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

The overview effect is the ultimate example of "seeing the forest for the trees." It’s the realisation that we live on a fragile planet, that we are all connected, and that this is all we’ve got—our one and only home.

Coined by author Frank White, this phenomenon is most vividly experienced by astronauts, who return to Earth profoundly changed by seeing our planet from space. Their reflections highlight the emotional and philosophical impact of this shift in perspective:

“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.”
— Ron Garan, Shuttle/ISS astronaut

This sketch is inspired by the iconic photograph of Earth taken from the moon during the Apollo 11 mission. The image captures the striking beauty and fragility of our planet and perfectly embodies the spirit of the Overview Effect.

This sketch appears in my book Big Ideas Little Pictures

The Overview Effect A Profound Shift in Perspective

What is a blue moon? A blue moon is a full moon that hits twice in a calendar month.

Given the cycle from full moon to full moon is about 29.5 days and months are generally 30 or 31 days a full moon appearing twice in a month doesn’t happen very often. And February never has a blue moon.

I like this definition, but it is apparently a modification of the original definition which is a little more complicated. The original blue moon definition is explained like this. Given the 29.5 day cycle, most seasons of 3 months have 3 full moons. But every now and then, a season would come by, like Sep, Oct, Nov in the sketch, where there would be 4 full moons. The 3rd of these was known as a blue moon, or a seasonal blue moon. See why I like the modern definition?

Here’s a good article that explains the subtleties and history of blue moons.

Check out more moon or astronomy explanations

What is a blue moon?

When it comes to the phases of the moon, it’s good to know your waxing (growing) from your waning (shrinking), and that a gibbous is the shape of the moon when you take a crescent of shadow from a circle of light, and that the hardest moon to see is a new one.

Or, as a reader passed on: If it’s light on the right, it’s gonna get bright. If it’s light on the left, it ain’t.

Like the moon? Apparently, so do I. Also see:

Phases of the moon

The moon seems smaller when up in the sky and bigger when on the horizon. I was surprised to learn that the real cause is still disputed.

The moon illusion

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.

Supermoon

It shuffles a little, but you’ll notice you can always see the face. The mathematics of it are quite hard to fathom.

Moon sketches