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Definition of orbit motion
Orbital motion or orbit motion is the motion of an object in orbit around another object and an orbit is the curved path of an object around another object in space.
An orbital motion is a motion over a repeating path taken by an object around another (celestial) object.
In general, orbits are elliptical. However, in the case of planets and moons, orbits are often close to circular, and we can treat those orbits as such.
Types of orbit motion
There are a few different types of orbital motions that we can talk about. In this article, we're going to consider circular orbits and non-circular elliptical orbits. Circular orbits have, as you would guess from the name, the shape of a circle. On the other hand, non-circular elliptical orbits have the shape of an ellipse that is squashed in one direction. You can think of an elliptical orbit as being a squashed circular orbit.
The main difference between circular and non-circular orbits, in this case, is the fact that circular orbits have a constant orbital radius, whereas non-circular orbits will have a distance to the orbited object that varies with time and place.
The physics behind circular orbits
To understand orbital motion, we need to understand how the physics of a circular orbit works. The first thing to note concerning a circular orbit is that the speed of the orbiting object remains constant. We're going to use the diagram below to help us understand the physics of a circular orbit around the Sun.
This diagram shows the Earth in two locations during its orbit around the Sun: \( E_1 \) and \( E_2 \). At location \(E_1\), the Earth has a velocity of \(v_1\). However, by the time it is at the point \(E_2\) in its orbit, its velocity has changed to become \(v_2\). This happens because velocity is a vector quantity, and the Earth is not travelling in a straight line: its velocity must change between the points \(E_1\) and \(E_2\). As was stated above, however, the speed of the Earth does remain constant: the magnitudes of the velocities \(v_1\) and \(v_2\) are the same.
So what gives? Well, a changing velocity means one thing is happening: acceleration. In this case of the Earth orbiting the Sun, the Earth is accelerating because it is under the influence of the Sun's gravitational pull. The attracting force due to gravity is labelled as \(F_1\) and \(F_2\) in the diagram above, and this Force causes the Earth to accelerate and thus change the direction of its velocity, following its (circular) orbital path.
Elliptical/non-circular orbits
An elliptical orbit is a type of non-circular orbit. Take a look at the GIF below to get a bit of an understanding of how an elliptical orbit works, and then we'll explain it in a bit more detail.
In the GIF above, you should be able to see that at the side of the orbit closest to the planet, the object moves faster, unlike at the end of the orbit farthest from the planet, where it moves slower. As we have said above, the planets actually orbit the Sun in an elliptical motion, however, the orbits are not as extreme as the one shown in this GIF. Because the orbits of the planet are close enough to be circles, we can treat the orbits as such in GCSE physics.
If objects have enough energy, they can have a trajectory that is not an orbit. Some comets in the Solar System have such a high speed that they will leave the Solar System, never to return again. Such objects follow hyperbolic trajectories.
Examples of orbit motion
There are lots of different kinds of objects in the Solar System, and most of them orbit some other object. There is one main rule we need to keep in mind: a larger object cannot orbit a smaller object. Remember, the Sun doesn't orbit the Earth but the Earth orbits the Sun!
Object (or body) | Object it orbits |
Planet | Star |
Moon | Planet |
Comet | Star or nothing |
Asteroid | Star |
Artificial satellite | Any object |
The main celestial objects we are going to continue looking at are planets, moons, and comets. Each of these objects has a slightly different form of orbital motion, with different orbit lengths and different speeds.
Orbits of planets
All planets have very similar orbital properties and therefore exhibit similar motions. Every planet has a slightly non-circular orbit around the Sun, however for the sake of calculating things, we consider their orbits to be circular. It's also important to note that all planets in the Solar System orbit the Sun in the same direction, as well as in roughly the same planes. In other words, if you looked towards the Solar System from its side, you would see all the planets orbiting across the equivalent of the Sun's equator.
There are some differences between the orbits of planets, though! They all have a different orbital radius, which means they are all at a different distance from the Sun. The closer a planet is to the Sun, the faster it travels and, therefore, the less time it takes to orbit.
Orbits of moons
Moons follow most of the same rules as planets orbiting the Sun, except they orbit planets. The main differences are that they generally don't orbit along the same plane and that some of them have very non-circular orbits. Again, in the case of circular orbits, the orbital radius determines how fast or slow the orbit of a moon will be, with closer orbits being shorter in time with a higher speed.
