Instead of being based on time alone, it’s based on astronomical angles and trigonometry. The other major definition that we sometimes use is also, albeit indirectly, based on the definition of Earth orbiting the Sun to make up a year: the parsec. Since the speed of light is a known and measurable constant, a “light-year” then arises as a derived unit of distance, and also only changes by very little over time it’s consistent over billions of years to the ~99.98% level. ( Credit: NASA/JPL-Caltech)Įven with all of the complex astrophysics taking place in our Solar System, then, it’s apparent that the duration of a year is probably the most stable large-scale feature that we could use to anchor our timekeeping to our planet. As the Sun loses mass via E = mc^2, the Earth slowly spirals outward, increasing its orbital distance by ~1.5 cm per year. The eccentricity, or the difference between the “long axis” and the “short axis” of our orbit, changes over time, while the Earth-Sun orbital period, which defines our year, changes slowly over the lifetime of our Solar System. The Earth orbits the Sun not in a perfect circle, but rather in an ellipse. This corresponds to the year lengthening by about 2 hours from the start of the Solar System until today. This has caused the year to lengthen, but only slightly: by about 2 parts in 10,000. This also pushes Earth out to distances a little bit farther from the Sun, and causes it to orbit slightly more slowly over time. The largest factor is the changing mass of the Sun, which has lost about a Saturn’s worth of mass over its lifetime so far. The variation in a year, however - or the time period required for Earth to complete a full revolution around the Sun - has only changed a little bit over the Solar System’s history. Some ~4 billion years ago, a “day” on planet Earth only lasted 6-to-8 hours, and there were over one thousand days in a year. As the Moon, Earth, and Sun all interact, the phenomenon of tidal friction causes our day to lengthen and the Moon to spiral away from Earth. But on a closer inspection, for a variety of reasons, the notion of days and years as we experience them on Earth don’t particularly translate very well into a universal set of axioms for marking the passage of time.įor one, the duration of a day has changed substantially over the history of planet Earth. ( Credit: Rob Carr/Wikimedia Commons)īased on this, it’s easy to understand why we came up with a system of timekeeping that is based around concepts such as a “day” and a “year,” as our activity on this planet is very tightly correlated with those periodic recurrences. At least, that’s what Jerry Bear thinks, writing in to ask:Īs the Earth orbits the Sun in an ellipse, it moves more quickly at perihelion (closest-to-the-Sun) and more slowly at aphelion (farthest-from-the-Sun), which leads to changes in the time at which the Sun rises and sets, as well as the duration of the actual day, over the course of a year. We know what the Universe is, we know that the part observable to us is a hair over 92 billion light-years in diameter, and we know that the hot Big Bang, which started off the Universe as-we-know-it, occurred precisely 13.8 billion years ago, with an uncertainty in these values of just ~1% or so.īut why, of all the ways there are to measure time and distance, do we use such an Earth-centric set of units, like “years” and “light-years”? Isn’t there a better, more objective, more universal way to do it? Surely there is. ![]() ![]() Questions like, “What is the Universe?” “How big is it?” and “Was it eternal, or did it spring into existence, and if so, when?” used to be some of the greatest philosophical mysteries, and yet the last 100 years have provided firm, scientific answers. There are a number of grand questions we can ask about the Universe that cut right to the very core of what reality actually is, and were some of the biggest head-scratchers for all of human history.
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