Facts

• The 7 base measurement units
• Why the sun can rise from the west
• Why are there 360 degrees in a circle
  
• How where the time units derived
  
• How were the 12 months named
  
• How were the 7 days named
  
• Why the earth has atmosphere
  
• Why the moon is always facing the earth

The 7 base measurement units

There are only 7 basic measurement units, and any other unit in nature can be drawn from these 7 units. 6 out of the 7 units has been corresponded to a natural procedure, with the exception of mass, which depends on a physical object.

Time (Second)
"The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom."

This means that if you want to count one second, you count the time of 9 192 631 770 periods of this radiation. Initially the second was defined as the 1/86400 of the mean solar day, until astronomers discovered that this duration was not constant. That is because the moon gravity adds 40 nanoseconds to each day, so that now the length of the mean solar day is 86400.002 seconds.

Length (Meter)
"The meter is the length of the path traveled by light in vacuum during a time interval of 1/299 792 458 of a second."

Initially (18th century) the meter was defined as the 1/10 000 000 of the distance from the equator to the north pole, but this distance was variable due to earth's rotation. Later the meter was defined in terms of an actual physical object, a bar, just before 1983 where the above definition was given. Since now a meter is defined through the speed of light, by measuring the speed of light is equivalent of measuring the meter.

Temperature (Kelvin)
"The Kelvin is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water."

The triple point of water is the temperature at which water exists simultaneously in the gaseous, liquid, and solid states.

Electric Current (Amber)
"The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 meter apart in vacuum, would produce between these conductors a force equal to 2 × 10^(-7) Newton per meter of length."

Amount of Substance (Mole)
"The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilograms of carbon 12. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles."

The Carbon 12 is the most common type of the carbon atom.

Intensity of light (Candela)
"The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 × 10^12 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian."

Mass (Kilogram)
"The kilogram is equal to the mass of the international prototype of the kilogram."

The Kilogram is the most problematic of the base units, since it is the only one that is defined in terms of a physical object. The reason that this definition remains is that it is quite accurate to measure a kilogram. Still there does not exist another method that can guarantee a better accuracy (1 part out of 100 000 000).

Why the sun can rise from the west

If you travel fast enough from west towards east just after the sun has set down (e.g., with an airplane), faster than the earth rotates, then you should see the sun rising back from the west. It is like inversing the rotation of the earth.

Why are there 360 degrees in a circle

In the ancient world (Babylonians etc) they preferred to use an arithmetic system with fractions that could be calculated easily. They commonly referred to the number 60, since 60 has 12 divisors: 1, 2, 3, 4, 5, 6, 10, 12, 15, 20, 30, 60. For example one could easily say the 1/60 or 5/60 of a certain quantity. In addition, earth rotates almost 360 times (6 times 60) around itself before returning the same position with respect to the sun. Since this motion draws (almost) a circle, it was comfortable to divide a circle into 360 units too.

How where the time units derived

Keep in mind that ancient people preferred to use numbers with many divisors: 12, 30, 360, etc. This was convenient for their calculations.

1 year
The time elapsed for a full rotation of the earth around the sun.

1 month
(Why 12 months in a year)
Almost the time elapsed for a full rotation of the moon around the earth. Since 1 year divided with one moon-month resulted approximately 12, this was an ideal combination.

1 week
(Why 7 days in a week)
A quarter of a moon-month.

1 day
The time elapsed for a full rotation of the earth around its axis.

1 hour
(Why 24 hours in a day)
This is one of the most difficult. Why a day should be divided into 24 time units and not 12 or 60? The answer lies into your palms. Excluding the thumb, each finger is composed of 3 bones, 3 "parts". So, excluding the thumbs, the fingers are composed of 8*3 = 24 different "parts". One could then refer to the time of the day only by pointing to a specific "part" of a specific finger.

Before 1300 AD, an hour was not a constant interval, but was the 1/12 of the time between sunrise and sunset. Only after the mechanical clocks were invented did the hour become a constant time unit.

1 minute
The 1/60th of an hour. 60 was just a convenient mathematical number. It does not correspond to any physical phenomenon.

1 second
The 1/60th of a minute. Same as above.

How were the 12 months named

Initial naming:

Later the following changes were made:

• Januarius was added (after the god Janus, the protector of the doorway and hence the opening of the year)
  
• Februarius was added (after Februaria, the time of sacrifices at the end of the year)
• Julius Caesar claimed the name of the fifth month.
  
• Caesar Augustus claimed the name of the sixth month because Julius Caesar did.
  (they both wanted their months to have 31 days!)

Note that the word month originates from moon.

How were the 7 days named

Why the earth has atmosphere

This is a combination of 2 reasons. First, gravity. The gravitational force keeps air molecules close to the earth. This reason would be enough in an ideal place, but not when there is the sun nearby. The sun creates the solar wind: trillions and trillions of charged particles traveling with great speeds arise from the sun and crush on the planets, every single second. Mercury has no atmosphere because of the solar wind since all the atmosphere it once had has been destroyed from the solar wind. Even Mars has less atmosphere than the earth because of the solar wind, which is a reason that even if life existed once in Mars, now the thinner layer of air cannot allow it.

Earth wouldn't have the atmosphere capable of holding life too, if it weren't for the second reason: magnetic field. Earth's magnetic field stops the solar wind from penetrating into the atmosphere, and therefore allows its existence.
 

Why the moon is always facing the earth

Probably you have noticed that the same part of the moon is always facing the earth (the other side of it being called the dark side of the moon - also a famous Pink Floyd album :-) ). It appears therefore to be standing still. What really happens is that the moon rotates around its axis at exactly the same speed as earth does. As a result when the surface of the earth turns a little bit, the moon has turned by the same bit and as a result the same people on the surface always see the same side of the moon.

Of course, the real question is why does the moon rotate at the same speed as earth. This is due to a procedure called mode locking, a nonlinear phenomenon common in many electrical and mechanical systems. The basic principle behind it is that when there are two systems oscillating (in the general sense) with certain frequencies, and these systems can be affecting one another, then they tend to alter their frequency of oscillation so that they finally match.

In the case of the moon and the earth, when the moon was first created it didn't have the same period as earth, and at that time different sides of it were visible from earth. However due to the gravitational attraction between them, the earth's much stronger attraction caused the moon to eventually alter its period so that it matched earth's. At this situation the systems locks, because it is the more convenient way of oscillating for both systems.