How do we measure distances in this mindbogglingly big universe?
You don’t need me to tell you that space is humongous. We’ve all found ourselves staring at the night sky in awe of its vastness and wondering what lies beyond the horizon.
Let’s try and understand how vast the universe actually is, so that we can better appreciate that measuring anything in space requires unequalled effort.
Imagine Earth is a ping-pong ball. On this scale, the Sun would be about the size of a Beluga whale and lie roughly four football fields from Earth. The Moon would be a marble just over a meter (4 feet) away from Earth.
What about the rest of the solar system? Well, the other inner planets, Mercury, Venus and Mars, would all be similar in size as Earth, or a bit smaller, and they would lie inside a radius of about 700 meters (2,300 feet) from the Sun.
The outer planets would be much farther away, starting with Jupiter at around 2.4 km (1.5 mi). If we travel twice as farther from the Sun, we find Saturn. The two planets would be about the size of a beach ball, Jupiter being rather larger.
Uranus and Neptune would be the size of a small bowling ball. They would lie respectively 9 km (5.6 mi) and 14 km (8.8 mi) from the Sun.
Pluto is 6 billion kilometers away from the Sun, almost 40 times farther than Earth! On our scale, Pluto is a small marble 18 km (11 mi) away.
Another distant dwarf planet in our solar system, Eris, is almost three times farther than Pluto, about 45 km (28 mi) in our model.
On our tiny scale, where everything is about 300 million times smaller than in reality, it would already be quite a walk to get to the outer bodies of our solar system.
But the edge of our solar system is defined by the region where the Sun’s magnetic influence ends, and is actually much, much farther away than Pluto or Eris.
From now on, we can no longer talk in kilometers or miles, we need to move on to Astronomical Units (AU). One AU is the average distance between the Earth and the Sun, and is about 150 million km (93 million mi).
The edge of the solar system coincides with the edge of the hypothetical Oort cloud, which is estimated at a whooping 100,000 AU from the Sun!
But that’s just the very beginning. Where is the closest star?
The nearest solar system to ours is called Alpha Centauri. Now, even AU will no longer be enough, we will have to start talking about light-years.
One light-year is 63,000 AU. Alpha Centauri is 4.3 light-years away from us. In our model, that’s 126,000 km (78,000 mi).
There are an estimated 300 billion stars in our galaxy alone. The Milky Way spans one hundred thousand light-years in diameter. In our reduced model that would be more than 3 million kilometers (2 million miles)!
The closest galaxy to the Milky Way is Andromeda, 2,5 million light-years away. On our scale, where Earth is a ping-pong ball, that represents 76 billion km (47 billion mi).
The observable universe contains an estimated 2 trillion galaxies. And how big is the observable universe? Its radius is about 46 billion light-years. I’m not going to try and convert that into kilometers on our scale because it makes my head hurt.
But you get the picture: the universe truly is mindbogglingly big.
So, you might wonder, and reasonably, how do we measure anything in space? Even short distances, like the distance to the Moon, how did we measure that?
Let’s find out!
5 ways to measure distances in the universe
Historically, all measurements were done with simple naked-eye observations. With this method, thinkers in Ancient Greece tried to construct a comprehensive model of the cosmos. They were able to deduct quite a few things about the Sun, the Moon and the closest planets.
Today, astronomers use different methods to measure distances to objects in space, depending on how far that object is.
For objects closest to us, those in our solar system, we can use the speed of light to calculate the distance with high precision. This is called the radar method.
1. Radar method
All electromagnetic radiation, whether it’s radio waves, microwaves, gamma rays, visible light or any other type, always travels at the same speed — the speed of light.
When we have a spacecraft at a distant object, we can send a powerful radar signal (radio waves) to that spacecraft. Since we know the speed it travels at and the time it takes to get there, we can easily calculate the distance to the object.
With the radar method, we’ve been able to measure distances to the other seven planets in our solar system, the Sun, the Moon, as well as Pluto. This method is very powerful because of its precision, but it has an obvious shortcoming — we can only use it to calculate the distance to the places we’ve visited.
