T e r m s page 1
A long list here
This page is NOT for readers who are content with everyday terminology.
If you have suggestions/requests for more terms to go on the page click here.
You will find mention of the following:
Aphelion
Astronomical unit
Dark energy
Dark Matter
Galaxies, stars and planets
Gravity
Light year
On this page.
Mass
Matter
Light
Perihelion
Photons some within other entries
Planck Constant
Quanta
Quantum
Quantum Mechanics
Quantum Theory
Radiation
Refractive Index foot of page
Space
Spacetime are here
Preamble
Astronomy, physics and astrophysics are all riddled with technical terms. The problem with definitions in books etc and via www is that explanations of X assume that you understand Y and Z.
.. light travels faster in vacuum than any other medium. This is because there is no obstruction in vacuum for the propagation of light and thus, the refractive index of vacuum is the lowest.
We don't need to know about the refractive index of vacuum.
If you do, it's at the foot of the page.
1
- The astronomical unit (AU) is a unit of length, roughly the distance from Earth to the Sun.
- However, that distance varies as Earth orbits the Sun, from a maximum .. to a minimum ... and back again once a year.
- Originally conceived as the average of Earth's aphelion and perihelion . .
- aphelion and perihelion here.
2
- Since 2012 it has been defined as exactly 149597870700 metres, or about 150 million kilometres (93 million miles)
- The astronomical unit is used primarily for measuring distances within the Solar System or around other stars. It's approximately 8 light-minutes.
- source
- Astronomical units are just a useful way to think about the solar system relative to the distance from Earth to the Sun, because it's easy to use. ... When you use AU, it is easier to understand the relative distances, and that Saturn is about ten times farther from the sun.
- source
- Why does it take about 8 minutes for sunlight to reach Earth?
- You might as well ask why it takes an hour from London to Brighton. Because the distance and speed = the time.
- Sunlight travels at the speed of light. See Light Year
- Photons emitted from the surface of the Sun need to travel across the vacuum of space to reach our eyes.
- The short answer is that it takes sunlight an average of 8 minutes and 20 seconds to travel from the Sun to the Earth.
- source
- About 8.5 mins as the earth orbit is not circular.
- Photons are units of light; a particle representing a quantum see below of light or other electromagnetic radiation.
- Photons. The property most readily measured is their energy. The different colors of light for example, are thought of as representing photons of different energy.
- source
- In physics, a quantum (plural: quanta) is the minimum amount of any physical entity (physical property) involved in an interaction.
- source
- Photons .. are particles which transmit light.
- In other words, light is carried over space by photons.
- source
- We study this website extract to glean Dark Energy before transferring to Dark Matter.
Dark energy
We still don’t know what it is. It is everywhere and we can’t see it. It makes up more than two-thirds of the universe, but we have no idea where it comes from or what it is made of.
Dark Matter here
Galaxies, stars and planets OU here
Gravity
Gravity (from Latin gravitas, meaning 'weight'..., or gravitation, is a natural phenomenon by which all things with mass or energy—including planets, stars, galaxies, and even light[2]—are brought toward (or gravitate toward) one another.
On Earth, gravity gives weight to physical objects, and the Moon's gravity causes the ocean tides.
The gravitational attraction of the original gaseous matter present in the Universe caused it to begin coalescing, forming stars—and for the stars to group together into galaxies—so gravity is responsible for many of the large-scale structures in the Universe.
Gravity has an infinite range, although its effects become increasingly weaker on farther objects.
Light Year
A light-year is the distance light travels in one year. How far is that?
Multiply the number of seconds in one year by the number of miles or kilometers that light travels in one second, and there you have it: one light-year.
It's about 5.88 trillion miles (9.5 trillion km).
What is the number of miles or kilometers that light travels in one second?
The speed of light in a vacuum is 186,282 miles per second (299,792 kilometers per second), and in theory nothing can travel faster than light. In miles per hour, light speed is, well, a lot: about 670,616,629 mph.
Try finding its speed not in a vacuum and brick walls spring to mind.
After a long while, the answer is here but read another view at the foot of the page.
Q: What’s a light-year?
A: The same as a regular year, but with fewer calories.
Mass
Mass, in physics, the quantity of matter in a body regardless of its volume or of any forces acting on it. The term should not be confused with weight, which is the measure of the force of gravity . . acting on a body.
Mass is not the same thing as weight.
