The motion of celestial bodies has been studied and interpreted for tens of thousands of years. Understanding how the Moon and Sun move helped our ancestors thrive across the changing seasons. Expanding that to the planets gave us an idea of our place in the universe. And the advent of the astronomy of precision several centuries ago, brought to light that we are a tiny fish in a very big pond. But how do astronomers work out where things are with respect to us?
There are different methods depending on when and what we are trying to map. And we are trying to map a lot of things in the universe. Some to keep our planet safe, some to understand our galaxy and its formation, and some to answer the most profound questions of cosmology.
Know Your Enemy
Celestial mechanics is complicated. There is no exact solution for the motion of three bodies in a gravitational field (the infamous three-body problem). So imagine when you have thousands of them, like in the Solar System. The way we do that is by measuring their positions over and over again.
For the planets and moons, we do not have to do this constantly; we know enough about them to know where they will be for quite a long time. But when it comes to smaller bodies such as asteroids, it is important to understand their precise orbits. This is doubly important for near-Earth objects, those asteroids and comets that come very close to our planet – because they might hit us.
These (relatively) small space rocks tend to be very dark. This size and color combination is not great for finding them, but this has not deterred astronomers. Using radio, infrared, and optical telescopes, over the last 25 years, 90 percent of planet-killer (larger than 1 kilometer) asteroids have been discovered.
But it is not just about finding them. Repeating observations is important because it allows us to understand and refine the orbits of those bodies, and work out where they have been and where they are going to be. An exciting recent development in this area is the use of radar technology, which allows for a greater precision of the orbital parameters.
Our Place Among The Stars
Radar would work at most to the orbit of Saturn – but we want to know our place in the Milky Way. The first approach is to work out the distance of stars using the parallax method, bringing some trigonometry to astronomy. Trigonometry is the study of the function of angles. You have your sine, cosine, and tangent.
This comes in useful in astronomy when you realize that you can create nice little triangles by working out how a star changes its apparent position in the sky as our planet moves around the Sun. The angles in question are very small, but with precise enough measurements, we can measure the distance of so many stars.
And no instrument is more precise for this job than the Gaia spacecraft. This European Space Agency mission has created the most precise map of the Milky Way, working out the position and motion of over a billion stars, but also exoplanets, comets, and asteroids in the Solar System, and even a few black holes.
This is not a photograph of the Milky Way, but the map reconstructed using data collected by the Gaia spacecraft. It looks like a photo because it is that good.
Image credit: ESA/Gaia
The Shape Of The Universe
But what about the wider cosmos? For a long time, we were only aware of our own galaxy. We had no way to measure things further afield (the parallax method is only as good as our precision). It was thanks to astronomer Henrietta Swan Leavitt that suddenly the universe was opened up to us. Her work shows that certain stars called Cepheid variables have a specific relationship between their period of variability and their intrinsic luminosity.
If you know how luminous an object is and you measure the apparent brightness here after the light has traveled such a long distance, you can work out that distance. This is the principle of the standard candles. By measuring the period of Cepheid variables, there was finally a way to measure extragalactic distances.
Not that astronomers knew there was something out there until Edwin Hubble found Cepheid variables in several “nebulae”, including Andromeda, and realized that they couldn’t be in the Milky Way. Those stars were in other galaxies.
But Hubble made another important discovery while studying these newly found galaxies, about a century ago. Almost all of them are moving away from us. It was the first evidence that the universe is expanding.
We know that they are moving away because we can measure their redshift. This phenomenon is similar to a Doppler shift, something we often experience on roads. If an ambulance is approaching with its siren on its pitch will be higher and once it passes and moves away from us its pitch will get lower. The siren is not changing, but the sound waves are compressed as it moves towards us and stretched when it moves away.
If you move fast enough, the same thing happens with light. Things moving towards us get bluer (their light waves are compressed) and away from us get redder. Redshift is not a true Doppler shift, however, because it is not the motion of the galaxies causing it, but the expansion of the universe.
Both the use of standard candles and redshift help us build a 3D picture of the universe, and that is where we hope to find answers to the biggest open questions in cosmology: what is dark matter and dark energy? These two components make up 95 percent of all matter-energy content of the universe. The rest is the regular matter that makes us.
Galaxies are not randomly distributed across space; they are organized in a structure called the cosmic web, which is shaped by all forms of matter and energy. Understanding what it is like precisely – such as with upcoming surveys, like the one planned by the Vera Rubin Observatory – will provide important insights into the mystery and hopefully, solutions.
Since antiquity, we have used the heavens to navigate our planet and map our way from here to there. It is only natural then that our map-making obsession has stretched from our pale blue dot to the farthest galaxies we have ever seen.
Source Link: How Do Astronomers Map The Universe?
