Monthly Archives: May 2013

Cranial nerves

The cranial nerves are a group of twelve pairs of nerves which emerge directly from the brain, rather than from the spinal column as is the case for most nerves.


The first two cranial nerves, the olfactory nerve (CN I) that transmits the signals that make up your sense of smell and the optic nerve (CN II) responsible for sight emerge from the cerebrum (the big grey wiggly bit of your brain); the other ten emerge from the brainstem.

The oculomotor (CN III), trochlear (CN IV) and abducens (CN VI) nerves control the muscles around the eye, moving it from side-to-side and tilting it up and down. The oculomotor nerve also controls the dilation of the pupil.

The trigeminal nerve (CN V) transmits sensations from the face to the brain and innervates (activates/operates) the muscles that you use to chew.

The facial nerve (CN VII) innervates the muscles that create facial expressions and carries information from your tastebuds on the rear anterior (front) two-thirds of your tongue. It is also responsible for sending the signals that cause two of your three salivary glands (the submandibular and sublingual glands) to activate.

The vestibulocochlear nerve (CN VIII) transmits sound to the brain and provides information about your position in space in relation to the force of gravity. It is essential for your sense of balance and allows you to move.

The glossopharyngeal nerve (CN IX) receives taste signals from the remaining one-third of the tongue and controls the activation of the remaining salivary gland (the parotid). It also innervates the stylopharyngeus muscle that is part of the process of swallowing food.

The vagus nerve (CN X) innervates most of the laryngeal and pharyngeal muscles, controlling the voice and helping with the swallowing of food. It also transmits signals from the tastebuds on the epiglottis.

The accessory nerve (CN XI) innervates the sternocleidomastoid and trapezius muscles that rotate and flexes the neck and head. The final cranial nerve, the hypoglossal (CN XII), innervates the tongue muscles (with the exception of the palatoglossus which is controlled by the vagus nerve) and thereby helps with speech and eating and swallowing food.

Disk drive sizes

In the International System of Units there are standard prefixes, based on powers of ten, used to indicate multiplication or division: kilo- to indicate multiplication by one thousand (103), mega- to indicate multiplication by one million (106), giga- to indicate multiplication by one billion (109), and so on.

But computer scientists don’t like powers of ten. The most basic unit of digital storage, the bit, is represented either as a one or a zero (with eight bits making a byte) and thus computer scientists are much happier working in binary, with powers of two rather than powers of ten. Standard binary prefixes do exist: kibi- for 210, mebi- for 220, gibi- for 230, etc.

SI Unit Size /B Binary Unit Size /B
kilobyte (kB) 1000 kibibyte (KiB) 1024
megabyte (MB) 1 000 000 mebibyte (MiB) 1 048 576
gigabyte (GB) 1 000 000 000 gibibyte (GiB) 1 073 741 824

The problem is that barely anyone uses the standard binary prefixes. During the “kilobyte era”, because 1000 and 1024 aren’t much different (2.4%) the difference was mostly ignored. But as file and hard disk drive (HDD) sizes have increased the difference between them has become more noticeable.

HDD manufacturers have stuck with SI (10x) sizes whilst operating systems calculate sizes in binary, but incorrectly use SI prefixes. A 256 gigabyte hard drive (i.e. one containing 256 billion bytes) will be reported by an operating system as being only 238 GB in size, a 6.9% difference. As HDDs becomes ever larger the problem will get worse: at the terabyte level the difference is 9.1% and at the petabyte level it is 11.2%.

Persuading operating systems to alter the way they report file sizes, thereby confusing users in the process, is unlikely to be a successful approach. A far better approach would be to persuade HDD manufacturers to change their marketing so that users purchasing a HDD receive the size they are expecting.*

* Though obviously, as a physicist, it causes me great mental anguish to abuse SI units in this fashion!

Solar eclipse types

The word “eclipse” is a general astronomical term that applies to any situation in which the view of one object is blocked by another, caused either by the second object’s shadow falling over the first, or by the second object coming in between the first object and the observer.

