On the scale of time

See also my time-line for an account of when particular things happened. Thechnically, that's concerned with positions (in time) while the following is concerned with intervals (separations in time). Note that there is a natural equivalence between length scales and time scales, via a factor of the speed of light; though we relate to the two rather differently. Likewise, aside from an arguable unit of cycles of something repeated, inversion yields an equivalence between times and frequencies. Note that the Planck unit of time, 0.13512e-42 s, is so ludicrously small as to be negligible for any physical process.

millennium, 1 k yr

Over the last 800 k yr, rates of change in atmospheric CO2 concentrations have (except very recently) been at most 30 parts per million over a thousand years – but 1989 to 2006 also saw increases of 30 parts per million.

mega year, 1 M yr

A million years seems a long time to most folk, but less so to geologists and cosmologists.

giga year, 1 G yr

Our solar system orbits the Milky Way galaxy in about a quarter G yr. The Milky Way and Andromeda are expected to pass close to one another in about 2 G yr and, after a second close pass, to fuse together in about 5 G yr – which is roughly how long our Sun is expected to last before turning into a planetary nebula.

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