And returning him safely to the earth

In 1961, John F. Kennedy told Congress: “I believe that this nation should commit itself to achieving the goal, before this decade is out, of landing a man on the moon and returning him safely to the earth.

The Moon landing on 20 July 1969 achieved the first part of that goal. The second part was yet to come (in 1970, that would prove to be the hard part).

But on 21 July 1969, at 17:54 UTC, the spacecraft Eagle lifted its metaphorical wings and took off from the Moon (well, the upper ascent stage took off, as shown in the photograph below). There followed a rendezvous with Columbia, a flight back to Earth, and an eventual splashdown on 24 July. Mission accomplished.


The Eagle has landed!

Fifty years ago, on 20 July 1969, at 20:17:40 UTC, the spaceship Eagle landed on the Moon. Here is the landing site – below, as it was, and above, as seen by the Lunar Reconnaissance Orbiter Camera (LROC). Clearly visible in the LROC image, and illustrated with inset photographs from 1969, are:

A tale of two arrivals

Fifty years ago, on 19 July 1969, the spaceship duo Columbia / Eagle entered orbit around the Moon, roughly 3 days and 4 hours after its launch, as part of the Apollo 11 mission. Eagle (with Neil Armstrong and Buzz Aldrin) went on the land on the moon on 20 July while Columbia (with Michael Collins) continued to orbit the moon. When he announced the space programme, Kennedy had said:

We choose to go to the moon. We choose to go to the moon in this decade and do the other things, not because they are easy, but because they are hard, because that goal will serve to organize and measure the best of our energies and skills, because that challenge is one that we are willing to accept, one we are unwilling to postpone, and one which we intend to win, and the others, too.

Much can be learned from doing hard things, and an enormous amount was learned from the space programme. Solar car teams also learn a great deal from doing hard things. Fifty years after Columbia and Eagle entered orbit, also after hard effort, the University of Michigan Solar Team’s solar car Electrum arrived in public view (at 17:30 Michigan time). They intend to win too!

A tale of two launches

Fifty years ago, on 16 July 1969, a Saturn V rocket carrying three men (Neil Armstrong, Buzz Aldrin, and Michael Collins), plus the command module Columbia, and the lunar module Eagle, took off from Florida, en route to the first human landing on the moon (above).

Fifty years later, the Dutch Vattenfall Solar Team launched their 10th solar car, Nuna X (below). Catamaran-style solar cars seem to have reached an optimum: the front of Nuna X looks a lot like Nuna 9 and the rear looks a little like Twente’s Red E and Agoria’s Punch 2 – but only on the left side! The new car weighs just 135 kg (298 lbs), which is probably the lightest ever solar car. I think that Twente, Agoria, and everybody else will have their work cut out trying to outrace this latest car from Delft!


The 2019 Eurovision Song Contest is on right now. Above (click to zoom) is a combined word cloud for the songs (or English translations of the songs).

From the point of view of getting into the final, it seems to be bad to sing about Heaven (Montenegro, Portugal), war (Croatia, Finland), cell phones (Belgium, Portugal), or cold (Latvia, Poland, Romania). On the other hand, it’s good to sing about lights (Germany, Norway, Sweden).

Good luck to everyone for the final!

International Nurses Day

Sunday (May 12) is International Nurses Day, a day which marks the contributions that nurses make all around the world. The day is in fact the birthday of Florence Nightingale, who was a pioneer nurse as well as a pioneer of medical statistics. Nurses are multi-talented!

Thank you, all you nurses, for your contributions to the world!

Revising the Metric System

Relationship between the new SI units (image produced using the igraph package of R)

On May 20, a major redefinition of SI (metric) units comes into force. In particular, the second, metre, ampere, mole, kilogram, kelvin, and candela will be defined as follows:

The second (unit of time)

As it is now, the second will be defined using ultra-precise caesium clocks. Specific microwave radiation from caesium atoms is defined to have a frequency of exactly 9.192 631 770 GHz. That is, counting 9,192,631,770 waves will take exactly one second.