In general use the term eclipse usually applies to one of two situations: a solar eclipse, in which the Moon obscures our view of the Sun; or a lunar eclipse in which the Earth’s shadow falls over the Moon. The planes in which the Earth orbits the Sun and which the Moon orbits the Earth are at an angle to each other, which is why there is not a total solar eclipse once every month.

There are three types of solar eclipse:

  • A total eclipse, in which the Moon appears the same size as the Sun and blocks light from it completely.
  • An annular eclipse, in which the Moon passes between the Sun and the Earth but because of the relative distances of each it appears slightly smaller than the Sun, causing a ring of light to appear.
  • A partial eclipse, in which the orbits of the Moon and Sun are such that the Moon only covers only part of the Sun.


A comparison of a total solar eclipse (left) and an annular eclipse (right). Only in the case of a total solar eclipse is the Sun’s corona (the white “cloud”) visible.


A diagram showing how the three different types of eclipse are formed.

There are also hybrid eclipses, which appear as total eclipses to some parts of the Earth and an annular eclipse to other parts. These are very rare, with the most recent in April 2005 and visible mainly from the Pacific Ocean and also Costa Rica, Panama, Colombia and Venezuela; and the next to occur in November 2013, visible from Central Africa (Gabon, Congo, Democratic Republic of the Congo, Uganda, Kenya, Ethiopia and Somalia).


Reaction wheels and pointing satellites

The Kepler telescope, a satellite telescope in orbit around the Sun* designed to look for exoplanets, has come to the end of its original mission. The end of the mission was caused by the failure of ‘s reaction wheels, which are used to point the telescope. Reaction wheels are electric motors connected to metals discs, usually with masses between . As the speed of the motors are altered the conservation of angular momentum imparts a force on the spacecraft, causing it to rotate around its centre of mass. lro-reaction-wheels

The aboard the Lunar Reconnaissance Orbiter.

Reaction wheels are usually employed in groups of three, the x-, y- and z-axes, enabling the telescope to be pointed accurately in . Kepler was fitted with , with all , and each acting as a spare for the other three. After wheels (#2) failed in July 2012 the spacecraft was still able to operate normally, but reaction wheel #4 began malfunctioning in January 2013, and whilst the wheel returned to working order initially, it failed completely on, leaving the spacecraft now unable to move and point properly. kepler-reaction-wheels

Reaction wheels aboard Kepler.

Reaction wheels are often confused with momentum wheels, but the two are very different. Momentum wheels are much heavier, and spin at much higher rates, and their role is not to point or steer the spacecraft but rather to use the gyroscopic effect to keep it in a fixed position when subjected to perturbing forces such as solar wind or radiation pressure. Satellites in orbit close around Earth can also use a device called a magnetorquer to control their position. By altering the flow of current through a set of coils (again usually three, with the spacecraft pushes against Earth’s magnetic field and the reaction force against this push causes the satellite to rotate.

* Technically an Earth-trailing heliocentric orbit.

Squat effect


The MS Oasis of the Seas is the world’s largest cruise ship (holding the title jointly with its sister ship, MS Allure of the Seas) and stands seventy-two metres above the water line. During the delivery process, as it was sailing from the STX Shipyard in Finland where it was built to its first stop, it had to pass underneath the Great Belt Bridge that connects the Danish islands of  Zealand and Sprogø, a bridge that stands only sixty-five metres above the waterline. Oasis of the Seas is equipped with telescoping funnels and ballast tanks, but this would only create a tiny margin of safety between the ship and the bridge.


Luckily, the ship was able to rely on something called the squat effect to help it make it under the bridge. The squat effect occurs when a ship moves through shallow water at speed. The water is forced underneath the ship, increasing its speed, and this causes the water pressure to drop (by Bernoulli’s Principle), thereby pulling the ship deeper into the water.

ship-no-squatIn deep water no squat effect is present.

ship-squatIn shallow water the squat effect causes the ship to sink lower into the water.

Oasis of the Seas approached the bridge at twenty knots (10.3 m/s; 23 mph), close to its twenty-two knot top speed and this was enough to pull it an additional thirty centimetres into the water, allowing it to pass safely under the bridge.