International Astronomical Union or IAU is the international association of astronomers, PhD and beyond, that are active in research and education in the field of astronomy. Apart from the research work, it is the authority recognized for assigning names and designations to celestial bodies and other surface features on stars, asteroids and planets among other bodies. IAU was formed over a century ago in July of 1919 in Paris, France.
They are the ones who define and recognize astronomical constants used in astronomy. For those who do not know, astronomical or universal or physical constant is a physical quantity that has a constant value in time. IAU’s system of astronomical constants includes astronomical units for mass, length, time as well as constants for constant of gravitation and speed of light.
While there are quite a few astronomical constants recognized by IAU and many other organizations across the globe, given below are the ten most important constants that are responsible for the major breakthroughs in human space exploration.
Hubble Constant
The first astronomical constant is one that most space lovers would be most aware of- the Hubble constant. It is 100% related to the Hubble telescope, which is one of the most important space instrument on Earth. The Hubble constant was calculated in the 1920’s by Edwin Hubble, the American astronomer who invented the Hubble Telescope. It has been a known fact for over a century that our universe is expanding at an accelerated speed. The Hubble constant is the unit that describes exactly how fast our universe is expanding and is changing continuously. In addition to that, there are a lot of discrepancies in the value of the constant in the space industry. As of 2019, the Hubble constant is 67.4km/sec/Mpc.
Solar Constant
Solar constant is the total energy radiated that is received by the atmosphere of the Earth from the Sun. The energy passes the surface of the planet at an angle of 90° at a specific distance from our home star. In simple words, solar constant is the unit at which radiant energy reaches the surface of the Earth from the Sun. The solar constant is approximately 1388 watts per square meter. NASA has launched various solar monitoring spacecrafts throughout the years to monitor radiations from the Sun.
Luminosity of Sun
In astronomy, luminosity refers to the total amount of light that is emitted by any celestial object in the universe in a set unit of time. Luminosity is different from brightness. Luminosity can also be called intrinsic brightness as well as absolute brightness, which means that it is the total energy that a star radiates in one second in every direction. Whereas brightness or apparent brightness of a star like Sun is in fact how bright the Sun actually appears to a viewer. The luminosity of the Sun has increased about 30% since its formation 4.5 billion years ago. Currently, the home star’s luminosity is 3.846 × 1033 ergs per second or 3.846 × 1026 watts.
Solar Mass
The solar mass or in simple terms, the mass of the Sun is equal to 2×1030 kg. It is a standard unit of mass in the unit system of astronomy. Solar mass (M☉) is not only used to demonstrate the masses of all the other stars in the universe, it also indicates the mass of galaxies, nebular and clusters. Isaac Newton was the first one to give an estimate of the mass of the Sun in 1687 in his work Principia. For comparison, the solar mass is 1047 times more than the mass of Jupiter and approximately 333000 time the mass of our planet.
Equatorial Radius of Earth
When it comes to the radius of the Earth, it is the distance from its center to a specific point on the surface. There are different ways to calculate the Earth’s radius and some of them do not have the same results as our planet is not a perfect sphere. However, IAU prefers the equatorial radius as a standard unit of measurement, especially in the fields of astronomy and geophysics. Equatorial radius is measured from the equator to one of the poles and the radius ranges from 6378 km to 6357 km.
Mass of Earth
Just like the solar mass, Earth’s mass is also a nominal unit of measurement of mass in astronomy. It is used to measure the masses of terrestrial planets as well as rocky planets like Venus, Mercury & Mars and exoplanets i.e. planets outside our solar system. The symbol of Earth mass is M⊕, which is equal to 5.9722×1024 kg. If you talk about the concentration of Earth’s mass, it is mostly accounted by oxygen and iron, which constitutes about 32% each as well as silicon and magnesium (15% each) and aluminium, calcium and nickel, which are about 1.5% each.
Sidereal Year (y)
The next constant in the top ten astronomical constants is Sidereal year, which is denoted by ‘y’. It is a unit of time that accounts the time taken by our planet to orbit our home star one time with respect to all the fixed stars in our universe. This also means that it measures the time taken by the Sun to come back to the same position after orbiting the ecliptic once. A sidereal year is equal to 365.256 363 004 Ephemeris days or 365 days 6 hours 9 minutes 10 seconds, which is 20 minutes 24.5 seconds longer than a tropical year because of the axial precession.
Parsec (pc)
Parsec, which is denoted by ‘pc’, is a unit of length or distance that is used to measure the distance between celestial bodies outside our solar system. A single parsec is equal to about 31 trillion kilometers or 19 trillion miles. In the everyday life of a professional astronomer, they use parsec or pc more frequently instead of constants like light years when it comes to denoting distances in astronomy. (1 pc = 3.26 light years)
Light Year (ly)
The international astronomical union defines a light year as “the distance that light travels in a vacuum in 1 Julian year”. Simply put, it is a standard unit of length, which is used to express astronomical distances. One light year is equal to approximately 9.4 trillion kilometers. While pc is mostly used by the professionals in their daily life, light year is used in popular and non-specialist science publications to express the distances to celestial object on a galactic scale.
Astronomical Unit (AU)
The last astronomical constant is not only the most standard unit of length in the field of astronomy but is responsible for exploration of our own solar system. AU is the distance from Earth’s surface to the Sun, which is equal to approx. 150 million kilometers. While the actual distance varies a few times every year, it is the average of the aphelion and perihelion. AU’s primary use is to measure the distance in our own solar system as well as around other stars,
