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Fundamental Units

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Fundamental Units of Physics And Astronomy

Length

  • Meter (m): The meter is the base unit of length in the International System of Units (SI). It is defined as the length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second.
  • Astronomical Unit (AU): The astronomical unit is a unit of length, roughly the distance from Earth to the Sun. It is used for measuring distances within the Solar System.
  • Light-Year (ly): The light-year is a unit of length equal to the distance that light travels in one year in a vacuum. It is used for measuring astronomical distances on a larger scale.
  • Parsec (pc): The parsec is a unit of length used in astronomy, equal to about 3.26 light-years or 3.09 × 10^13 kilometers. It is used for measuring large distances in space.

How a Parsec is Defined

Let familiarize ourselves with the concept of Parallax, which is the apparent shift in the position of an object when viewed from different locations. The parallax angle is the angle between the two lines of sight to the object from two different locations.

This makes a Parsec quite interesting, as it is a unit of length determine by how we find the distance to a star. The distance at which a star would have a parallax angle of one arcsecond is defined as one parsec.

Time

  • Second (s): The second is the base unit of time in the International System of Units (SI). It is defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the cesium-133 atom.

We don’t really have a standard unit of time in astronomy, but we often use seconds, minutes, hours, days, years, etc.

Mass

  • Kilogram (kg): The kilogram is the base unit of mass in the International System of Units (SI). It is defined as the mass of the International Prototype of the Kilogram (IPK), a platinum-iridium cylinder kept at the International Bureau of Weights and Measures in France.
  • Solar Mass (M☉): The solar mass is a unit of mass equal to the mass of the Sun, approximately 1.989 × 10^30 kilograms. It is used for measuring the masses of stars and other celestial objects.