The stars in space are not actually in fixed positions, despite appearing that way. They move constantly with a very slow speed, that is almost imperceptible to the human eye.
Add that to how far away the stars are from us, and you begin to understand why they appear to remain still.
What is stellar parallax?
This is a scientific term used to describe relative displacement. It is used by astronomers to work out the closest estimate of a distance between objects in space. It uses basic principles of geometry to measure the apparent movement of celestial objects against more distant stars as the Earth rotates.
At 2 different points in the Earth’s rotation, for instance, June and December, a measurement is taken of the angle of the line of sight to the star in question. These measurements will always be taken 6 months apart as this is when the Earth is in the complete opposite position to the first measurement.
Astronomers know that in a 6 month period, the Earth will have traveled 2 AU (astronomical units), or about 93 million miles. With this information, you can work out the parallax angle of the star and the distance away it is.
They will use the formula tan (angle) = 2 AU / distance or distance = 2 AU / tan (angle). The diagram below should help you to understand this method more clearly.
Stars that are closer to the Earth will have a larger parallax than stars that are further away.
Parallax is not just a phenomenon seen in space, it can be applied to everyday life too. Imagine you are driving down a country road at a great speed.
The fields and hedges beside you appear to be moving incredibly fast, whipping in and out of your field of vision. If you look ahead, say to a church spire or a mountain range, this will not appear to be moving much, if at all. This is down to the parallax effect too.
All of the objects that you see are moving with the same amount of speed relative to your speed in the car. The closer objects appear to move faster while the more distant ones remain motionless.
How do the stars move?
When you look up to the sky with the naked eye, you can only see stars that are held within our own galaxy. This galaxy is constantly orbiting a black hole, meaning that we are trapped in a never-ending circular orbit.
Every star and planet in our galaxy is subject to the laws of gravity which plays a key role in our orbit. The combined mass of all celestial bodies in our galaxy is so large, and this mass is primarily distributed near the center.
This creates an intense gravitational pull that draws everything into a circular orbit. This means that not only is our galaxy orbiting the black hole, but the stars and planets within the galaxy are orbiting independently too.
As well as the different pulls from inside and outside of our galaxy, stars also have a random galactic motion. They follow a smooth trajectory that is determined by the balance between the central galaxy gravitational pull and the gravitational field that is in the immediate vicinity of the star.
Does this mean that every star moves differently?
Yes, it does! Every celestial body has a slightly different gravitational pull which adapts the path of motion. The Earth rotates around the sun as this has a large gravitational pull.
In turn, the gravitational pull of the black hole is strong enough to cause our entire galaxy to move around it. This is largely due to the huge amount of mass that a black hole holds.
Are star charts accurate?
Star charts, also known as sky charts, star maps, or sky maps, can be trusted to be fairly accurate. They are reprinted roughly every 50 years to ensure the location of all celestial bodies is up to date and correct.
As technology has advanced, this has allowed astronomers to be much more accurate with their readings. Modern charts are available online using data from NASA and you can even get personalized star maps printed now!
What about constellations?
There are many constellations that even people with no astronomy knowledge can identify. The most commonly known constellations include the Big Dipper, Orion the Hunter (including Orion’s Belt), and Ursa Major.
These have been recorded and seen for centuries, but why is this if all of the stars are constantly moving? The constellation patterns remain the same as the distance between stars appears to remain constant.
The constellations will move as a group through the sky and may change rotation but the standard format will never alter.
There are 88 original constellations recorded, primarily by Ancient Greek astronomers. These are very useful for navigation and time-telling purposes.
What about the North Star?
The North Star, also known as Polaris, is commonly used as a navigational tool marking the north pole of the sky. Many explorers will use this in place of a compass during the night when visibility is reduced.
Surprisingly, the North Star does in fact move in the sky. In actuality, it is not marking the exact point of celestial north and is roughly ¾ degree offset. This means that it actually creates a small circle around the exact celestial north every 24 hours.
Polaris is not the only North Star to have existed. Every 26,000 years the Earth’s axis traces a circle on our celestial sphere due to a motion known as precession. This means that the North Star changes over time.
In Ancient Egyptian times, close to the building of the pyramids, the North Star was a star known as Thuban, found in the Draco the Dragon constellation. In 12,000 years, scientists predict that a star in the Lyra constellation, known as Vega will be the new North Star.
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