Planning

Learning goal

You will be able to plan a Milky Way night before setting out. You will know in which months of the year, at which hours of the night and in which direction of the sky the brightest region of the Milky Way appears, and bring it together in the same frame with the landscape of a dark place, a hill or a tree. By reading together the hour when the sky really darkens after sunset, the moon phase and the moon's rising and setting times, you will find a dark span of time when the moon isn't in the sky. With apps that show the sky over a map you will see the direction in advance, and look at the forecast for cloud, humidity and clarity. If you will add a person to the frame, you will decide in advance whether to photograph them in the last light of twilight or in the dark with a small lamp, and plan the way to show their skin in its real tone.

The question

A month later your friend goes back to the dark plateau they found on the map. This time the sky is lit by the bright light of the moon; most of the stars are erased, the Milky Way isn't visible at all. When they write to the observer at the club, the answer is surprising: tonight is a full moon, and what's more, in winter that bright central region of the Milky Way isn't visible at night at all. Your friend realizes that the sky has a calendar too. When and where is the Milky Way visible?

Context

In the planning lesson you learned the sun's path, stating a direction with an azimuth angle, and the moon phase (Lesson M.2). In the previous lesson you saw how city lights brighten the sky and how to find a dark place from maps (Lesson G.3). While studying the last stages of daylight, you also told apart blue hour, when the sky's blue, scattered light lingers after sunset (Lesson 5.4).

Finding a dark place is only half the plan; you need to be in that place on the right night and at the right hour. The Milky Way doesn't stay in the same place in the sky all year; the moon rises and sets at different hours every night; after sunset the sky takes time to darken fully. This lesson takes up choosing a Milky Way night in advance by laying these three calendars on top of each other.

Simple explanation

When is the Milky Way visible?

The galactic core is the region of the Milky Way that looks brightest and densest in the sky, where stars, gas clouds and dust lanes are most crowded. The bright, branching central part you see in photos is this. In the northern hemisphere this region appears in the south of the sky, close to the horizon. In the winter months it is in the daytime sky and isn't visible at night; in spring it appears toward morning in the east and southeast, in the summer months it reaches its best position in the south toward midnight, and at the start of autumn it starts to set in the southwest in the evening.

That is why, to photograph the galactic core, you need to know which month of the year it is and at which hour of the night the south will be open and dark. In winter, other, fainter regions of the Milky Way are in the sky; they can be photographed too, but they are less bright. Choosing a place with an open south and no city light in that direction is important for showing the galactic core together with the landscape.

When is the sky really dark?

Astronomical darkness is the time when the sun has gone eighteen degrees below the horizon and no trace of daylight remains in the sky. After sunset the sky stays bright for a while; even after blue hour has passed, a faint brightness remains on the horizon for a while. Faint stars and the Milky Way appear fully only in astronomical darkness. Its start time shifts with latitude and calendar; in summer at northern latitudes it comes an hour and a half to two hours after sunset and ends the same amount of time before morning.

The new moon window is the few days around the new moon when the moon isn't in the night sky or appears very small and briefly. At full moon the moon is in the sky all night and erases faint stars. At new moon, on the other hand, the moon rises and sets with the sun and the night stays dark. A few days before or after the new moon too, by looking at the moon's rising and setting times, dark hours can be found when the moon is below the horizon; if a thin crescent sets early in the evening, the rest of the night is dark.

How is all this combined?

Apps that show the sky over a map or a landscape view show where the Milky Way, the moon and the sun will be on a night and at an hour you choose. By putting a pin on the place you chose and sliding the date and time back and forth, you can see when the Milky Way will stand above a hill, a rock or a tree. This direction, read as an azimuth angle, is confirmed in the field with a compass.

The plan is completed with the weather: cloud cover, humidity and the clarity of the air. A clear sky isn't enough; air carrying moisture and dust makes stars fainter. If you will add a person to the frame, plan in advance how you will light them too: a frame shot in the last light of blue hour, while there is still some brightness in the sky, shows the face with natural light but also carries a cold blue tone; in the dark, a small, warm and dimmed lamp lights the face without spoiling the stars. On light skin the lamp easily shines too bright, on dark skin it may fall short; trying out and noting the lamp's distance and duration in advance saves time in the field.

