# Change in Day Length with Latitude

As the Earth moves around the Sun, the length of the day (defined as the time between sunrise and sunset) changes. The extent to which it changes depends on latitude, as shown in the graph below:

As you can see, the length of a day changes far more during the year at higher latitudes than at lower latitudes. (Latitudes beyond 66°33′ are not shown because the Sun does not always rise or set at these latitudes.) The graph runs from one winter solstice to the next, with the two equinoxes clearly visible in March and September.

It’s quite interesting to look at by how much the length of a day changes every day. This graph would have the same shape as the previous one, but not if we look at percentage change. In a way, this gives an impression of how quickly it appears that “the nights are drawing in”.

At higher latitudes the length of day changes quite noticeably in early January and mid-November.  In some situations two adjacent days are different in length by nearly five minutes, and at some points the day loses nearly fourty minutes over the course of a single week.

# Base Bleed Artillery

Most of the drag on an artillery shell comes from friction between the nose of the shell and the air, as the shell pushes air out of the way at very high speeds. But some of the drag on a shell comes from the sucking effect of the vacuum left behind the shell as it pushes air in front out of the way faster than air can move to fill the space left behind.

To combat this, many artillery shells employ a system called base bleed in which the shell produces gas at its rear to fill this vacuum. This gas produces very little thrust, but by reducing the effect of the vacuum it increases the range of the shell enormously, typically by around 30%. On the diagram on the right (taken from this patent) the top image shows a view from below, with the gas generator’s exhaust labelled “5”. The housing of the gas generator is labelled “1” and the casing of the shell “2”. The igniter that starts the gas generator is labelled “4” and the fuel charge that produces the gas is labelled “6”.

# The Difference Between Centre of Mass and Centre of Gravity

Many people assume that the terms “centre of mass” and “centre of gravity” are synonymous, but this is not the case.

Centre of mass is the point at which the distribution of mass is equal in all directions, and does not depend on gravitational field. Centre of gravity is the point at which the distribution of weight is equal in all directions, and does depend on gravitational field.

A toy bird balances when a pivot is placed at its centre of gravity.

The centre of mass and the centre of gravity of an object are in the same position if the gravitational field in which the object exists is uniform. In most cases this is true to a very good approximation: even at the top of Mount Everest (8848 metres) the gravitational field strength is still 99.6% of its standard value. You are unlikely ever to experience a difference between centre of mass and centre of gravity, as the gravitational field in which you find yourself is extremely uniform.

The object on the left, in a uniform gravitational field, has overlapping centres of gravity and mass. For the object on the right, in which the gravitational field is stronger towards its base, the centre of gravity is below the centre of mass. Approaching a black hole the gradient of the gravitational field would be infinitely “steep”, leading to an incredible difference in gravitational field and death by spaghettification for anyone falling into a black hole.

# Rotational Kinetic Energy and Moment of Intertia

The kinetic energy of an object moving in a straight line is easy to understand:

$E_\textrm{k} = \frac{1}{2}mv^2$

where $E_\textrm{k}$ is kinetic energy, $m$ is the mass of the object and $v$ is the object’s velocity.* If the mass of the object doubles, the kinetic energy doubles; and if the object’s velocity double the kinetic energy is quadrupled.

Kinetic energy is the energy that an object has due to the fact that it is moving, and therefore an object that is rotating must have kinetic energy. But if it’s rotating, staying in one place, then its velocity is zero and thus we can’t use the equation above.

The kinetic energy of a rotating object is given by:

$E_\textrm{k} = \frac{1}{2}I\omega^2$

where $E_\textrm{k}$ is the rotational kinetic energy, $\omega$ is the angular velocity (i.e. the angle turned through divided by the time taken, measured in degrees per second or radians per second). The complicated part of the equation is $I$, which is the moment of inertia.

The moment of inertia can be thought of as the rotational equivalent of mass; the degree to which an object resists a change in its (rotational) motion. The moment of inertia depends on the shape of the object and the way it rotates. For example: the moment of inertia of a rod rotating about its centre, and a rod rotating about its end, are very different.

The moment of inertia of the rod on the left, rotating around its end is $I = \frac{ml^2}{3}$, where $m$ is the mass of the rod and $l$ is its length. The moment of inertia of the rod on the right that rotates around its centre is $I = \frac{ml^2}{12}$ and thus if the rods have the same lengths and masses, and rotate at the same rate, the kinetic energy of the rod on the left will be four times greater.

# 2013-2014 Review

MrReid.org launched on 30th August 2007. In 2012 I published a Five Year Review, and last year I reviewed 2012-2013, and I thought I’d follow up this year with a review of 2013-2014.

## September

September, like many months this past year, was a quiet one for posts. I looked at where all the elements came from, the differences in the order of subjects, objects and verbs in sentences in various languages, and finally worked out the difference between “tons” and “tonnes”.

Full September archive

## October

I was quite pleased with a post about tritium illumination and telomeres and aging, but my favourite post from October was one about the different types of average. I also wrote about an actual method of turning lead (or something else) into gold.

Full October archive

## November

I liked all my posts from November. The post about different sorting algorithms was probably a bit too long, and could maybe have been a bit clearer, but the ones on the different gases used when breathing underwater and on RAID array types were just right.

Full November archive

## December

The post about the EURion Constellation and counterfeit money was easily my favourite from December, and one of my favourites from the whole year. I also liked Earth Sandwich and the post about inequality and the Gini Coefficient.

Full December archive

## January

There was nothing really outstanding from the month of January, but I did enjoy researching Unconventional Nuclear Weapons.

Full January archive

## February

February was very quiet, with only two posts: one on converting from miles to kilometres by using the Fibonacci sequence which was more popular than I’d expected it to be, and one on decibel weighting and the difference between white and grey noise.

Full February archive

## March

March was not a good month for MrReid.org.

## April

April was much better than March, in that I actually made some posts. MrReid.org joined Twitter, and I wrote another personal favourite post discussing what “Five Sigma” data is. I also corrected a very common misconception, writing about the difference between real and apparent weightlessness.

Full April archive

## May

May was another busy month, with a definite typographic theme towards the end. My personal favourite was Why Tokyo Looks Different From Space, but I was also pleased with Ranking Ratings and UPC Barcodes.

Full May archive

## June

An amazing coincidence in June, when an issue I wrote about in a MrReid.org post (Cousins or Siblings?) came up in an A Level biology final exam question a few days later. I also wrote about where space starts, and the shape of rocket engines and the creation of “shock diamonds”.

Full June archive

## July

July wasn’t a brilliant month, because I was very busy at work. I’m still not quite sure I explained why we can’t get all the way to absolute zero quite well enough, but it’s a post I quite like.

Full July archive

## August

I found The Composition of Earth’s Atmosphere with Elevation very interesting, but I have a feeling I might be the only one. I was also very interested by hydraulic fuses, and I finally worked out the difference between an assault rifle and a carbine. I also explained why it feels hotter when the air is more humid.

Full August archive

Here’s to 2014-15. I hope you’ll keep reading.