It's mindblowing to me to think at any moment, over 6,000 satellites are whipping around the Earth at terrific speeds and somehow not drifting off into space or crashing into Earth. How does this work?

An orbit around Earth is a delicate balance between the momentum of a satellite traveling at speed parallel to the Earth's surface and the constant tug of gravity towards the Earth.

If the satellite travels too slowly, gravity wins, eventually pulling the satellite back to Earth. If it travels too fast, gravity isn't strong enough to hold it, and the satellite will shoot off into space. But at just the right speed—which depends on its distance from Earth—gravity will pull the satellite towards Earth, but the satellite's speed ensures it will keep missing and instead continue its journey orbiting around Earth. In this case, gravity acts as a centripetal force, a force acting towards the centre, enabling satellites to stay in orbit.

Newton had a thought experiment known as Newton's Cannon where he imagined firing a cannonball off a high mountain at different velocities and what the resultant trajectories would be.

Most orbits are slightly elliptical or oval-shaped. For example, the elliptical nature of the Moon's orbit gives us the joys of things like supermoons.

Orbit

Redshift refers to light being 'shifted' towards the redder end of the spectrum — longer wavelengths — as objects move away from each other. If a light source is moving towards us then light is blueshifted, shifting towards the bluer end of the spectrum, shorter wavelengths.

Imagine waving a spring back and forth to create a wave and then starting to run away – the waves would be stretched longer. Because the universe is expanding 🤯 distant galaxies are moving away from us faster than nearer ones — imagine how the chocolate chips in a cookie move away from each other as the cookie bakes and grows in an oven. By comparing the redshift of light seen from distant galaxies with what we would expect to see it's possible to use redshift to determine how far they are away.

Redshift is an example of the Doppler effect, or Doppler shift, in action. It's more commonly known by the stretching or compressing of soundwaves as, say, an ambulance moves towards or away from you, or how the sound in front of a moving aircraft eventually can produce a sonic boom.

Redshift

Russell's teapot is a memorable example illustrating how, if you are going to make claims that are difficult to verify, the burden of proof lies on the one making a claim — not on any skeptics to disprove it.

The example he gave was claiming there's a china teapot in elliptical orbit somewhere between Earth and Mars that's too small to spot with a telescope. He suggests it would be unreasonable to expect anyone to believe such a patently unfalsifiable claim.

Bertrand Russell original invoked the teapot in the context of religion and the existence of an unverifiable God, in a similar vein to the Flying Spaghetti Monster and Carl Sagan's invisible dragon. It can however be applied much wider than that.

I learned about it from a helpful commenter on the BS Asymmetry Principle, also known as Brandolini's Law.

Russell's Teapot

Astronauts returning from space have repeatedly shared experiences like this:

“You develop an instant global consciousness, a people orientation, an intense dissatisfaction with the state of the world, and a compulsion to do something about it. From out there on the moon, international politics look so petty.”— Edgar Mitchell, Apollo 14 astronaut

“When we look down at the earth from space, we see this amazing, indescribably beautiful planet. It looks like a living, breathing organism. But it also, at the same time, looks extremely fragile … Anybody else who’s ever gone to space says the same thing because it really is striking and it’s really sobering to see this paper-thin layer and to realize that that little paper-thin layer is all that protects every living thing on Earth from death, basically. From the harshness of space.”— Shuttle/ISS astronaut Ron Garan

The overview effect, coined by Frank White, is kind of the mother of seeing the forest for the trees. It’s the realisation that we live on a fragile planet, we are all connected, and this is all we’ve got.

The sketch is based on the image of earth from the moon taken on the Apollo 11 mission.

The overview effect

Meteor: A rock burning up as it enters the atmosphere. Also known as a shooting star.

Meteorite: Parts of a meteor that made it all the way to the ground. Look out!

It was a long time before I really knew the difference between these things.

Asteroids, meteors and meteorites

A supermoon is when the Moon appears at its largest. Because most orbits are actually ellipses—an oval shape—rather than circular, the moon changes how close it is to Earth each day. A supermoon occurs when the Moon is at least 90% of perigee, the point at which it is closest to Earth. The point it is furthest from Earth is known as the apogee—for our moon, that's around 50,000km further away than when it's at the perigee.

According to NASA, a supermoon can appear around 17% larger and 20% brighter than the faintest moon of the year.

Super moons are a treat for photographers with big zooms and aspirational-looking cityscapes or yachts on the horizon (or ET on a flying bike). Hat tip to Science Llama.

Space sketches