Astronauts on the International Space Station see sunrise far more often than people on Earth. Instead of one sunrise each day, they see about sixteen because the station races around Earth in about 90 minutes. Each orbit carries the ISS from the dark side of Earth into sunlight, creating a new orbital sunrise.
This makes orbital sunrise a clear example of how motion, light, and shadows work in space.
The Sun is always shining on one side of Earth, while the other side lies in Earth’s shadow. As the ISS follows its curved path around Earth, it repeatedly enters the shadow and then exits back into sunlight. The moment it leaves the shadow, astronauts see the Sun rise over Earth’s edge, called the limb.
Since a full day has 1440 minutes, a 90 minute orbit gives about 1440 ÷ 90 = 16 sunrises per day.
Understanding Astronautics: Orbital Sunrise
An orbital sunrise does not look exactly like a sunrise from the ground. Earth appears as a huge curved horizon, and the Sun comes from behind that curve. Before the bright disk appears, sunlight passes through long paths in the upper atmosphere.
Air molecules scatter shorter blue wavelengths more strongly, leaving orange, red, and pink light near the edge of Earth. This glowing band is called the atmospheric limb.
It can look thin from orbit because astronauts are viewing the atmosphere from the side. The change from darkness to direct sunlight happens very quickly compared with the slow dawn seen from one place on Earth.
The station remains in orbit because it is constantly falling toward Earth while moving sideways extremely fast. Gravity pulls it inward, but its forward motion carries it around the planet instead of straight down. This balance sets the shape and timing of the orbit.
A lower orbit needs a higher speed and has a shorter trip around Earth. A higher orbit moves more slowly and takes longer to complete one circuit.
Orbital sunrise is therefore connected to gravity as much as to sunlight. Without gravity bending the station’s path, it would travel away in a straight line and would not repeatedly pass through Earth’s shadow.
Not every orbit produces the same amount of darkness. The result depends on the angle between the station’s orbital plane and the direction of the Sun. At some times of year, the orbit carries the station deeply through Earth’s shadow.
At other times, the Sun is positioned more nearly above the orbital plane. Then the station may stay in sunlight for much longer. During rare periods, it can avoid the full shadow for many orbits.
This changing pattern matters to mission planners because sunlight provides power through solar panels. It also changes how much heat different parts of the station receive.
Frequent light changes affect people and equipment. Astronauts do not try to sleep whenever the view outside becomes dark. They follow planned schedules, use controlled lighting, and protect regular sleep habits.
Their eyes and body clocks still respond to light, so timing matters. Cameras must handle a fast shift from faint city lights to intense sunlight. Spacecraft surfaces expand when warmed and contract when cooled, so engineers design materials for repeated temperature changes.
When learning this topic, pay attention to the difference between Earth rotating and a spacecraft orbiting. Both can create a changing view of the Sun, but they happen for different reasons and on very different timescales.
Key Facts
- The ISS completes one orbit around Earth in about 90 minutes.
- Number of ISS orbits per day = 1440 minutes ÷ 90 minutes = 16 orbits.
- Astronauts see about 16 sunrises and 16 sunsets each Earth day.
- Orbital sunrise happens when the ISS moves from Earth’s shadow into sunlight.
- The ISS travels about 28,000 km/h, fast enough to circle Earth many times per day.
- Earth’s shadow is caused when Earth blocks sunlight from reaching part of the ISS orbit.
Vocabulary
- Orbit
- An orbit is the curved path an object follows around a planet, moon, star, or other body in space.
- International Space Station
- The International Space Station, or ISS, is a large spacecraft where astronauts live and work while orbiting Earth.
- Orbital Sunrise
- An orbital sunrise is the moment a spacecraft moves out of Earth’s shadow and back into sunlight.
- Earth’s Shadow
- Earth’s shadow is the dark region behind Earth where sunlight is blocked by the planet.
- Limb
- The limb is the visible curved edge of a planet or moon seen against space.
Common Mistakes to Avoid
- Thinking the Sun rises because it moves around the ISS. This is wrong because the ISS is moving rapidly around Earth while the Sun stays in nearly the same direction during one orbit.
- Counting only one sunrise per day in space. This is wrong because a sunrise happens every time the ISS exits Earth’s shadow, which occurs about once per 90 minute orbit.
- Assuming Earth’s shadow means the Sun turns off. This is wrong because the Sun keeps shining, but Earth blocks the sunlight from reaching the ISS for part of its orbit.
- Using 24 instead of 1440 when dividing by 90 minutes. This is wrong because 24 is in hours, and the orbit time is given in minutes, so the units must match.
Practice Questions
- 1 The ISS completes one orbit in 90 minutes. How many orbits does it complete in 24 hours? Use 24 hours = 1440 minutes.
- 2 If astronauts see one sunrise each orbit, how many sunrises would they see during a 6 hour period? Use 1 orbit = 90 minutes.
- 3 Explain why astronauts on the ISS can see many sunrises in one day even though people on Earth usually see only one.