# How Long Do ISS Cosmonauts Live?

Astronauts on the ISS live in Coordinated Universal Time (UTC), not Greenwich Mean Time (GMT), as is often incorrectly claimed. GMT time fluctuates relative to UTC by 0.9 seconds.

## Instructions

### Step 1

In 1999, the launch of the Russian Zarya module marked the start of the most ambitious space project of our time: the beginning of the creation of the International Space Station (ISS). The predecessors of the ISS were the Soviet space stations, which had no analogues and do not exist. Longest worked in the orbit "Mir".

Since then, many curious people have been interested in: how long do astronauts live on the ISS and in orbit in general? But why does such a question arise at all? We need to understand in more detail, because almost all the answers in well-known sources are incorrect.

### Step 2

Even the ancient Babylonians invented for the convenience of calculations to divide the circle into 360 equal parts - degrees, each degree - into 60 arc minutes, and every minute - into 60 arc seconds. The adjective "angular" is not in vain.

The earth makes a complete revolution in 24 hours, this is a day. Then there are 15 degrees of the circumference of its equator per hour of time, in which there will be 900 arc minutes and 54,000 arc seconds. In geometry, astronomy, geodesy and cartography, navigation, aviation and astronautics, angular minutes and seconds are called minutes and seconds of the arc.

Since the reckoning of time is associated with the rotation of the Earth, then confusion is possible with minutes and seconds. To avoid it, you need to remember: there are 60 minutes in an hour and 3600 seconds of time. Thus, when the Earth rotates, one minute of time corresponds to 15 angular ones along its equator, and a second of time corresponds to 15 angular ones. Such confusion arose because people learned how to accurately count the time thousands of years later than the Babylonian priests - to observe the sky.

### Step 3

Until the last century, time all over the Earth was counted from noon at the Royal Observatory of England in Greenwich. Noon was marked with a special astronomical instrument - a passage instrument. This world time was called GMT, Greenwich Meridium Time; Meridium in Latin is noon.

Local time, as the Earth rotates from west to east, is ahead of GMT to the east of Greenwich and lags behind to the west. The magnitude of the time discrepancy depends on the geographical longitude of the place, that is, on the distance in degrees / arc minutes / arc seconds of the noon line of the place (its meridian) from the Greenwich meridian. The midday line is the shadow of a thin, straight vertical pole at exactly noon, when the Sun is exactly south of the observation point.

For example, noon in Moscow comes 2 hours 10 minutes and 29 seconds earlier than Greenwich. Then the longitude of Moscow is 37 degrees 37 minutes (angular) to the east. People did not know how to accurately determine longitude at any time, until a special watch appeared that for a long time kept an even course - chronometers. Before that, it was determined from case to case, according to regularly recurring astronomical phenomena.

For convenience in everyday use, local time is rounded to the nearest hour, and the entire globe is conventionally divided into 24 time zones. The middle of the zero belt falls exactly on Greenwich. Time zones east of Greenwich are considered positive; to the west - negative. The time zone of Moscow is +2 GMT. If they write or say simply GMT, then this is Greenwich time: 0 GMT.

The inconveniences of standard time for the needs of sufficiently fast transport (railway, aviation) became immediately apparent: it is impossible to control the movement of trains or airplanes if the time in them along the route is constantly changing. As a result, as early as the 19th century, transport workers always exhibited their capitals in standard time in their systems. On slow transport (sea vessels, for example), the clock was changed to local time upon arrival at the port. But every ship always had a chronometer adjusted to GMT.

### Step 4

However, the Earth does not rotate perfectly evenly, and the development of science and technology required more accurate measurements of time than astronomy could provide. Then, namely in 1955, the atomic clock appeared. Modern examples of atomic clocks go or lag behind by a second in 3 trillion years, or 3000 million years.

On the basis of atomic clocks, time standards were developed, and according to them, the unified world time UTC was set. This abbreviation is not exactly deciphered. The British, since the Greenwich Observatory has long been a sensor of the exact time for the whole world, suggested the name CUT (Coordinated Universal Time). The French, remembering that the first, even before Greenwich, the exact time began to measure the Paris Observatory, insisted on TUC (Temps Universel Coordonné). Both mean in translation Universal, or Universal, Coordinated Time. In the end, the World Telecommunication Union (it is in charge of time standards) assigned the designation a neutral, easy-to-remember and unique designation UTC.

The starting point for UTC was simply obtained: we spotted GMT at the moment when the factors knocking it down mutually neutralized each other. Over many decades of observations, their causes and magnitudes have been clarified very accurately. Simply put, astronomers caught the moment when astronomical (professionally speaking - ephemeris) time in Greenwich coincided with world time, and immediately turned on the atomic clock.

### Step 5

Astronomical (ephemeris) Greenwich time UTC slowly, over months and years, fluctuates relative to UTC by plus or minus 0.9 seconds. In everyday life, this is insignificant, but already with maneuvers in orbit, accuracy in thousandths is required, and in scientific experiments - in millionths and billions of a second.

In addition, astronauts in orbit cannot use any standard time, since the spacecraft orbits the Earth in about an hour and a half. Astronauts need to tie their time to some point on Earth. Until the 90s of the last century, Soviet cosmonauts lived at +2 UTС, Moscow time. American - UTC Houston. In joint projects, for example, Soyuz-Apollo, they worked according to zero, absolute UTC.

The ISS from the very beginning lives in UTC time, and not according to GMT or simplified versions of world time (UT with indices) for everyday use, as they often write. And not only the ISS. In astronautics, it is still tacitly accepted to do everything in UTC time. Five countries are already flying into space: Russia, the United States, France, China and Iran. The number of space powers will undoubtedly expand. Each of them has its own ephemeris time and, in order not to get confused and not interfere with each other, binding all actions to universal UTC is absolutely necessary.

And one more widespread inaccuracy: the local home time of the ISS control centers in Moscow and Houston is +2 UTC and –5 UTC, respectively. Uninformed informants often argue that the time differences between Moscow and Houston from Greenwich are equal to each other in magnitude. That this is not so, it is clearly visible at least on the map.