Measurement on a Cosmic Scale
Space is Really Big
On a human scale, cosmological distances are almost unimaginably vast and require their own system of units to make them manageable and meaningful. Many such units, as used by astronomers on Earth, have little or no meaning on Calmarendi, being based around the dimensions of the Solar System and the time measurement system used on Earth. However, in this document, we use these Earthly units because those are what most readers will be familiar with and, although Calmarendian science does have a similar system, astrophysics is rarely a subject of day-to-day conversation, even amongst the great and the good.
The Astronomical Unit
Notionally, the Astronomical Unit (AU) is taken to be the mean distance between the Earth and Sol and is defined as being 149,597,870,700 metres exactly. Approximately 150 million kilometres is close enough for rock 'n' roll. The equivalent unit on Calmarendi, the Calmarendian Orbital Unit (COU), is approximately 4.44 AU or 665 million kilometres.
The Parsec
On Earth, the parsec (pc) is defined as the unit of length equal to the distance at which a star would have a heliocentric parallax of one second of arc, approximately 31 trillion kilometres or 3.26 light years. Sol, the Earth and the star form the three angles of a right angled triangle: the right angle is at the Sun, the angle at the star is the parallax angle and the side opposite the parallax angle is the distance from Earth to Sol. This distance, by definition, is one Astronomical Unit (AU) and thus, by simple trigonometry, the length of the adjacent side – the distance from Sol to the star – can be calculated: a parallax angle of 1" defines the parsec and gives rise to an interstellar distance of 206,265 AU.
On Calmarendi the parsec is defined in the same way and, as angles are measured the same way they are on Earth but the planet’s orbital semimajor axis is 4.44 AU, we get a distance 4.44 times as long for any given parallax angle. Thus a Calmarendian parsec is 915,815 AU or 206,265 COU.
The Light Year
Whilst the parsec is the favoured unit of interstellar distance measurement amongst astronomers and astrophysicists , the light year (ly) is more common colloquially. A light year is defined as the distance light travels, in a vacuum, in one year.
The Calmarendian equivalent of the light year is the Cycle of Light (CL) and is, unsurprisingly, defined as the distance light travels in a Calmarendian cycle. As we have seen elsewhere, as near as makes no odds, in absolute time one Calmarendian Cycle lasts exactly seven Earth years. Thus, in the course of a Calmarendian Cycle, light will travel seven of your Earth light years. If they knew where to look and had the means to measure its distance, Calmarendians might describe Earth as being 1.7 million Cycles of Light away from them.
665 million kilometres
137 trillion kilometres
66 trillion kilometres
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