Time zones are used on Earth to divide the planet into different regions and to coordinate the measurement of time among different locations. Time zones are based on the rotation of the Earth on its axis, which takes approximately 24 hours to complete.
Orbiting spacecraft typically experience many sunrises and sunsets in 24 hours, or the case of Apollo program astronauts traveling to the moon, none. Thus, it is impossible to calibrate time zones concerning the sun and still respect a 24-hour sleep/wake cycle. A common space exploration practice is using the Earth-based time zone of the launch site or mission control. This keeps the sleeping cycles of the crew and controllers in sync. The International Space Station typically uses Coordinated Universal Time (UTC).
In outer space, time zones are unnecessary because no fixed points of reference are tied to the rotation of the Earth. Instead, time in outer space is based on the International Atomic Time (TAI), a scale based on the frequency of electromagnetic radiation emitted by certain types of atoms. TAI is used to measure time in outer space because it is stable, accurate, and not affected by the gravitational forces that can distort the flow of time.
To keep track of time while in outer space, astronauts and travelers use special clocks synchronized with TAI. These clocks are often based on the resonance frequency of a specific type of atoms, such as cesium or hydrogen, and can measure time with an accuracy of a few nanoseconds per day.
In addition to using specialized clocks, astronauts and other space travelers may also use mission elapsed time (MET) to keep track of the passage of time while in outer space. MET measures the amount of time that has passed since a specific event, such as the launch of a spacecraft or the start of a mission. MET is often used in conjunction with TAI to help astronauts and other space travelers coordinate their activities and track the progress of their missions.
Timekeeping on Mars can be more complex since the planet has a solar day of approximately 24 hours and 39 minutes, known as a sol. Earth controllers for some Mars missions have synchronized their sleep/wake cycles with the Martian day because solar-powered rover activity on the surface was tied to light and dark periods. The difference in day length caused the sleep/wake cycles to slowly drift with respect to Earth’s day/night cycles, repeating approximately once every 36 days.
Time zones on Earth
- Central America
- Indian Ocean
- North America
- South America