Visual evidence

In an app your friend puts a pin on the plateau. They find the week the new moon falls in the coming August; that night the sun sets at eight in the evening, astronomical darkness starts at half past nine, and the moon is below the horizon all night. The galactic core stands in the south around ten, above a lone tree at the edge of the plateau.

A week ahead they look at the cloud and humidity forecast: clear and dry. In the field, by day, they confirm the tree's direction with a compass. At ten at night the bright center of the Milky Way rises right above the tree; the sky dark, thousands of stars and a branching band of light.

They first photograph Selin in the last light of blue hour, beside the tree; her skin takes on a slightly cold blue tone. At night they light her in the same place for two seconds with a palm-sized warm lamp softened with a piece of card; they hold the lamp at the distance they tried out in advance, and her skin is in its warm brown, the galactic core behind her.

Common mistakes

Summary

The galactic core, the brightest region of the Milky Way, appears in the south of the night sky in the northern hemisphere from spring to autumn and isn't visible in winter; faint stars appear fully only in astronomical darkness, which starts when the sun goes eighteen degrees below, and in the new moon window when the moon isn't in the sky. An app combines these calendars over the chosen place, the direction is confirmed in the field with a compass, and the weather forecast completes the plan; the light for a person entering the frame is also planned for the skin's real tone by trying it out in advance.

Mini assignment

In this assignment you will plan a Milky Way night in the coming months.

1. Mark the dark place you found in the previous lesson in an app and choose a night in the coming months that falls on a new moon, when the galactic core will be visible.
2. For that night write down the hour astronomical darkness starts, the moon's rising and setting times and the hour the galactic core will come above a foreground element.
3. If you will add a person to the frame, decide whether to photograph them at blue hour or in the dark with a small lamp, and try out the lamp's distance at home in advance.

Check yourself

• On the night you chose, is the moon below the horizon?
• At which hour does the galactic core meet your foreground?
• Did you try out the light in advance for the skin of the person in the frame?

Glossary

Mini quiz

1. In the northern hemisphere, in which season is the galactic core not visible at night?

• A) Summer
• B) Winter
• C) Spring
• D) Early autumn

2. When does astronomical darkness start?

• A) When the sun goes eighteen degrees below the horizon
• B) At the moment the sun sets
• C) At the start of blue hour
• D) At midnight

3. Why is a full-moon night unsuitable for Milky Way photography?

• A) The moon hides the galactic core
• B) The camera doesn't work in moonlight
• C) The moon is in the sky all night and erases faint stars
• D) The Milky Way disappears at full moon

4. How do you find when the Milky Way will come above a tree?

• A) By guessing in the field
• B) By looking only at the weather forecast
• C) By looking at the sun's direction in the daytime
• D) With an app that shows the sky on a map, confirmed in the field with a compass

Answers

1. B — In winter the galactic core is in the daytime sky and stays below the horizon at night.
2. A — When the sun goes eighteen degrees below, no trace of daylight remains in the sky.
3. C — A full moon lights the sky all night and makes faint stars invisible.
4. D — The app shows the direction and hour in advance, the compass confirms the real direction in the field.

Summary card

• Galactic core: in the northern hemisphere from spring to autumn, in the south.
• Wait for astronomical darkness; the sun eighteen degrees below.
• New moon window, or the hours when the moon is below the horizon.
• See the direction and hour in an app, confirm with a compass in the field, check the weather.
• Try out in advance the light for the person entering the frame.

Bridge to the next lesson

After the frame with the tree, your friend looks at the enlarged frame: the galactic core is in place, but all the stars have become small lines; in the thirty-second exposure, not points but stretched smudges. In a frame they tried with a long lens the lines are even longer. Why do stars turn into lines? The next lesson takes up the earth's rotation, the longest time needed to keep stars like points, and the simple and more precise rules that calculate this time.