Above your head is a thin blanket of air that gets thinner and thinner until it fades into space. There's no dome and no water up there. Gravity pulls everything toward everything else, and a satellite is just something moving sideways so fast that as it falls, it keeps missing the Earth.
We've measured the sky directly with balloons, aircraft, rockets and spacecraft. People have lived above the "firmament" without a break since November 2000, on the International Space Station. You can see it yourself, without a telescope.
Genesis says God made a raqia, a solid "firmament", to hold back "the waters above" (Genesis 1:6–8), and that Noah's flood came when "the windows of heaven were opened" (Genesis 7:11). Other verses say the Earth is fixed and "should not be removed for ever" (Psalm 104:5). Some flat-earth groups go further and say satellites are fake.
1. Fly up through the air
Drag the blue line up and down. The numbers come from the U.S. Standard Atmosphere, a model built from decades of balloon, aircraft and rocket measurements.1
- Half the air is below 5.5 km. At the top of Everest, each breath holds only about a third as much oxygen as at sea level.
- The ozone layer (about 15–35 km up) absorbs most of the Sun's ultraviolet light. Life on land depends on it.
- Temperature goes down, up, down, then way up as you climb. Each layer is defined by that change.
- There's no edge. The air just gets thinner. By convention, space begins at the Kármán line, 100 km up.
- Thin as an apple's skin. If Earth were an apple, the layer holding almost all the air would be thinner than its skin.
Take a sealed bag of crisps (potato chips) on a plane, or up a mountain road. It puffs up as you climb, because the air outside presses on it less. The air pressure you just read off the chart is what's doing that.
2. What gravity is
Gravity is the pull between anything with mass. Newton described it as a force that weakens with distance; Einstein explained it as mass bending space and time. Both make predictions we test every day.
- Everything falls at the same rate if air doesn't get in the way. In 1971 astronaut David Scott dropped a hammer and a feather on the Moon. They hit the ground together.2
- Gravity gets weaker with distance, but never switches off. On the ISS, 400 km up, gravity is still about 90% as strong as on the ground. Astronauts float because they're falling around the Earth with their station.
- Gravity holds the air down. That's why it thins with height. A world with weaker gravity, like Mars, holds on to much less air.
Drop a book and a flat sheet of paper side by side: the paper flutters behind. Now lay the paper flat on top of the book and drop them together. The book blocks the air, and the paper falls just as fast.
3. Newton's cannon: how an orbit works
Isaac Newton imagined a cannon on a mountain so tall it pokes above the air. Fire slowly and the ball falls back. Fire faster and it lands farther away. Fire fast enough and the ground curves away beneath it as fast as it falls, so it never lands.3 Move the slider:
This is real physics, calculated step by step with Earth's actual gravity. A circular orbit 300 km up takes about 7.7 km/s, roughly 28,000 km/h. To leave Earth for good from there takes about 11 km/s.
4. Satellites at their real heights
Thousands of working satellites circle Earth. How high they fly decides how fast they go: the closer in, the faster the lap.
| Orbit | Height | One lap takes | What's there |
|---|---|---|---|
| Low Earth orbit | about 400–2,000 km | about 90–120 minutes | The ISS (about 400 km), Starlink, Earth-watching and weather satellites |
| GPS (medium orbit) | 20,200 km | about 12 hours | About 31 GPS satellites; your phone listens to at least four at once4 |
| Geostationary | 35,786 km | 23 h 56 min, the same as one turn of the Earth | TV and weather satellites that hover over one spot |
| The Moon | about 384,400 km | 27.3 days | Earth's one natural satellite |
- GPS only works because Einstein was right. The satellites' clocks run about 38 microseconds a day fast compared with clocks on the ground (they move fast, which slows them, but feel weaker gravity, which speeds them up more). Without that correction, positions would drift by kilometres every day.5
- People have lived up there since November 2, 2000. The ISS crew circles Earth about 16 times a day.6
The ISS is the third-brightest thing in the night sky after the Moon and Venus. NASA's Spot the Station tells you when it will pass over you. It looks like a bright, steady star gliding across the sky in a few minutes. A pair of binoculars on a satellite night will show you a few more.
5. So where is the firmament?
The Bible's writers pictured the sky the way their neighbours did: a solid vault, with windows that let rain through, holding back an ocean above (Genesis 1:6–8, Genesis 7:11, Job 37:18). Babylonian myth had the same picture: in the Enuma Elish, the god Marduk splits the sea monster Tiamat in two and uses half of her body to make the sky, with guards posted so her waters can't escape. See The Bible's cosmos for the Hebrew details.
The sky is air that thins into space within about 100 km. Rockets pass through it daily; people have lived above it since 2000. No solid dome and no ocean above it has ever been found.
"The firmament" describes something real that God made on the second day of creation.
Many Christians say raqia just means "expanse", and that Genesis uses everyday language, the way we still say "sunrise". Some point to Job 26:7 ("hangeth the earth upon nothing") and Isaiah 40:22 ("the circle of the earth") as ahead of their time. The skeptic's reply: the same books also describe pillars, windows in the sky and water above it, which match their Babylonian neighbours, not modern science. "Circle" (chug) means a flat circle, not a ball. A book that gets the sky right only when read loosely, and wrong when read plainly, looks human-written.
Sources
- NOAA, Layers of the Atmosphere; U.S. Standard Atmosphere, 1976 (NOAA/NASA/USAF). Figures on this page are rounded; the boundaries between layers move with latitude and season.
- NASA NSSDC, The Apollo 15 Hammer-Feather Drop (1971).
- Isaac Newton, A Treatise of the System of the World (1728), the "cannonball" thought experiment. The simulation integrates Newton's law of gravity with Earth's real mass (GM = 398,600 km³/s²) and radius (6,371 km); air resistance is ignored.
- U.S. government, GPS: Space Segment: orbit height 20,200 km, two orbits per day.
- Neil Ashby, "Relativity in the Global Positioning System", Living Reviews in Relativity 6 (2003).
- NASA, Spot the Station; NASA, International Space Station facts (continuous crew since November 2, 2000; about 16 orbits per day).