2. Stellar parallax
Then how do we measure distances outside our solar system, for example to stars? For nearby stars, we use something called parallax.
The idea of parallax is simple and dates back to Ancient Greece: When objects are observed from two different angles, closer objects appear to shift more than farther ones. You can see this for yourself.
Hold one finger at arm’s length and close one eye and then the other. Your finger will appear to shift slightly against the background. By measuring this small change and knowing the distance between your eyes, you can calculate the distance to your finger.
Consider that the Earth moves in its orbit around the Sun, allowing us to look at nearby stars from slightly different locations — just like your two eyes are at slightly different locations. By knowing the size of Earth’s orbit and measuring the angles of the light from the star at two points in the orbit, the distance to the star can be derived.
The farther the star is, the smaller the angles. With our current technology, we can use parallax to measure distances to stars about 100 light-years from Earth. For further stars, the parallax angle is too small and this method fails.
3. Cepheid variables
For stars in our galaxy that are farther away, or stars in nearby galaxies, we use Cepheid variables. These are types of stars which brighten and dim periodically. This behavior allows them to be used as cosmic benchmarks for distances of up to a few tens of millions of light-years.
The first to discover the relationship between the luminosity and period of Cepheid variable stars was an American astronomer Henrietta Swan Leavitt at the beginning of the 20th century.
She noticed that 25 of these stars in the Small Magellanic cloud would brighten and dim periodically. Leavitt was able to measure the period of each star and determine that the brighter the Cepheid, the longer its period. A Cepheid period is regular — it doesn’t change with time. And once you know its period, you can deduce its brightness.
Cepheids are abundant and very bright. They exist in our galaxy, but also in other nearby galaxies. To calculate the distance to a given galaxy, we first locate the Cepheid variables in this galaxy and determine their period.
Leavitt’s data states that a given period has a unique brightness associated to it. From the period and Leavitt’s plot we can then calculate the distance to the stars.
With this method we were able to determine the distance to the Virgo cluster — 56 million light-years.
Leavitt’s discovery changed astronomy because it allowed scientists to accurately measure distances on inter-galactic scale.
4. Supernovae
For larger distances astronomers can no longer use parallax or Cepheid variables. They have to turn to methods that use “standard candles” — objects whose absolute magnitude is thought to be very well-known.
For this to work, we need extremely bright objects, such as a supernova explosion. A special type of supernova, type 1a, has very regular brightening and dimming. When the maximum brightness at a distance of 1 light-year is calculated, it is found to be about the same for all such supernovae.
To calculate a distance to a far-away galaxy, we first find a type 1a supernova in that galaxy and measure its observed brightness. If we compare it to the known maximum brightness, we can calculate the distance to the galaxy in question. With this method, we can measure distances to objects up to one billion light-years away.
5. Redshift and Hubble’s Law
To measure distances beyond one billion light-years, we can no longer count on observations alone — we need to combine them with theory.
The theory used to determine these enormous distances is based on the discovery that the universe is expanding.
In 1929, Edwin Hubble found that the universe is expanding and that all galaxies are moving away from each other.
What’s more, the farther they are, the faster they are moving away from Earth. The velocity of the galaxies has been determined by their redshift, a shift of the light they emit to the red end of the spectrum.
To determine an object’s distance, we only need to know its velocity which we can measure thanks to the Doppler shift. Putting this velocity into the Hubble equation, we can then determine the distance.
This method is based on observation and theory. This means that if the theory is not correct, the distances determined this way are also completely wrong.
Most astronomers believe that Hubble’s Law is true for a large range of distances in the universe.
Let’s recap!
You now have a better understanding of the size of the universe and the considerable effort it takes to measure distances in space.
We use radar signals to measure distances to objects in our solar system we’ve traveled to. With parallax we can measure distances to close stars.
Cepheid variables are special types of stars used to calculate how far are more distant stars in our galaxy and in nearby galaxies.
We use supernovae explosions to find out the distances to distant galaxies.
Finally, if we need to measure something truly far away, we can only count on redshift and Hubble’s Law.
With new technologies popping up every year, we will someday undoubtedly be able to use new methods to an even greater precision.
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