choose the white balance for the face.

Bridge to the next lesson

In the dark sky over the plateau the faint band of the Milky Way rises from the horizon, but when your friend goes to the same place the next month, the sky is lit by the full moon and the band isn't visible. The observer at the club tells them that the bright center of the Milky Way isn't visible at night at all in some months of the year. When and where is the Milky Way visible? The next lesson takes up planning a Milky Way night using the moon phase, the season and apps that show the sky in advance.