Orbits of comets
Some comets orbit a star, so we will talk about those comets. The orbit of such a comet is usually highly elliptical, in other words, the orbit is stretched out, and this has some very interesting effects on the motion of the comet. The stretched nature of a comet's orbit causes extremely large changes in speed as the comet's distance from the Sun changes. At its furthest point from the Sun, the comet will be the slowest, and at its closest point to the Sun, the comet will be the fastest. On top of this, not all comets orbit in the same plane or even the same direction, they're all over the place!
Orbit motion of galaxies
As we've discussed, a lot of celestial objects have some form of orbital motion, as they interact with something else. This holds even in galaxies, massive groups of stars held together around some galactic centre that is generally something with an extremely large gravitational pull. Look at the picture of the milky way below, taken by the European Space Agency's Gaia satellite, and notice how the majority of the stars are clustered along the centre line of the image. This is known as the galactic plane, and as we can see, most stars orbit the galactic centre along this plane.
Now whilst we do know a lot about what's out there in the universe, we certainly don't know everything. For example, we have explained how orbital motion is calculated in this article, however, scientists are unable to work out why the orbital speeds of some stars are not what we calculate that they should be. Scientists are not sure why this happens, but believe it could be some unknown particle or field that we haven't discovered yet, and have given the unknown cause of this unexpected orbital speed the name dark energy.
Stability of orbit motion
Consider a satellite in orbit around a planet. For it to remain in a steady circular orbit, the speed of the satellite needs to be a specific value. If the speed of the satellite is too low, then it will fall back towards the Earth, and if the speed of the satellite is too high, it will move off into space, away from the Earth's gravitational pull.
Calculating orbital speed
The speed of the orbit of a satellite can be calculated using a few steps. The first step is to calculate the distance travelled in one orbit. This is calculated using the equation
\[\begin{gather}d=2\pi r, \\(\text{distance}=2\pi\times\text{radius})\end{gather}\]
where \(d\) is the distance travelled by the object and \(r\) is the radius of the orbit. The second step is to calculate the average speed using the following equation:
\[v_\text{average}=\frac{d}{T}.\]
In this case, the time \(T\) is how long it takes the satellite to complete a single orbit.
Let's calculate the orbital speed of an object around the Sun. We'll use Mars as our satellite, as it has a nearly circular orbit. In the case of Mars, its average distance from the Sun is 228 million km, and it takes 687 days to make a complete orbit. The first thing we can do is combine our two equations:
\[v_\text{average}=\frac{2\pi r}{T}.\]
We get this equation by taking our equation for calculating distance and putting it into our equation for calculating speed. The next thing to do is plug the values we have for radius and time to orbit into the equation above to get:
\[v_\text{average}=\frac{2\pi\times 228\cdot 10^6\: \mathrm{km}}{687\:\mathrm{days}}=2.1\cdot 10^6\:\frac{\mathrm{km}}{\mathrm{day}}.\]
Here we have the final answer, which tells us that Mars' orbital speed is 2.1 million kilometres a day.
Orbital Motions - Key takeaways
- An orbital motion is the motion of an object around another object.
- Most celestial objects have some form of orbital motion as they interact with other celestial objects.
- Planets generally have slightly non-circular orbits. However, we can assume them to be circular, which makes orbital speeds easier to calculate.
- Moons have orbits around planets, some of them are circular and some of them are far from circular.
- Comets have highly elliptical orbits, in which they are fastest when they are close to the Sun and slowest when they are farthest from the Sun.
- Orbits are stable when the orbital speed is between two speeds. Too slow, and an object falls back towards a planet, but too fast, and the object will fly off into space.
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Frequently Asked Questions about Orbital Motions
What is orbital motion?
An orbital motion is a motion over a repeating path taken by an object around another (celestial) object.
What type of motion is orbiting?
Orbiting is the motion in an orbit: it repeats itself as it follows a closed curve around another object.
What is the orbital motion of the galaxy?
The orbital motion of the galaxy Milky Way galaxy is due to the fact that most of the stars within it have an orbit around the centre of the galaxy. The galaxy itself does not have an orbit around any other point in space, as far as we know.
What are two motions that cause orbiting?
The two motions that cause orbiting are the forward motion that the orbiting object has and the pull of the gravity of the object that is being orbited work together to create an orbital motion.
What causes an object to orbit?
Objects will orbit each other because of the gravitational attraction between them.
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