The metre (unit of length)

As it is now, the metre will be defined using the speed of light, which is defined to be exactly 299,792,458 metres per second. That is, the metre is the distance travelled by light in one 299,792,458th of a second (where the second is defined as above).

The ampere (unit of electric current)

The definition of the ampere (amp) has been greatly simplified, taking account of the connection between electricity and electrons. The ampere is a coulomb of electric charge flowing past a given point per second, and the charge on a single electron is now defined to be 1.602 176 634 × 10−19 coulombs. Thus an ampere is about 6,241,509,074 billion electrons flowing past a given point in a second.

As a consequence of this new definition, two important natural constants which used to have defined values (the permeability of free space and the permittivity of free space) now have experimentally determined ones. This will require rewriting pretty much every physics and electrical engineering textbook.

The mole (unit of amount of substance)

The mole represents Avogadro’s number of atoms, molecules, or other particles. Previously, Avogadro’s number was defined to be the number of carbon atoms in 12 grams of pure carbon-12. It is now defined to be exactly 6.022 140 76 × 1023.

The kilogram (unit of mass)

Until 2019, the kilogram was defined by the mass of a specific metal cylinder held in Paris. This has been felt to be unsatisfactory for many years. The current definition uses the fact that the energy of a light photon (in joules) is its frequency times Planck’s constant h, which is defined to be exactly 6.626 070 15 × 10−34.

In practice, a Kibble balance will be used to measure weights by balancing them against an electrically produced force. Units derived from the kilogram include:

  • The newton (unit of force): the force needed to accelerate 1 kilogram at a rate of 1 metre per second squared
  • The pascal (unit of pressure): 1 newton of force per square metre
  • The joule (unit of energy): the energy used in applying a force of 1 newton over a distance of 1 metre
  • The watt (unit of power): 1 joule of energy per second
  • The volt (unit of electric potential): the amount of electric potential across a resistance producing 1 watt of heat per ampere of current
  • The ohm (unit of electrical resistance): the resistance which produces 1 ampere of current when 1 volt of electric potential is applied

See also what NIST has to say about the kilogram.

The kelvin (unit of temperature)

Temperature in degrees Celsius was originally measured on a scale with 0 °C being the freezing point of water and 100 °C the boiling point (at standard pressure). The lowest possible temperature turned out to be absolute zero, −273.15 °C. In 1954, the two fixed points on the scale were changed to −273.15 °C (0 kelvins) and the triple point of water, 0.01 °C (273.16 kelvins).

This definition proved unhelpful for calibrating thermometers intended for very high temperatures, and the current definition uses the fact that the average translational kinetic energy (in joules) of a moving atom of a monoatomic ideal gas is (3/2k T, where T is the temperature of the gas in kelvins, and the Boltzmann constant k is defined to be exactly 1.380 649 × 10−23.

The candela (unit of luminous intensity in a given direction)

The definition of the candela remains what it has been, except that it is influenced by the change in definition of the kilogram (and hence the watt). A light source that emits monochromatic yellowish-green light at a frequency of 540 THz (roughly 555 nm wavelength) is taken to emit 683 lumens per watt, and a light source that uniformly radiates 1 candela in all directions has a total luminous flux of 4π lumens (the constant 683 reflects the human ability to perceive light). The lux is a lumen per square metre.

The dream

When the metric system was first introduced, the metre was defined in terms of the world (1/10,000,000 of the distance between the Equator and the North Pole, measured via Paris). Today, the metric system carries that philosophy to its ultimate conclusion, with all units except the candela defined in terms of the universe. Five of the units are defined in terms of fundamental physical constants: the speed of light (first measured by Rømer in 1676), the charge on the electron (first measured directly by Robert A. Millikan in 1909), the Avogadro constant (measured several ways by Jean Perrin around 1910), and the Planck and Boltzmann constants (first defined by Max Planck around 1900).

The redefined metric system is a little difficult to grasp without understanding modern physics, but fortunately most of us will just keep on using exactly the same measurement instruments as we have done for years.