AAA: How do we know how far away stuff is?

When you look up at the night sky, it's easy to wonder: how do we actually know how faraway the moon, the stars, and the galaxies are? After all, we can't stretch a tape measure across space! 

Astronomers have developed clever methods to unlock the vast scales of the universe, step by step - building whats known as the Cosmic Distance Ladder. 

From bouncing radar signals off nearby planets to measuring the ancient light of the universe itself, each 'step' of the ladder helps us reach further into the cosmos.

Read more to discover how we measure the unimaginable - and just how far our reach really goes.

The Cosmological Distance Ladder

In astronomy we talk about things in are in our visible view like the moon andd galaxies at the edge of the known universe. But since we cant take a measuring stick and physically measure the ddistance to these objects, a very reasonable question is:

How do we measure distances in space?

There are a number of techniques astronomers use. Each one helps us reach a little farther, building on the pervious, which is why we imaginatively call the whole system the "cosmological distance ladder". We rarely measure distances directly - instead we work them out indirectly using observation planets, stars and galaxies that we can actually measure. 

Fig 1: A diagram of the Cosmological Distance Ladder

The Solar System

At the bottom of the ladder is our Solar System. Here, we can use radar: we send out a pulse of radio waves toward an object - like the Moon or Neptune - andd wait for the reflection to return. Because we know radio waves travel at the speedd of light, the time it takes tells us how far awayy the object is.

There's even a mirror left on the Moon by astronauts so that we can bounce lasers off it and measur ethe Moons distance with incredible precision. (Fun Fact: the Moon is moving away from us by about 4 inches every year!) We also track spacecraft as they travel through space, giving us extremely accurate measurements of the planets they pass or land on. 

Near Stars

For stars beyond our Solar System, we use a method called parallex. This is like holding your thumb out at arm's length and closing one eye, then the other - your thumb seems to move! Astronomers do the same thing with nearby stars, using Earth's orbit around the Sun to get two different viewing angles six months apart. The apparent shift tells us the distance. 

Because these stars are so far away, we measure their ddistance in light years - the distance light travels in one year, which is nearly 6 trillion miles! Even our nearest star is 24 trillion miles away, so we just say its 4 light years instead. This is much more digestable for our understanding. Parallex works well for stars upto a few thousand light years away.

Within The Milky Way 

To explore further into the Milky Way, we use standard candles - objects like Cepheid variable stars that pulse in a predictible way so we can identify them, and shine with a known brightness. It's like seeing a candle up close and another far away - you can tell which is further away just by how dim it looks. Because we've used parallex to measure the distances to nearby Cepheids, we can use them to estimate distances to far more distant ones - stretching across the Milky Way. 

Nearby Galaxies

Outside our galaxy, the Milky Way, it gets a bit harder. Our closest galaxy is over 2.5 million light years away. So we have to use a similar approach as standard candles but instead of using stars we need to use supernova - an exploding star. Specifically, a type 1a supernova which is a type of explosion that has a predictable brightness. Imagine you have a whole bag of the same size balloons. If you blow them up too much they will all explode at the same size. Type 1 supernovas do this, and when they do explode they can outshine all the other stars in their galaxy so we can see them and measure their brightness.

Faraway Galaxies

At the top of the ladder, we're peering deep into the universe. Light from distant galaxies stretches as the universe expands - a phenomenon called redshift. The more redshifted the light, the farther away the galaxy is. By comparing the amount of redshift to the known rate of expansion of the universe (called the Hubble constant) we can estimate how far the light has travelled. This method allows us to map the universe out to billions of light years, approaching the very edge of what we can observe. 

The actual eddge of the observable universe

The oldest light in the universe - the Cosmic Microwave Background - is the final step. This light comes from just 380,000 years after the Big Bang and has been travelling through space ever since. It's stretched so much by the expansion of the universe that we now detect it as microwaves, rather than visible light. 

This gives us a boundary: the edge of the observable universe, about 13.8 billion light years away (though the actual size of the universe may be far larger).

These techniques - from radar to redshift - make up the Cosmic Distance Ladder. Each rung buildd on the one below it, allowing us to map our universe step by step. Together, they form the backbone of mordern astronomy helping us understand the size, shape and age of the universe. And it all begins right here, in our own backyard.