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

From the combination of sol (sun) and sistere (cause to stand or stop) the solstice marks the highest or lowest points of the sun’s path in the sky each year.

In summer, for instance, the sun’s arc in the sky stops its daily ascent at the longest day of the year. There it reverses, beginning its descent to the autumnal equinox — with equal day and night — and eventually to its lowest point at the year’s shortest day, the winter solstice. At that point, its descent finished, the sun once again begins its slow climb towards the summer solstice.

Solstice

It’s pretty hard to get to grips with the scale of the solar system when Venus, the closest planet, feels really far already. But we’re practically squished together compared to Uranus and Neptune — they are a really, really, really long way away.

James O'Donoghue has made some brilliant videos about the solar system that explain this and more.

Solar system planets — distances to scale

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

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?

The Earth’s tilt causes the changing seasons. As the Earth revolves around the sun, the tilt of its axis—23.5 degrees—causes the two hemispheres, North and South, to be either tilted towards the sun or away from it at different times of the year.

In Winter, because light from the sun hits at a greater angle and closer to the poles, the solar energy gets spread over a larger area. This makes it weaker and cooler and gives rise to colder weather (aka winter).

In summer, the sun's light hits more directly and is closer to the equator, so it’s concentrated on a smaller area, leading to hotter days.

Because of the tile of the earth, seasons are reversed in the two hemispheres, so if it’s winter in the Northern Hemisphere, it’s summer in the Southern and vice-versa. The further from the equator you go, the bigger the effect of the seasons. As an example, temperatures in Ecuador stay relatively constant throughout the year, while the poles experience extremes of 24-hour daylight or 24-hour night and great freezes and thaws.

A version of this sketch, along with a number of other planetary and seasonal ideas, features in my book Big Ideas Little Pictures

Seasons

The Goldilocks Zone is the range around a star that’s not too hot or too cold to support liquid water. Liquid water is vital to sustaining life like we have on Earth, so fortunately for us, Earth sits in the Goldilocks Zone.

If we lived too close to the sun, we’d roast. The average temperatures on Mercury and Venus are hot enough to boil water (and then some).

And on Mars, the next planet farther out than Earth, the average temperature sits at -85 degrees F (-65 degrees C)— cold enough to freeze water solid.

If you want to look for planets that might support life as we know it in distant star systems, looking for worlds occupying Goldilocks Zones is a promising start.

This sketch, along with plenty of others in the "Starry-Eyed Surprises" section, features in my book Big Ideas Little Pictures

Know your space objects: comets, asteroids, meteors, meteorites

The Goldilocks Zone

I like this radius. It explains some of why asteroids are lumpy and bumpy, kind of like potatoes, and yet planets and moons are smooth round balls.

From the potato radius size, about 2–300km, the mass of the rock itself gets large enough that it is pulled in on its own gravity and consequently starts to round itself off.

I learned about this from Brian Cox’s show Forces of Nature.

Not sure about asteroids, meteors and meteorites?

The potato radius

The clarity of the night sky. A low number means clear, black and amazing stars. Higher numbers mean gradually more urban environments with increasing light pollution. At 9 you’ll barely spot a star being pretty much in Piccadilly Circus or Times Square.

More detail needed? John Bortle, Light Pollution And Astronomy: The Bortle Dark-Sky Scale, Sky and Telescope, July 18 2006.

Bortle Scale

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

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

Where the Northern Hemisphere simply allows you to find the North Star, around which all the stars will rotate, finding your way in the Southern Hemisphere is a little more involved. Unlike in the North, there’s no obvious star due south in the Southern Hemisphere. Instead, you have to use two handy indicators: the Southern Cross — easily spotted, fortunately — and two stars known as The Pointers. Find where these intersect, as in the sketch, and you’ve found due south.

Or, if you prefer, you can follow the direction the Southern Cross is pointing for a distance equal to 4.5x the length of the Southern Cross to arrive at the spot.

Incidentally, I wondered about reliable methods of finding south in the Northern Hemisphere and north in the Southern Hemisphere. And I think the answer is to find north or south and turn around.

Use the Southern Cross to find South

In the Northern hemisphere you can use this handy technique to find the North star, or pole star, that is always almost due North. Follow the last two stars of the Big Dipper, known as the plough in the UK, to a prominent star. Handy for a navigation sanity check at night.

The North star is the star that stays fixed in those neat star trails photos with all the stars rotating in a circle as the earth turns.

In the Southern hemisphere you can use the Southern Cross and the Pointers to find South, but it's not quite as simple as in the North.

Find the North star

Aligned with Earth’s axis of rotation. Appears largely fixed above the poles. The Northern Hemisphere has a Pole Star that will point you North, and the Southern hemisphere’s Pole Star will point you towards South. Handy. Also responsible for some great photography worth checking out.

Pole star

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.

Astronomy sketches