Weight has meaning only when an object having a specific mass is placed in an acceleration field, such as the gravitational field of the earth.
At the earth's surface, a kilogram mass weighs about 2.2 pounds, for example.
But on Mars, the same kilogram mass would weigh only about 0.8 pounds, and on Jupiter it would weigh roughly 5.5 pounds.
Mass (symbolized m) is a dimensionless quantity representing the amount of matter
in a particle or object. The standard unit of mass in the International System is the
kilogram .... .
Mass is measured by determining the extent to
which a particle or object resists a change in its
direction or speed when a force is applied.
If you'd like to see an explanation video combining
mass and matter find
" See a physics video comparing mass to weight:"
here.
Matter
has many definitions, but the most common is that it is any substance which has mass and occupies space.
All physical objects are composed of matter, in the form of atoms, ...
Light an extension of 3 - Light here
- Light travels as waves .... like the ripples in a tank of water. The direction of vibration in the waves is at 90° to the direction that the light travels.
- Light travels in straight lines*, so if you have to represent a ray of light in a drawing, always use a ruler.
- *Not so. It could pass near to a star and its line is curved by gravity.
- Unlike sound waves, light waves can travel through a vacuum (empty space). They do not need a substance to travel through, ...
- Light travels extremely quickly. Its maximum speed is approximately 300,000,000 miles/second, when it travels through a vacuum.
- The very large difference between the speed of light in air (almost 300,000,000 m/s) and the speed of sound in air (343 m/s) explains why you:
- see lightning before you hear it - see a firework explode before you hear it - see a distant door slam before you hear it - source
- Imagine you’re in a park, looking at a leaf on the branch of a tree. We know light bounces off the leaf to your eye to tell you it’s green – but what is light, exactly?
- We can trace the first steps towards understanding light’s makeup to a benchtop in Copenhagen in 1820, where Danish scientist Hans Christian Ørsted was giving a lecture on electricity.
- A compass happened to be sitting near the battery he was using in his demonstration and he noticed the compass needle suddenly jerking when he switched the battery on or off.
- This meant electricity and magnetism were related – or, as it was more formally described later, a changing electric field creates a magnetic field.
- Then 11 years later, English scientist Michael Faraday discovered the opposite rang true: that a changing magnetic field also creates an electric field.
- It was the Scottish physicist James Clerk Maxwell who collected these ideas about electricity and magnetism (plus a few others) and pulled them together into one coherent theory of “electromagnetism”.
- But Maxwell’s most celebrated insight was when he combined the work of Ørsted and Faraday to explain the essence of light.
- He realised that a changing electric field could create a changing magnetic field, which would then create another electric field and so on. The result would be a self-sustaining electromagnetic field, endlessly repeating, travelling incredibly fast.
- How fast? Maxwell was able to calculate this too, at about 300,000,000 metres each second – pretty close to what had recently been measured for the speed of light.
- And so this is what light is: an electric field tied up with a magnetic field, flying through space.
- You can think of the two fields as dance partners, wrapped up in an eternal embrace. To keep self-generating, both electric and magnetic components need to stay in step. It takes two to tango.
Planck constant
is a physical constant that .. relates the energy carried by a photon [unit of light]... and is the basis for the definition of the kilogram.
In physics, a quantum (plural: quanta) is the minimum amount of any physical entity (physical property) involved in an interaction.
Quantum mechanics
is a fundamental theory in physics which describes nature at the smallest scales of energy levels of atoms and subatomic particles.
Quantum theory
is the theoretical basis of modern physics that explains the nature and behavior of matter and energy on the atomic and subatomic level. ...
Planck wrote a mathematical equation involving a figure to represent these individual units of energy, which he called quanta. See his Constant on this page.
Radiation in physics
The emission and propagation of energy in the form of rays or waves.
The energy is radiated or transmitted in the form of rays, waves, or particles.
A stream of particles or electromagnetic waves that is emitted by the atoms and molecules of a radioactive substance as a result of nuclear decay.
More on refractive index
It is widely known, however, that the speed of light is variable when it is not moving through a vacuum.
The ratio of the speed of light through a given medium and through a vacuum is called the medium's refractive index or optical density.
Some media have such a high refractive index that they can slow light to the speed of a person walking, or even bring it to a temporary standstill.
At least two media are capable of slowing down light tremendously: Einstein-Bose condensates, and hot rubidium gas. These have both been used to halt light entirely. This was first achieved on a temporary basis in experiments conducted in 2001.