Just how big is everything

"Space is big, really big, you just wont believe how vastly mind bogglingly huge it is. I mean you may think its a long way down the street to the chemist but that's just peanuts to space. Listen....."

A word of caution this article contains really big numbers so I will use some simple scientific notation, it looks like this 1*10^2 which basically means 1 *10 *10 or 100, another example 5*10^3 would be 5*10 *10 *10 or 5000 (basically it is the first digit followed by a number of zeroes matching the number after the ^) also each of the units below is a thousand times larger than the one before it.

one, thousand, million, billion, trillion

The simplest to expalin unit of measurement used on the scale of galaxies or the universe is the light year, so I will use that. Light (in a vacuum) travels at 3*10^8 meters per second or 300 000 Km per second, for scale the earth is only around 12 700Km across. A light year is the distance travelled by light in a vacuum in one year and is about 10 000 000 000 000 or 10 trillion Km, the nearest star to our sun is around 4.5 light years away, our galaxy 1*10^5 light years across and the visible universe 9.2*10^9 light years. An interesting point to mention here is that obviously the light we receive shows us what these objects were like when the light left them, as such we see things as they were millions or even billions of years in the past. This means very distant galaxies look very different from near ones and the limit of the observable universe is caused by seeing back so far in time that the light at this distance was emitted long before any stars formed at the point when the universe finally became cool and spread out enough for it to be able to transmit light.

The visible universe (as far as we can see) contains a minimum 170 billion(1.7*10^11) galaxies, the smallest of which contain around 10 million(1*10^7) stars and the largest 100 trillion(1*10^14), giving a low estimate of around 1700 000 000 000 000 000 000 stars in the universe.

Take a few minutes to get your head around these numbers and watch this video attempting to show the length scales involved and this one demonstrating the relative sizes of some of the stars in our galaxy. Each of those behemoths is to the naked eye at most a single dot of light or completely invisible due to the shear distance between Earth and it. Some of those minuscule points of light in the sky every night are even entire galaxies, a hundred billion glowing giants so far off that combined they are barely even visible and most galaxies are utterly undetectable without incredibly powerful telescopes. If we then turn that logic on its head we get this, and bearing in mind that the photo was taken from near Saturn, a distance which is less than insignificant on a galactic scale. Understanding just how small our world is is one of the most incredible and humbling experiences I can think of.

Opening quote taken from The Hitchhikers Guide to the Galaxy by Douglas Adams, everyone should go read it.

Earths shape

I was revising for an exam the other day in the physics resource room and somehow we ended up having a short conversation about conspiracy theorists and the one name I remember cropping up was the Flat Earth Society. So I have decided to do a bit of a rant/proof on this. This post will just be a bit on the shape of the Earth and I will go into the evidence later in the week.

Long ago people assumed the Earth was flat, perfectly reasonable since apart from all the hills and valleys no overall curving trend was observable to the people at the time living their day to day lives. Even today a flat Earth is a sufficiently good local approximation for people who don't travel exceptionally long distances, although there are plenty technologies that do rely on it to work but I'm discussing direct observation at this stage so that's beyond the point.

Things started to change around 330BC when the first evidence started to emerge for a spherical Earth, this was an important discovery because long distance navigation such as crossing oceans is going to be substantially different depending on if the ocean is flat or curved and yes by the time Columbus came along everyone knew full well the Earth was certainly not flat, give the people of the time a little credit.

That is all well and good and for most of us this will be the most accurate model we need but it is not the end of the story, a more accurate picture, and one important for satellites was first proposed by Newton in 1689. Since the Earth rotates there will be a 'centrifugal force' that will push the material outwards more strongly the further it is from the axis of rotation(actually its not a force as such, just inertia, the material further from the axis is moving faster so it is harder to change its direction). The end result of this is that the radius of the earth to one of the poles is 6 356.8km but the radius to the equator is 6 378.1km. This means the polar diameter is about 40km less than the equatorial diameter, this slightly flattened sphere is called an oblate spheroid.

It is even possible to make more detailed observations which have found that the South pole is 40m closer to the centre of the Earth that the North pole, so it actually has a very slight pear-shape, with various lumps and dents roughly matching the continents and oceans. However having said that its relative variations from an oblate spheroid(0.17%) are less than a pool ball from a perfect sphere(0.22%).

All that is actually a nice example of how science works, start with a basic idea, such as "as far as I can see directly the earth is flat" and then develop the idea "actually its a sphere it just looks flat when your standing on it because its bloody huge" and further development "wait its a slightly squashed sphere" and this continues to whatever accuracy is needed. Since the maths involved gets evil very fast as the idea is refined the secret is to use the simplest model that is detailed enough for the task at hand. Treating the Earth as an oblate spheroid is vital to keep satellites where they are needed, but its a bit of a headache when you just want to find somewhere on a map of your town, at that level even the gross oversimplification(to the point of outright wrong) of a flat Earth will do the job. Of course the key point of it being science it you also have to prove the ideas.

http://www.scientificamerican.com/article.cfm?id=earth-is-not-round an article covering spherical Earth onwards

http://ocw.mit.edu/courses/earth-atmospheric-and-planetary-sciences/12-002-physics-and-chemistry-of-the-terrestrial-planets-fall-2008/lecture-notes/MIT12_002f08_lec27_28.pdf the kind of maths involved in getting this lovely detailed understanding of the Earths shape.