Satellites

Why satellites stay up

A satellite is always falling towards the Earth, but it also moves sideways so fast that the ground curves away beneath it as quickly as it falls. That endless falling-and-missing is an orbit. Lower satellites must go faster: the ISS, about 420 km up, travels at about 28,000 km/h and goes round the Earth every 93 minutes, about 16 times a day.

Why the track shifts

The orbit stays almost fixed in space (it slowly turns) while the Earth rotates underneath it. So each time the ISS comes round, the ground below has moved about 2,500 km east, and its track appears further west. Over a day the tracks cover most of the planet between 51.6° north and south.

Three kinds of orbit

Low orbits (a few hundred km) are close enough for detailed photos and for people to live in. Polar sun-synchronous orbits (about 800 km) pass over the poles and cross each place at the same local time every day, which is ideal for weather and Earth-observation satellites. Geostationary orbits (35,786 km above the equator) take one sidereal day, 23 hours 56 minutes, the time the Earth takes to turn once relative to the stars, so the satellite seems to hang still over one spot; that is where the weather satellites that photograph whole hemispheres live.

How the positions are calculated

The app downloads each satellite's orbit, described by a few numbers called orbital elements, and computes the position on your device with SGP4, the standard method used for satellite tracking. Because the orbit is known, it can show where a satellite will be at any time, which is how it predicts when one passes over you.

What the toggles show

Stations

Places where astronauts live and work, such as the International Space Station and China's Tiangong. The ISS is often the brightest object in the night sky after the Moon, and can even outshine Venus; Tiangong is fainter but still easy to see. You can watch them with your own eyes at dusk or dawn, when they are in sunlight but your sky is dark.

Weather

Blue: geostationary weather satellites that photograph a whole hemisphere every 10 to 15 minutes (Himawari for East Asia, GOES for the Americas, Meteosat for Europe and Africa). Green: polar weather satellites that pass over the poles and scan the whole Earth twice a day in strips.

Telescopes

Hubble orbits about 520 km up, above the blurring of the atmosphere, which is why its pictures are so sharp. It has been observing since 1990.

Navigation

GPS (USA), Galileo (Europe), BeiDou (China) and GLONASS (Russia): about 150 satellites roughly 19,000 to 23,000 km up, most circling the Earth about twice a day. Your phone measures how long their signals take to arrive from at least four of them and works out where you are, usually to within a few metres.

Starlink

Thousands of small internet satellites, most a few hundred kilometres up, arranged in shells of many orbital planes so that several are always above any place. The layer shows the current number above you. Each point with a fading tail is one satellite and the path it has just travelled. Such large constellations are changing the night sky, which astronomers are studying closely.

Magnetic

Shows the Earth's magnetic field: short needles pointing along the field, coloured by its strength (weak over the South Atlantic, strongest near the magnetic poles), the path of the magnetic poles since 1590, and a card with what a compass does where you are. Tap Learn on the card for how the field is made.

Aurora

Shows NOAA's forecast of where aurora may be visible in about the next hour, as a glowing ring around each magnetic pole, and the current Kp storm level. Tap Learn on the legend to see why it forms.

Tracks

Shows or hides the orbit lines. The selected satellite always keeps its line.